Development of Organic Hop Production in the Pacific Northwest

Final Report for SW09-050

Project Type: Research and Education
Funds awarded in 2009: $123,465.00
Projected End Date: 12/31/2012
Region: Western
State: Washington
Principal Investigator:
Dr. Kevin Murphy
Washington State University
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Project Information


As Proposed

Seventy-five percent of the hops produced in the U.S. are grown on 28,000 acres by approximately 65 farmers in the Yakima Valley of Washington State. Oregon represents another approximately 15% of the U.S. hop acreage. Hops are a perennial high-value crop that, under current production standards, requires large quantities of pesticides and nitrogen fertilizer to achieve high yields and good quality. In response to increasing demand for organic hops and the rising costs of fertilizer and crop protection chemicals, hop growers in the Pacific Northwest have begun to plant organic hops. Hop yields, however, often show dramatic decreases under organic or low-input management due to increased insect and disease pressure. Research in low-input and organic systems is needed to identify suitable hop cultivars and evaluate the ability of cover crops to suppress weeds, build soil quality, supplement nitrogen, provide habitat for beneficial insects and optimize nitrogen fertility and irrigation management. The tasks of this project include: 1) evaluate hop varieties in organic and low-input systems for insect (aphids and mites) and disease (powdery and downy mildew) resistance, cone quality and yield; 2) evaluate different in- and between-row cover crop options to enhance soil fertility, enhance weed suppression and provide habitat for beneficial insects; and 3) develop an educational handbook for hop growers that focuses on sustainable hop production.

Field trials focusing on hop agronomy and varietal selection will occur on certified organic ground on several cooperator farms in Washington State. These field trials emphasize a whole systems approach that includes the use of cover crops, intercrops, fertility and irrigation treatments, beneficial insect monitoring and diverse hop varieties to optimize hop cone yield and quality while suppressing weeds, disease and detrimental insects. We anticipate that results from this trial will have positive short-range outcomes, including the identification of varieties that perform well in organic systems and the increase in producer familiarity with and knowledge about sustainable hop production. Medium-range outcomes will be evaluated through the successful development of integrated hop farms, a change in acreage of organic hop production and a documented exchange of chemical usage in conventional hop production for biological disease and insect control methods. Field days will complement trade, extension, online and academic journal publications in an effort to reach the maximum number of producers, extension, researchers and industry professionals. This project was conceived and designed by PNW hop farmers in close communication with Washington State University researchers and extension personnel. Significantly, the hop growers in the area intend to use portions of this research on their conventional ground to reduce inputs in their quest toward regional agricultural sustainability. This project offers a multi-institutional, multi-state team of farmers, scientists and extension specialists the opportunity to improve the production of low-input and organically grown hops.

After Implementation

The demand for organically grown hops represents a small, but increasing, amount of total hop (Humulus lupulus L.) demand in the United States. This is due in part to legislation recently passed by the National Organic Standards Board which, as of January 2013, requires certified organic hops to be used in beer labeled organic. Hops, like many crops, face greater challenges when grown under organic practices. High nitrogen requirements, along with disease, weed and pest pressure all contribute to these challenges. Cover crops are an important component of aiding in weed control and increasing soil quality in organic production of many annual and perennial crops, but little research has investigated the use of cover crops in organic hop production. The objective of this study was to evaluate different cover crop treatments for their ability to suppress weeds and alter the weed populations within a certified organic hopyard in the Yakima Valley of Washington State. The experiment examined seven different interrow cover crop and tillage strategies against a control of existing vegetation to determine their effect on weed populations. A split-plot design with three replicates was used. The design included four hop variety treatments and eight total groundcover treatments. The response to different groundcover management strategies were examined for biomass, plant density and percent groundcover for both cover crops and weeds in the interrows of the hopyard. Cover crop treatment varied in their effect on weed biomass, weed plant density and groundcover. The control treatment consistently had among the highest weed biomass, weed plant density and percent groundcover by weeds than that of the applied cover crop or tillage strategies. Groundcover treatment had no effect on hop yield. Results indicate that cover cropping systems are a viable strategy for assisting with the control and management of groundcover within an organic hopyard.

Project Objectives:

1. Formalize an advisory committee of organic and low-input hop growers and establish grower roundtable discussions in Washington State.
2. Identify high quality hop varieties optimally adapted to low-input and organic production systems.
3. Evaluate in-row cover crops for weed suppression, disease and insect control, fertility enhancement, creation of beneficial insect habitat and positive varietal interactions.
4. Evaluate drive-row intercrops for fertility management, beneficial insect attractants, impacts on disease and insect pressure, drought resistance and soil quality effects.
5. Conduct effective outreach through field days on growers’ fields in Washington and publication of results in a wide range of media.
6. Develop an educational product for growers focusing on organic, low-input, biologically diverse hop production.


Hops (Humulus lupulus L.) have a long history of being commercially propagated for the flower of the female hop plant, which is used in the beer brewing industry (Burgess, 1964). Global hop production reached 151,850 MT in 2009, of which 42,000 MT were harvested in the United States valued at over $336 million (FAO, 2010; USDA, 2010). United States hop production is concentrated in the Yakima Valley region of south-central Washington. The region produces nearly 80% of U.S. hops (USDA, 2012). Organic hop production makes up a small, but increasing, percentage of U.S. hop production, in which over 12,000 ha of hops were harvested in 2011 (USDA, 2012). The organic hop harvest is projected to reach nearly 200 ha in 2013, up significantly from an estimated 51 ha in 2010 (AOHGA, 2012).

Weed control remains one of the most important problems in organic agriculture, including organic hop production (Lotter, 2003; Bàrberi, 2002; Turner, et al. 2011; Organic Farming Research Foundation, 2004). Conventional methods of weed control in hops include herbicides in the hop row, in addition to tillage and mowing in the hop interrow, the space between hop rows. Hand weeding in organic hops is labor intensive and expensive, but is necessary to maintain adequate yields. Weeds compete both indirectly and directly with crops, primarily for light, water and nutrients (Zimdahl, 2007). The perennial root system of a well-developed hop plant is large and complex, and can grow over four meters deep and up to five meters laterally (Beatson et al., 2009; Burgess, 1964). The large rootsystem suggests that older, well-established plants are unlikely to be influenced by competition from weeds, but changes to soil fertility or leached allelopathic chemicals from plant residue may impact growth and available nutrients in a wide radius around well established plants. Hops, a climbing vine, generally only compete with weeds when becoming established, or early in the growing season. However weeds compete directly with hop plants for water and nutrients. For optimal production, hops require approximately 150 kg ha-1 N, annually (Gingrich et al., 1994).

The use of cover crops to help control weeds, improve soil quality and attract beneficial arthropods is well documented in both annual and perennial agricultural systems (Hartwig and Ammon, 2002; Abawi and Widmer, 2000; Grasswitz and James, 2009; Ramos et al., 2010; Thorup-Kristensen et al., 2003; Baumgartner, et al., 2008). The dynamics of crop-weed and weed-cover crop interaction are complex, but several important themes have been described by Gallandt et al. (1999): weed suppression increases as cover crop biomass increases, weed control by cover crops is species specific, and complete weed control is rarely achieved. Suppression of weeds by cover crops is accomplished through several mechanisms. Initially, cover crops or their residue can physically prevent seedling emergence by occupying the space in which weeds would otherwise inhabit. Cover crops also inhibit weed germination and growth by altering light, temperature and water availability. Once weeds emerge, cover crops compete directly with the weeds for resources and physical space. Cover crop residues can also contain allelopathic chemicals which are phytotoxic to plants, and these organic residues can lead to increased soil microbial activity and reduction in the viable seedbank (Gallandt, et al. 1999; Camaal-Maldonado, et al. 2001).

Weed control is costly and labor intensive in organic hop production and represents one of the critical challenges that organic hop producers face. In this on-farm study, our objective was to evaluate different cover crop treatments for their ability to suppress weeds and alter the weed populations within the hopyard. Cover crop treatments were chosen based on their ability to increase soil organic matter and fertility, survive compaction, thrive in a variety of temperature and moisture regimes, and out-compete and suppress weed establishment.


Click linked name(s) to expand
  • Lori Hoagland
  • Scot Hulbert
  • Melissa Matthewson


Materials and methods:

The experiment was conducted in a certified organic commercial hopyard in the Yakima Valley of Washington State (N 46? 21.6´ Lat., W 120? 28.2´ Long.) from 2010 to 2012. Soil at the site is a Naches series loam soil (fine-loamy over sandy or sandy-skeletal, mixed, superactive, mesic Aridic Argixerolls), well-drained and deep soil with loam over sand (USDA, 2012). The site received 86 mm of precipitation during the 2011 experiment, between planting and biomass harvest, and 134 mm during the 2012 experiment. Precipitation and temperature data was attained through the Washington Agricultural Weather Network’s AgWeatherNet (AgWeatherNet, 2012). Hop cuttings were propagated in a greenhouse and were planted in the field in early August 2010. Three varieties (Cascade, Centennial and Willamette) were included along with a no-plant control in which the hop hills were left empty. Hills were spaced 1 m apart with 15 hills between trellis poles. Rows were oriented north to south with 3.6 m between them. Trellis height was 5.5 m. Hop plants were drip-irrigated beginning in May and continued throughout the growing season as needed. Irrigation rate was dependent on climatic factors.


The experimental design was a split-plot, where the whole plot consisted of a completely randomized design with three replicates each and a one way treatment structure (variety at four levels). The subplot design structure was a randomized complete block design with a one way treatment structure (cover crop at eight levels). Analysis of variance (ANOVA) was used to determine differences in response variables among treatments. The no-plant control was used in the hop treatments for the purpose of assessing whether or not there was a significant shading effect on the groundcover plant communities from the hop plants themselves.

Cover crop treatments were planted in the interrows, with each treatment spanning the length of seven hop hills and covering the interrow on either side of the hop row. The interrow area planted on each side of the hop row was 3.6 m x 8.2 m. Growing degree days (GDD) were calculated using a base of 0?C (AgWeatherNet, 2012). Cover crops were planted May 11, 2010 (857 GDD), April 1, 2011 (371 GDD) and April 7, 2012 (400 GDD) (Table 1). The hop row was a 0.6 m strip in which the hops are planted oriented north-south. The hop row was not planted to a cover crop. Experimental hop rows were separated by existing, commercially managed hop rows. Before each cover crop planting, the seed bed was prepared by applying two passes of a roto-tiller (Northwest Tillers, 3715 W. Washington Ave, Yakima, WA). Cover crops were planted by hand-broadcasting seed that was then hand raked in to a depth of 10-15 mm before being compacted with a custom built roller-packer that matched the width of the drive-row. Cover crops were irrigated via overhead sprinklers to supplement soil moisture and moderate spring rains in 2010 and 2011 for approximately three weeks until plants were established. Supplemental overhead irrigation was also applied whenever temperatures exceeded 29 ?C. The primary purpose of the temperature-dependent overhead irrigation was to cool and humidify the hopyard to alleviate pest pressure from two-spotted spider mites (Tetranychus urticaeKoch) that thrive under hot, dry conditions (Grasswitz and James, 2009). Plots were permanent and treatments remained in the same locations throughout the course of the experiment.

In 2011, an outbreak of downy mildew (Pseudoperonospora humuli Miyabe & Takah.) G. W. Wilson, brought on by an unseasonably cool and wet spring, was exacerbated by overhead irrigation. Therefore, overhead irrigation was not applied in the early 2012 season due to the downy mildew pressures and costs associated with control in 2011. Overhead irrigation was also shut off from June 15, 2011 to July 10, 2011. Soil moisture and natural precipitation was used to establish the cover crops in 2012. Due to lack of irrigation, cover crop stand establishment was sporadic in early spring, resulting in less than 10% emergence for all treatments. The site received over 20 mm of precipitation between June 3 and June 5, which caused a flush of new cover crop and weed emergence. Between June 5 and 8 of 2012, four 0.8 m2 areas were marked in each plot and previously established weeds and cover crops were removed, leaving only plants that had emerged with recent rains. These locations were used for all sampling that occurred after June 6, 2012, including percent cover, plant density and biomass collection. Two drip irrigation lines of 18 mm diameter and emitters at 1 m intervals were placed in each drive row and situated to ensure irrigation of marked quadrats and the surrounding drive row. Overhead irrigation was used again in mid-June when temperatures began to exceed the 29 ?C threshold. Drip irrigation continued to supplement overhead irrigation, ceasing in late August 2012.

The cover crop treatments included: periodic tillage (TIL), perennial rye and red clover (PRyeC), medic and subterranean clover mix (MED), annual clover mix (CLO), fescue roadway mix (ROAD) and soil builder mix (SOB). A control of only periodic mowing (NONE) was also implemented (Table 1). TIL consisted of tillage at three different times annually with existing vegetation only and no cover crops planted. Tillage was performed before cover crop planting in May 11, 2010, April 1, 2011 and April 7, 2012. TIL was also tilled in mid-June in 2011 (1126 GDD) and in late July, 2012 (2311 GDD) due to delayed establishment, and in late October after harvest was complete. PRyeC was planted with perennial ryegrass (Lolium perenne L.) and red clover (Trifolium pretense L.) in mid May 2010 and early April 2011 and 2012. The combination was planted at a rate of 50 kg ha-1 (seed composition by weight: 50% perennial ryegrass and 50% red clover). MED consisted of a mixture of ‘Paraggio’ barrel medic (Medicago truncatula Gaertn.) and ‘Antas’ subterranean clover (T. subterraneum L.) that was planted in mid May 2010 and early April 2011 and 2012. The combination was planted at a rate of 50 kg ha-1 (seed composition by weight: 50% barrel medic and 50% subterranean clover). CLO was a combination of clover, subterranean clover, and medic consisting of ‘Lightning’ and ‘Nitro’ Persian clover (T. resupinatum L.), rose clover (T. hirtum All.), crimson clover (T. incarnatum L.), ‘Dalkeith’, ‘Trikkala’, ‘Campeda’, and ‘Denmark’ subterranean clover and ‘Paraggio’ barrel medic. The CLO mixture was planted in mid May 2010 and early April 2011 and 2012 at a rate of 50 kg ha-1 (seed composition by weight: 5% ‘Lightning’ Persian clover, 7% ‘Nitro’ Persian clover, 10% rose clover, 15% crimson clover, 10% each of ‘Dalkeith’, ‘Trikkala’ and ‘Campeda’ subterranean clover, 15% ‘Denmark’ subterranean clover and 18% ‘Paraggio’ barrel medic). ROAD was a mixture of tall fescue (Festuca arundinacea Schreb.), strawberry clover (T. fragiferum L.), white clover (T. repens L.) and Kentucky bluegrass (Poa pratensis L.) which was planted in mid May 2010 and early April 2011 and 2012. It was planted at a rate of 280 kg ha-1 (seed composition by weight: 60% tall fescue, 18% strawberry clover, 18% white clover, and 5% Kentucky bluegrass). SOB was planted with a combination of ‘Cayuse’ oat (Avena sativa L.), hairy vetch, purple vetch (Vicia Americana Muhl. Ex Willd.), bell bean (Vicia faba L.) and ‘Biomaster’ peas (Pisum sativum L.) in mid May 2010, early April 2011, and 2012. The SOB mixture was planted at a rate of 240 kg ha-1 (seed composition by weight: 10% ‘Cayuse’ oat, 10% hairy vetch, 20% purple vetch, 40% bell bean, and 20% ‘Biomaster’ peas) (Table 1). The NONE treatment allowed existing species to grow, which were maintained only through periodic mowing.

Cultural Practices

Hops were cut back at the beginning of each growing season using a custom built mechanical pruner which removes the plant growth at the immediate soil surface in early April of 2011 and 2012. Hop bines were trained twice each season, the first approximately one month after pruning and the second occurred one week later. The area directly surrounding the hop plants were hand-weeded four times throughout each growing season, with the first weeding occurring at training, and the following occurring every four weeks through July. Interrows were mowed using a rotary mower (Bush Hog 3208, 2501 Griffin Ave. Selma, AL), set to 30cm height with side discharge to deposit the removed material into the hop-row. Mowing began in late April and was performed once per month until the first week of September 2011 and the first week of August 2012.

Statistical Analysis

Univariate analysis was used to determine normality of the data. Transformations were used to best meet the assumptions of constant variance and approximate normality of the data, which was confirmed through the univariate analysis. The data for plant density and biomass were log10 transformed to normalize the data. A value of one was added to all data points before taking the log10 in order to account for observed zeros in the data. Percent cover data were transformed using an arcsin transformation as described by Ahrens, et al. (1990). Analysis was performed on transformed data and means were reported in their raw form.

Effects of treatment, year and hop variety on cover crop and weed biomass, as well as plant density, were analyzed using ANOVA. Sampling date was also used in the analysis for percent groundcover. A mixed model was used to determine the effects of different cover crop treatments on weed populations. ANOVAs were conducted using the ‘mixed’ procedure in SAS(SAS Institute, Cary, NC) with the Satterwaite as the denominator degrees-of-freedom method. A repeated measures statement was used with the ‘mixed’ procedure to evaluate percent cover over the six sampling dates in each season. Least squared means (LSMeans) were reported when performing multiple comparisons, and letter groupings were determined using Fisher’s protected LSD method of multiple comparisons. Letter grouping via means separation was performed using the pdmix800 macro in the mixed procedure (Saxton, 1998). Means sharing the same letter were considered not significantly different from one another, and α = 0.05 was used for all statistical measures.


Percent groundcover by species was estimated over six dates in both 2011 and 2012. Accumulated growing degree days for sampling were calculated starting at zero days after planting (DAP). Sampling dates one through six for 2011 were: (Date 1) May 5 (281 GDD), (Date 2) May 25 (541 GDD), (Date 3) June 15 (872 GDD), (Date 4) June 29 (1124 GDD), (Date 5) July 21 (1556 GDD), and (Date 6) August 8, 2011 (1949 GDD). Sampling dates one through six for 2012 were: May 9 (408 GDD), May 24 (637 GDD), June 14 (975 GDD), July 4 (1339 GDD), July 24 (1802 GDD), and August 9 (2175 GDD) (Table 2). Sampling was performed using 1 m2 quadrats in 2011, and the first two sampling dates of 2012, while 0.5 m2 quadrats were used for the final four sampling dates in 2012. A quadrat was randomly placed at two locations on each side of the hop row and percent groundcover was visually determined for each species within the quadrat for a total of four random subsamples per subplot in 2011. In 2012, the quadrats were placed within the four permanently marked and prepared areas. Species were grouped together when timely visual identification was not applicable (eg. clover mix, or medic mix spp.).

Plant densities were taken August 16 through 18, 2011 (2131 GDD) and August 18 through 20, 2012 (2412 GDD). Six randomly placed 0.1 m2 quadrats per plot, with three on each side of the hop row, were used for sampling in 2011. In 2012, six 0.1 m2 quadrats per plot were placed within pre-established sampling areas, with three subsamples on each side of the hop row. Plants were counted by species or group of species when timely visual identification of individual species was not applicable.

Biomass samples were taken once per year in 2011 and 2012. Samples were taken October 6 through 8 in 2011 (3091 GDD), and October 3 through 5 in 2012 (3218 GDD). 1 m2 quadrats were randomly placed with one on either side of the hop row in 2011, and 0.5 m2 quadrats were used in 2012, utilizing one of the previously marked sampling sites in 2012 for a total of two subsamples per subplot per year. Biomass was removed at the soil surface and separated by species or group of species as appropriate. Samples were dried at 45 ?C for a minimum 96 hours in order to ensure they were completely dry before being weighed. Relative abundance of harvested species was determined as a percentage of total biomass per treatment.

Hop cones were harvested at maturity for each hop variety. Harvest occurred from September 7 through 18, 2011 and September 9 through 13, 2012. Five of the seven hop plants in each subplot were harvested. Hops plants were removed from the field during harvest, and hop cones were picked using a stationary Wolf picker (Wolf Spezialmaschinen, Germany). Yield was reported in kg of dry cones per hop hill. Hills in which a plant failed to establish were not counted when calculating average yield per hop hill.

Research results and discussion:

1. Successfully obtained, evaluated and analyzed all years of yield data, including green weight, cone weight (green and dry), pounds per string, pounds per hill and mg/cone on 16 varieties grown on an organic hop farm in the Yakima Valley. Varieties include: Cascade, Centennial, Chinook, Cluster, Fuggle, Galena, Glacier, Mt. Hood, Newport, Nugget, Perle, Santiam, Sterling, Tettnang, Vanguard and Willamette. This data is currently being written up for publication to growers and scientists

2. Obtained second year quality data, including alpha acid, beta acid and H.S.I on the 16 varieties (see #1 above) grown on an organic farm in the Yakima Valley.

3. Graduate student Erin Hightower is analyzing and writing up this data. We expect to have a manuscript sent out for publication in summer 2014.

4. Obtained , evaluated and analyzed all years of yield and quality data on three varieties that were grown on one organic farm in the Yakima Valley in a cover crop trial. These varieties included Cascade, Centennial and Wilamette.

5. Obtained year weed suppression and cover crop data from each of eight cover crop treatments tested on one organic farm in the Yakima Valley in 2012. This data includes stem counts of weed and cover crop species and biomass of weed and cover crop species.

6. Graduate student Sam Turner successfully published the cover crop trial data in his MS thesis, and our hop team is in the process of publishing two papers from his thesis (one chapter was published in Agronomy Journal in 2011). One will be sent to a peer-reviewed academic journal and the other will be published as a WSU extension paper in 2014.         

For detailed results and discussion, please see below.

Weed Communities

Weeds present at the time of biomass harvest included the following: barnyardgrass (Echinochloa crus-galli P. Beauv.), redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenopodium album, L.), hedge bindweed [Calystegia sepium (L.) R. Br.], green foxtail [Setaria viridis (L.) Beauv.], kochia [Kochia scoparia (L.) Scribn.], western witchgrass (Panicum occidentale Scribn.), Canada thistle [Cirsium arvense (L.) Scop.], western yarrow (Achillea millefolium L.), common mallow (Malva neglecta Wallr.), hairy nightshade (Solanum sarrachoides Sendt.), and common purslane (Portulaca oleracea L.) (Table 3).

Grass weeds dominated the plots at the time of biomass harvest for many of the treatments and years. TIL had the highest relative abundance of grass weed species with an average of 72% over the two years; the next highest was SOB with 37%. Barnyardgrass and redroot pigweed consistently had the highest abundance across years and treatments, with these two species having a minimum of 23.5% and a mean of 50.1% of total biomass (PRyeC in 2012) for non-tilled treatments. Cover crop abundance exceeded that of weeds for the following treatments/years: PRyeC in 2011 and 2012, MED in 2012, CLO in 2012 and ROAD in both 2011 and 2012 (Table 3).

Perennial weeds included hedge bindweed, Canada thistle and western yarrow. Perennial weeds represented a maximum of 10% of total biomass (NONE, 2011) and a mean of 2.3% of total biomass per year across all treatments. Perennial weed biomass represented a mean of 6.9% of total biomass for the TIL treatment that includes tillage and 2.0% of total biomass for the other treatments (Table 3).


Biomass was compared using the following response variables: total cover crop biomass, total weed biomass, broadleaf weed biomass and biomass of grass weed species. The most abundant individual weed species were also incorporated into the biomass analysis; the most abundant species were barnyardgrass, green foxtail, redroot pigweed, common lambsquarters and Kochia (Table 3).

There was no three way interaction present for year by variety by treatment for any biomass response variable. A two way interaction for year by cover crop treatment was present for total cover crops, total weeds and grass weed species (P ≤ 0.05). The two way interaction justified the presentation of separate data for each year. The differences between years were likely due to differences in weather pattern and irrigation regime.

Total cover crop biomass increased for treatments PRyeC, MED, CLO and ROAD from year 2011 to 2012. Total cover crop biomass under PRyeC was 158% higher in 2012 compared to 2011 (39.0 to 100.6g m-2), was 112% higher under MED (52.0 to 110.1g m-2), was 25.5 times higher under CLO (3.0 to 79.5), and was 184% higher under ROAD (36.7 to 104.4g m-2) in 2012 compared to 2011. Total weed biomass was 141% higher under NONE (68 to 163.7g m-2), and 72% higher under SOB (78 to 133.8g m-2) in 2012 compared to 2011. Biomass of grass weed species increased 110% under the NONE treatment (23.7 to 49.7g m-2), and decreased under the TIL by 36%, (58.6g m-2 to 37.6g m-2) from 2011 to 2012. Biomass of broadleaf weeds increased under NONE by 157% (44.3 to 114g m-2) and by 94% under SOB (46 to 89.2g m-2) over the same period (Table 4).

Total weed biomass remained the same between years for all cover crop treatments with the exception of SOB and NONE, in which weed biomass increased. Cover crop biomass increased from 2011 to 2012 for all treatments involving cover crops with the exception of SOB, which did not increase. The increase in cover crop biomass and no change in weed biomass for PRyeC, MED, CLO and ROAD suggest that these treatments impacted weed populations compared to the SOB and NONE treatments.

There was a significant year effect (P ≤ 0.05) for total cover crops, grass and broadleaf weeds biomass across treatments. From 2011 to 2012, total cover crops and broadleaf weed biomass increased, while biomass of grass weed species decreased (Table 4). Treatment effects were significant across years when comparing biomass, but due to differences in weather patterns, irrigation and farmer-driven hopyard management strategies between years, treatments were analyzed separately for each year. Total cover crop biomass was highest under the PRyeC, MED and ROAD treatments in 2011 (39.0 g m-2, 52.0 g m-2, and 36.7 g m-2, respectively), and highest under PRyeC, MED, ROAD and CLO treatments in 2012 (100.6 g m-2, 110.1 g m-2, 104.4 g m-2, and 79.5 g m-2, respectively). Total cover crop biomass was the lowest under NONE, TIL, and SOB (0, 0, and 0.1 g m-2, respectively) with no significant differences among them in both 2011 and 2012 (Table 4).

Biomass oftotal weeds was lowest under PRyeC and ROAD treatments in 2011 (30.9 g m-2 and 32.4 g m-2, respectively). Weed biomass was highest under the remaining treatments (NONE, TIL, MED, CLO, and SOB), and there no difference between weed biomass in these treatments in 2011. In 2012, total weed biomass was lowest under TIL, PRyeC, MED, CLO, and ROAD treatments. The highest total weed biomass was present under the NONE and SOB treatments (163.7g m-2 and 133.8 g m-2). In 2011, SOB had a total weed biomass of 78.0 g m-2, 252% higher than that of PRyeC. In 2012, NONE had a total weed biomass of 163.7 g m-2, 395% higher than that of PRyeC (Table 4). Total weed biomass was higher in 2012 than in 2011 (80.2 g m-2 and 58.1 g m-2, respectively). Total cover crop and total weed biomass were moderately negatively correlated across years (R = -0.37, P <0.0001).

Biomass of broadleaf weeds was highest under NONE, MED, CLO, and SOB in 2011 and highest under the NONE and SOB treatments in 2012. Broadleaf weed biomass was lowest in PRyeC, TIL, and ROAD treatments in 2011 and lowest in TIL in 2012, followed by PRyeC and MED. Grass weed species had the highest grass weed biomass under the TIL treatment in 2011, and the highest grass biomass under NONE, TIL, and SOB in 2012. The lowest grass weed biomass was under the PRyeC treatment in 2011, and PRyeC, MED, CLO, and ROAD in 2012. Grass weed species were less impacted by tillage than were broadleaf species. NONE consistently had the highest relative biomass of weed species in all categories, being among the highest for broadleaf weed biomass in 2011 and 2012, and biomass of grass weed species in 2012 (Table 4).

The two most abundant weed species were redroot pigweed and barnyardgrass (Table 3). Barnyardgrass had the highest biomass under TIL and CLO in 2011, and NONE, SOB, TIL, and ROAD in 2012. Redroot pigweed had the highest biomass under NONE, SOB, and CLO in 2011, and NONE, SOB, MED, and CLO in 2012. Suppression of broadleaf weed species appears more variable than that of grasses when comparing weed biomass among treatments.

Treatments that consistently provided both high cover crop biomass and reduced weed biomass include PRyeC, MED, and ROAD. Mid season tillage did not consistently reduce weed biomass at the end of the season during biomass harvest, with TIL having among the highest total weed biomass in 2011, and among the highest grass weed biomass in 2011 and 2012. The effects from different treatments cannot be attributed to differences in irrigation or tillage regimes between years.

Plant Density

Plant counts were taken for the same responses as those analyzed for biomass, and counts were used to calculate density. There was no three way year by hop variety by cover crop treatment interaction for total weeds, cover crops, broadleaf or grass weeds. There was also no hop variety by cover crop treatment interaction for any species group. Canada thistle was sparsely populated throughout the field and subplots with a maximum of 5 plants m-2 under the Cascade variety.

There was a year by cover crop treatment interaction present for plant density in all categories (P ≤ 0.05). PRyeC, CLO and ROAD all increased in total cover crop plant density from 2011 to 2012. From 2011 to 2012 PRyeC increased from 232 to 356 plants m-2, CLO from 66 to 149 plants m-2, and ROAD from 129 to 439 plants m-2 from 2011 to 2012. Total weed density decreased for treatments TIL, CLO, ROAD, PRyeC and MED from 2011 to 2012. Total weed density across treatments decreased 38% from 2011 to 2012. From 2011 to 2012, total weed density decreased from 171 to 50 plants m-2 under the TIL treatment, from 126 to 93 plants m-2 under PRyeC, from 143 to 104 plants m-2 under MED, from 165 to 101 plants m-2 under CLO and from 160 to 89 plants m-2 under the ROAD treatment. Density of grass weeds decreased significantly for all treatments except for NONE from 2011 to 2012. Broadleaf weed density remained unchanged from 2011 to 2012 for all treatments except for ROAD, in which broadleaf weed density decreased by 37% (Table 5).

Density of total cover crops was highest under PRyeC and MED treatments in 2011 at 232 plants m-2 and 209 plants m -2, respectively. In 2012, plant densities were highest under PRyeC and ROAD treatments at 356, and 439 plants m-2, respectively. The lowest total cover crop densities were present in NONE, TIL and SOB in 2011, and NONE and TIL, followed by SOB in 2012. Total weed densities were highest under the NONE treatment for both 2011 and 2012 at 237 plants m-2 and 234 plants m-2. The lowest weed densities were found in the PRyeC and MED treatments (1236 and 143 plants m-2, respectively) in 2011. Lowest weed densities were found under TIL (50 plants m-2), followed by PRyeC, MED, CLO and ROAD, which were not significantly different from one another in 2012. Broadleaf weed densities were highest under the NONE treatment in 2011 with 101 plants m-2, and the NONE and SOB treatments had the highest broadleaf weed densities in 2012 (104 and 71 plants m-2 respectively). TIL had the lowest broadleaf weed densities in 2011 with 19 plants m-2, while TIL and ROAD had the lowest broadleaf weed densities in 2012 (15 and 29 plants m-2, respectively). Grass weed species had the highest densities under NONE, TIL and SOB treatments in 2011 (136, 152, and 130 plants m-2, respectively), while NONE maintained the highest weed density in 2012 with 129 plants m-2. Lowest plant densities of grass weeds were associated with PRyeC, MED and ROAD treatments in 2011 (89, 102, and 113 plants m-2, respectively). TIL had the lowest grass weed density in 2012, with 36 plants m-2 (Table 5).

Total cover crop plant density increased significantly across treatments from 2011 to 2012, while plant densities decreased for total weeds, and grass weed species between years. Broadleaf weed density did not change significantly from 2011 to 2012 across cover crop treatments. Weed species barnyardgrass, common lambsquarters and kochia significantly decreased, while green foxtail and redroot pigweed increased from 2011 to 2012.

Plant density followed a similar trend to that of biomass with respect to different cover crop treatments and subsequent response variables investigated. Overall, PRyeC, MED and ROAD treatments consistently had high cover crop biomass and plant numbers compared to other cover crop treatments. TIL had among the lowest weed densities during both years, which suggests that tillage had an impact on the number of plants that were able to establish during the growing season, even though weed biomass under TIL was among the highest among treatments examined. SOB continued to demonstrate poor competitive ability, with similar responses to those of the control (NONE) under management conditions of an organic hopyard.

Percent Groundcover

Observations for percent groundcover were taken over six dates throughout each growing season. Percent groundcover data was taken in order to evaluate the ability of different cover crop treatments to successfully establish, grow over time, and displace weed species within a stand. There was a significant interaction for sampling date by year for total cover crops, total weeds, broadleaf, grass weed species, bare ground, redroot pigweed, common lambsquarters and kochia. A sampling date by cover crop treatment interaction and a year by cover crop treatment interaction were also present for all response variables (P ≤ 0.05 for each).

Consistent with other measures of the plant community within the trial, a year by cover crop treatment interaction within groundcover had a similar pattern to that of biomass and plant density. Total weed groundcover decreased significantly across sampling dates under the PRyeC, MED, CLO and ROAD cover crop treatments between years. This decrease can be attributed to slow stand establishment in 2012 compared to 2011 from the absence of overhead irrigation early in the season. Cover crop groundcover decreased for the PRyeC, MED and CLO treatments, but increased for the ROAD treatment (Table 6). The increase in cover crop groundcover for the ROAD treatment corresponds to the reported increases in both ROAD biomass and cover crop plant density in 2012. Groundcover of grass weed species decreased for all cover crop treatments, and broadleaf weeds only decreased under the ROAD treatment from 2011 to 2012, with broadleaf weeds remaining unchanged in other treatments between years. The greater reduction in grass groundcover from 2011 to 2012 suggests that grass weed species, particularly barnyardgrass which commonly inhabits moist environments, may have been more negatively impacted by the lack of irrigation in 2012 than were broadleaf weeds (Barrett & Wilson, 1981). Bare ground increased for all cover crop treatments from 2011 to 2012. An increase in bare ground reflects the later and slower emergence in 2012. There was no hop variety by cover crop treatment by sampling date interaction present for either year (P ≤ 0.05).

A sampling date by cover crop treatment interaction was observed for percent groundcover (P ≤ 0.05).  Groundcover of total weeds increased significantly for all treatments between date 1 and date 2 (34 and 51 DAP). Between date 2 and date 3 (51 and 72 DAP), total weed groundcover increased significantly under NONE and ROAD treatments, and decreased under TIL, with the decrease under TIL was due to a tillage event. Between date 3 and date 4 (72 and 89 DAP) all treatments except for MED and ROAD increased significantly in total weed groundcover. The timing of this increase coincided with the peak of the hop growing season, occurring in late June. Between dates 4 and 5 (89 and 110 DAP), only SOB increased significantly. From date 5 to date 6 (100 and 127), all treatments decreased significantly except for TIL, which exhibited no change in total weed groundcover. NONE had the highest percent groundcover of total weeds from date 2 through date 6 (51 to 127 DAP). Total cover crop groundcover remained zero throughout the sampling dates for TIL and NONE, in which no cover crops were planted. Total cover crop groundcover peaked earlier in the growing season near date 4 (89 DAP), than weed groundcover which peaked closer to date 5 (110 DAP). Cover crop groundcover of ROAD and PRyeC had the slowest decline of all cover crop treatments late in the season, but MED and CLO achieved higher total cover crop groundcover during mid season (51 to 110 DAP) (Table 2).

A cover crop treatment effect was significant for percent groundcover of all species groups tested (P ≤ 0.05). Years were analyzed separately due to the differences in management and cover crop establishment between years (Table 2). Groundcover for total cover crops was highest under the MED (34%) treatment in 2011, and highest in the MED (21%) and ROAD (24%) treatments in 2012. Groundcover by cover crops was not analyzed for NONE and TIL because those treatments do not include cover crops. Cover crop groundcover was lowest under ROAD (7%) in 2011 and under PRyeC (16%), CLO (15%), and SOB (14%) in 2012. Total weed groundcover was highest under NONE in both 2011 and 2012 (50% and 36%, respectively). Total weed groundcover was lowest under MED (23%) and TIL (25%) in 2011, and TIL in 2012 (14%) (Table 6). For non-tilled treatments, total weed groundcover was lowest under MED in 2011 (23%) and ROAD, MED, and SOB in 2012 (17%, 17%, and 19%, respectively). Broadleaf weed cover was highest under NONE (27%) and ROAD (22%) in 2011, and highest for NONE (27%) in 2012. Broadleaf weed cover was lowest under TIL in both 2011 (6%), and 2012 (10%). Groundcover of grass weed species was highest under NONE in (23%), and ROAD (23%) treatments in 2011, and highest under NONE (9%) in 2012. Grass weed groundcover was lowest under PRyeC, MED, CLO, and SOB treatments in 2011 (12%, 9%, 12%, and 12%, respectively), and was lowest in TIL, PRyeC, and MED treatments in 2012 (5%, 4%, and 3%, respectively). Groundcover of grass weeds was much lower in 2012 compared to 2011, presumably due to reduced irrigation in the early season and subsequent delayed establishment. Finally, bare ground was highest under the TIL in both 2011 (75%) and 2012 (86%) (Table 6).

In 2012 there was a delayed establishment of cover crops due to limited irrigation. Cover crop treatment was significant across dates and years for percent groundcover; however, due to the significant sampling date by cover crop treatment and year by cover crop treatment effects, robust conclusions cannot be inferred. Total weed groundcover was highest under the NONE control treatment and lowest under the tillage treatment, TIL. MED had the lowest level of weed groundcover, but was not significantly lower than CLO or SOB, which were not significantly lower than PRyeC. SOB more effectively reduced total weeds in terms of percent groundcover measurements than in plant density or biomass results, probably because repeated measurements were taken over time, and SOB peaked in biomass before plant density sampling or biomass collection. Cover crop cover was highest under MED, CLO and SOB.

Impacts on Production

Hop yield was analyzed to determine whether different cover crop treatments had an effect on hop yield. A year by hop variety interaction was present for hop yield. A year effect was also present for hop yield (P ≤ 0.05 for each). The different cover crop treatments had no effect on hop yield, and there was no cover crop treatment by year interaction.

The hop variety ‘Centennial’ increased from the lowest yield in 2011 (1.06 kg hill-1) to the highest yield in 2012 (1.93 kg hill-1). ‘Cascade’ yield increased from 1.29 kg hill-1 to 1.80 kg hill-1 from 2011 to 2012. ‘Willamette’ yield did not increase from 2011 to 2012, yielding 1.60 kg hill-1 in 2011 and 1.82 kg hill-1 in 2012. Hop yield in 2012 was higher than that of 2011, with a mean of 1.85 kg hill-1 in 2012, and a mean of 1.31 kg hill-1 in 2011 (data not shown). Higher overall hop yields in 2012 were expected because the hop plants were more mature and better established.

The lack of significant effects on yield from the different cover crop treatments suggests that the cover crops did not negatively effect, or compete with the hop plants. This is likely due to intrarow management being similar among treatments, and intrarow weed populations were not examined in this experiment. Young hop plants were used in this experiment, which would have less developed root systems than older plants, further reducing the impact of interrow management on the hop plants. This, in addition to the use of drip irrigation to minimize competition for water may have also negated any direct and immediate impact from the interrow cover crops. Similar findings are common in vineyards, with little impact on yield from interrow cover crops when water is not limiting (Baumgartner, et al., 2008; Sweet and Schreiner, 2010). However, research suggests that differences in vineyard yield among groundcover treatment can become apparent after multiple years (Tesic, et al., 2007).


Increased cover crop biomass can reduce weed biomass by increasing groundcover and physical competition with weeds. Increased groundcover reduces seed germination and impedes weed seedling emergence, directly competes with weeds for available nutrients, light, and water (Bàrberi, 2002; Bond & Grundy, 2001; Zimdahl, 2004). Increased cover crop biomass is generally associated with increased control of weeds (Gallandt, et al. 1999). High plant biomass is associated with reduced light transmittance to the soil surface, which also reduces weed germination and establishment. For example, 548 g m-2 of hairy vetch biomass and 501 g m-2 of cereal rye biomass was associated with 90% light extinction at the soil surface (Teasdale, 1998). Light extinction from associated biomass may be a good indicator of the suppressive ability of cover crop biomass (Teasdale, 1998). Furthermore, higher biomass should be correlated with increased allelopathic compounds and subsequent impact on weed species (Creamer, et al. 1996; Liebman and Davis, 1999; Gallandt, et al. 1999).  

Cover crop treatments in this organic hopyard varied in their ability to establish and suppress weeds. PRyeC, MED and ROAD treatments were among the treatments to produce the highest amounts of cover crop biomass in both years. PRyeC and ROAD were associated with the lowest total weed biomass in 2011 and among the treatments with the lowest total weed biomass (along with TIL, MED, and CLO) in 2012. Timing of tillage had an expected impact on weed biomass, which was observed through the differences in total weeds under the TIL treatment from 2011 to 2012. No hop variety effect was found for any response variables that could be attributed to a shading effect from the hop plants. However, shading from adjacent hop rows may have been responsible for this non-significant effect of hop variety on cover crop or weed biomass, plant counts, or groundcover.

Plant counts and percent groundcover further corresponded with biomass findings, while taking into account differences in growth habit and plant size. The PRyeC and MED treatments had the highest number of cover crop plants in 2011 and the PRyeC and ROAD had the highest total cover cropcounts in 2012. Total weed counts were lowest under PRyeC and MED in 2011, and TIL had the lowest total weed counts in 2012. Non-tilled treatments with the lowest total weed counts in 2012 were PRyeC, MED, CLO and ROAD. The MED treatment had the highest cover crop groundcover in 2011, and MED and ROAD had the highest total cover crop groundcover in 2012. The TIL and MED treatments had the lowest total weed groundcover in 2011, and TIL had the lowest weed groundcover in 2012.

PRyeC, MED and ROAD consistently had among the highest cover crop establishment, along with the lowest weed biomass, density and groundcover of the treatments without tillage. Hop yield did not differ significantly between cover crop treatments, suggesting that interrow cover crop and tillage systems can be implemented without fear of reduced yield; however, long term studies are probably needed to determine the possible impacts of groundcover treatments on organic hop production.

Research conclusions:

2013 marked the frist year that organic hops were required to be used in certified organic beer. Therefore, the transition from conventional to organic hops will be a major issue in 2013 and, beyond that, will need to be solved by brewers and hop farmers. Our research data are in the process of final dissemination of major results. In particular, our research addresses the question of varietal selection in organic hopyards and weed management through cover cropping in organic hopyards.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Turner, S.F., C.A. Benedict, H. Darby, L. Hoagland, P. Simonson, J.R. Sirrine, K. Murphy (2011). Challenges and opportunities for organic hop production in the United States. Agronomy Journal 103: 1645-1654.

Murphy, K.(2010). Breeding for genetic and market diversity: Examples from hops, quinoa and buckwheat. EUCARPIA Low-input and Organic Plant Breeding Conference, Paris, France, Dec. 2, 2010.

Murphy, K. (2012). Organic hop production, variety trials and cover crop systems in the Yakima Valley of Washington State. USDA-NIFA-OREI Project Directors Workshop. Washington, DC, October 2, 2012.

Murphy, K.(2011). Participatory breeding methods for a diversity of crop pollination strategies: Examples in wheat, quinoa, buckwheat and hops. Northern Plains Sustainable Agriculture Society’s 2011 Annual Conference, Fargo, ND, Feb 4, 2011.

Murphy, K. (2010). Plant Breeding and agronomy of organic hops. American Hop Convention. Yakima, WA, January 23, 2010.

Hoagland, L., K. Murphy, S. Turner, J. Perrault, R. Sirrine, B. Tennis (2013). Cover crop mixtures build soil quality in organic hop orchards. American Society for Horticultural Science Annual Conference, Palm Springs, CA, July 23, 2013.

Turner, S. and K. Murphy (2012) Evaluating cover crops for weed suppression in organic hopyards. WSU Academic Showcase, Pullman, WA, March 30, 2012. 

Turner, S. and K. Murphy(2011). Evaluating cover crops for weed suppression in organic hopyards. CSSA-ASA-SSSA Annual Meeting, San Antonio, TX, October 2011.

Murphy, K. Quinoa and Hops Variety Trial Field DayWSU Organic Farm Field Day, Pullman, WA, July 2011


1. Presented research results at the Organic Hop Field Day in Toppenish, WA in August, 2012.
2. Presented a poster of our research results on organic hop weed management at the International Humulus Symposium in the Czech Republic. Turner, S. and K. Murphy (2012). Cover cropping systems for organic hop production in the Yakima Valley, USA. Zatec, Czech Republic, September 11, 2012.
3. Presented a poster of our research results on cover cropping with hops at the WSU Academic Showcase. Turner, S. and K. Murphy (2012) Evaluating cover crops for weed suppression in organic hopyards. WSU Academic Showcase, Pullman, WA, March 30, 2012.
4. A publication on the effect of cover crops on weed suppression in organic hopyards is in final stages of preparation and will be submitted to a peer-reviewed journal for publication. Once published, this information will be disseminated in other farmer oriented publications.

Project Outcomes

Project outcomes:


Farmer Adoption

Farmer adoption was not measured specifically as a part of this project.  Organic hop acreage has increased significantly from 2008 to present, and the varieties grown are selected by farmers based on consumer demand and agronomic suitability. The need for cover crops in organic hopyards is apparent to organic growers, and most of the organic hopyards have some form of cover crop cultivation.  Species mix is ever-changing, site-specific, and an area in which most of the oragnic hop growers are now experimenting.


Areas needing additional study

Cover crops in Yakima Valley hopyards are used primarily to prevent erosion and improve soil quality. This study suggests that they may also be helpful in suppressing weeds and, subsequently, creating a more manageable interrow stand. The differences in irrigation regime from 2011 to 2012 most likely had an impact on success of the different treatments and establishment of different species, but the overall trends were similar from year to year. Ultimately, interrow cover crops may be able to reduce weed densities in the intrarows by influencing dispersal from interrows to intrarows, eventually reducing the need for labor and hand weeding in the intrarow. A grass/legume mixture such as that used in the ROAD and PRyeC, or a pure legume mixture such as MED, may be the most beneficial in terms of reducing weed pressure, while improving soil quality and contributing to fertility in an organic hopyard.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.