Cover Crops for Hop Production in Semi-arid Yakima Valley, Washington

See the complete report to view figures: Del-Moro.-2019.-Cover-Crops-for-Hops-Final-Report-SARE-FW15-044

Ground Cover

The most representative ground cover measurements were made after the cover crops were clearly established. We considered establishment as visibly planted rows with tillers or branching, and around 4-in of height. Barley reached this threshold as early as Nov. 2014 (Fig. 1). Siberian wheatgrass was established by the time our first spring measurements were made in 2015 (Fig. 2), and streambank wheatgrass required until the summer of 2015 to fully establish (Fig.3). Hard fescue was much slower; it was not until the fall of 2015 that most plots were established well enough to represent ground cover (Fig. 4). Alfalfa unfortunately did not establish at any time during 2015, even after re-planting in the spring. We observed only occasional, isolated sprouts. Therefore, the yellow-blossom alfalfa was removed from our study, assuming it would not germinate quickly enough to provide benefit in hop production. We disced the alfalfa plots in the fall of 2015, but then they took and interesting and surprising turn. By summer of 2016 the alfalfa plots germinated, and by the summer of 2017 half of the alfalfa plots showed significant ground cover (Fig. 5). Measurements were still not recorded as the crop had been dismissed as a viable option due to lack of germination and establishment in the first two years. 

Most ground cover measurements began in April and continued through September each year. Table 3 displays the season-long ground cover results for each ground cover treatment and each year. Weedy ground cover in the control plots was minimal during 2015 when they were disced regularly throughout the growing season (Figs 6A and B). Weeds in the control plots quickly emerged and established significant ground cover through 2016 and remained through 2017 (Figs. 6C and D).

After establishment, barley quickly became the quickest and most consistent ground cover of all of the species tested. Barley produced seed heads quickly after its first mow, with most plots completely headed by 15 May 2015 (Fig. 7A). After the second mow, barley quickly re-produced seed heads then senesced completely by Jul. 2015 (Fig. 7B and C). By Oct. 2015, barley had visibly re-seeded and new plants had completely emerged, although none had emerged in the original 6-ft planted swath of the drill (Fig. 7D). All of the new barley plants were on the outside of swath, in the area where tractor tires travelled and all the way into the hop row. We disced and replanted the barley plots in Nov. 2015. In 2016, barley achieved the best ground cover before heading and senescing (Figs. 8A-D). Seed heads were produced in May 2016, and the barley was once again fully senesced in Jul. 2016. Overall vigor of the barley was reduced in 2016 compared to 2015. This may be caused by barley’s allelopathic nature, or simply because not enough moisture was available after replanting.  Regardless, the barley plots still resulted in the lowest season-long weed cover percentage in 2016.

The perennial grasses displayed an opposite trend to barley over the course of the study. Siberian wheatgrass maintained the most consistent season-long ground cover during every year of the study out of every cover crop treatment (Figs. 9A-D).  Streambank wheatgrass followed a similar trend as Siberian wheatgrass, but was overall less vigorous (Figs. 10A-D). Hard fescue ended with the highest season-long average ground cover of 72.3% in 2017, and the lowest percentage of weeds. Hard fescue requires at least one year to establish before true ground cover benefits can be realized (Figs. 11A-D). The perennial grasses also differed in how much seed they produced. Hard fescue seeded early in 2016 (Fig. 11A), then produced only minimal seed at any point in the study thereafter. The wheatgrasses produced seed heads in the summer of both 2016 and 2017. During and after seed production, the vigor of the wheatgrasses was slightly reduced. Some plots developed a chlorotic appearance until temperatures cooled again. The hard fescue on the other hand remained dark green, and continued to grow and spread throughout the summer. The fescue thrived in the shade of the hops in the otherwise hot temperatures in August and September, then slowed after the hops were harvested. Vigorous growth returned as soon as the temperature cooled.

Table 3: Average Season-long Ground Cover of Cover Crops and Weeds

Soil Qualities

Soil temperature measurements were initiated in the summer of 2016 and continued through 2017. The temperature measurements were not taken at exactly the same time from every plot, but were collected within the same 30-min time frame from each block. Therefore we calculated the difference in soil temperature recorded from cover cropped plots and control plots from the same block. The average results by year are presented in Table 4. Surface soil temperature was not significantly different among any of the cover crops in 2016; however in 2017, we observed that the hard fescue was consistently 1 to 3 °F lower in temperature than the control plots, especially on the hottest days. The fescue resulted in a significant season long average decrease in soil temperature of 2.14 °F compared to control plots. The effects of soil temperature differences on hops, either beneficial or detrimental, are inconclusive in this study.

Table 4: Average season-long difference in surface soil temperature vs. control

Several soil test variables were significantly different among all of the treatments according to an analysis of variance at a 95% confidence level (Appendix), including organic matter, pH, nitrate, potassium, boron, manganese, iron and aluminum. Organic matter was perhaps the most interesting result. All of the perennial grasses resulted in significantly higher organic matter levels than the control, with 0.25% more on average in both of the wheatgrasses, 0.23% more organic matter on average in the fescue plots, and though the increase was not significant, 0.17% more organic matter on average in barley plots. Assuming our sandy loam soil has a bulk density of about 1.55 g/cm³, this 0.25% gain is equivalent to over 10,000 lbs more organic matter than the control plots. This roughly 5-ton gain in the top 1-ft of soil cannot be attributed entirely to the degradation of mowed debris on the soil surface, but more likely contributed from exudates of deep, dense root systems common to grass species and to a lesser degree, microbial remains.

Soil pH was significantly higher in barley and Siberian wheatgrass plots compared to control and hard fescue plots. This same general trend occurred with potassium, sodium, boron and zinc; all of these soil minerals seemed to increase steadily with the grainier grasses. These are many of the same minerals required for the reproductive stage of plants, including flowering and seed production. Additionally, grasses require more potassium than herbaceous plants, and even more so if they are classified as a cereal. It is for this reason that it is not surprising to see the most elevated levels of potassium, sodium, zinc and boron from barley plots which grow and produce seed rapidly, while the lowest levels outside of the control plots were from fescue, which rarely produced seed.

Economic Analysis

Outside of this study, the farm planted tall fescue in other hop yards. As of the 2018 growing season, we observed the tall fescue germinate much more rapidly than hard fescue and achieve excellent ground cover in the first year of planting. While we were initially concerned that tall fescue would cost more to manage from mowing, it tended to lodge as soon as it reached a height of over 12-in. The farm also planted cereal rye as a cover crop in several additional hop yards. Cereal rye establishment and growth was very similar to what we observed from barley.

The most common interrow management in Yakima Valley hop production is still cultivation of the soil to remove and prevent weeds. The next most common interrow management is to allow the weeds to grow, but with occasional mowing to shorten their height and prevent the spread of seeds. Cover crops are an increasingly common management option for hop growers in the Yakima Valley. The preferred cover crop option among hop growers is cereal rye.  

To evaluate the economics of cover cropping, we compared two cover crop systems with two non-cropped systems: 1) an annual grain, 2) a perennial grass, 3) disc cultivation and 4) mowed weeds. Current seed prices of an annual grain range from $0.27 to $0.75 per lb, the lower and cheaper end of the spectrum including grains such as cereal rye and wheat, and the higher end of the spectrum includes barley. Price varies more among perennial grasses. Most varieties of tall fescue are currently $2.00 per lb, hard fescue is $2.70 per lb for the variety, “Durar”, Siberian wheatgrass is $3.70 per lb, and ‘Sodar’ Streambank Wheatgrass is $5.40 per lb. Cereals should be planted at 100 lbs per acre to achieve the greatest benefit in ground cover. Most perennial grasses are recommended to be planted at 10 lbs per acre, but as cover crops in the typical soils of Yakima Valley, the planting rate should be doubled to achieve a denser stand in the first year. AS of 2018, we estimate that it costs approximately $6.00 per acre to operate a tractor for any purpose in a hop yard, including labor and fuel.

Our economic scenario also assumes that the hop yard will be removed 5 years from the initial fall planting of the cover crops. New hop cultivars are released almost yearly, and as the market changes, it is increasingly uncommon for a hop yard to remain planted to the same cultivar for a time frame longer than 5 to 8 years. This is a factor when considering a perennial cover crop as it will add complication to the removal of hop plants since root diggers travel perpendicular to planted rows and cannot move as efficiently through a cover cropped interrow.

The planting, maintenance and removal cost estimations of 4 interrow management programs in a 5-year hop production cycle are presented in Table 5.

Table 5. Cost estimation per acre of interrow management programs in a 5-year hop yard, beginning with fall planting after harvest as Year 1, and ending with cover crop removal in fall at the end of Year 5.

Additional Cost Considerations:

• If fescue were planted as the perennial grass, it would likely not require 3 mowings per growing season. One mow is all that would be required in the first year, and two would be plenty in the years after.
• How high the hops are hilled up and what pruning methods are used to smooth them down will drastically affect a grower’s decision on cover crops.
  • Our standard production practice used to be cross-cultivation (down as well as across rows) to smooth out hills for efficient and safer string tying, as well as mechanically pruning the plants. Adjusting our standard practice to fit cover crops required at minimum an investment of time, and from some farms can require an investment in new equipment.
• What pests become more prevalent when cover cropping?
  • Rodents such as a voles were a severe economic pest in the 2017 growing season. Voles and their trails have been sighted more frequently in cover cropped hop yards on many farms.
  • Loopers and other lepitdopteran larvae also tend to be more prevalent in hop yards that have any interrow vegetation growing. Nearly all hops regardless of ground cover are currently treated at least once per season with a larvicide, however, due to increased pressure from the use of pesticides that are safer for beneficial arthropods.
  • Other pests and diseases may be correlated with cover crops, but at present, we don’t have sufficient evidence of the effects a cover crop has on diseases or other pests in hops.

Benefit Considerations

What are the values of the benefits of satisfactory ground cover in a hop yard?

• Reduction of arthropod pests:
  • Two-spotted spider mites are the most economically impactful pest of hops, and they thrive in dusty conditions for a variety of reasons presented in the proposal of this research. How would reducing dust in a hop yard impact a mite management program? Miticide sprays may be more effective, or fewer sprays may be required. While working in hops throughout the Yakima Valley, I have observed hop yards that are cover cropped generally require at least one less miticide spray than hop yards with disc cultivated interrows year-after-year. The correlation and significance between cover crops and two-spotted spider mites is still being developed. However if a farm can eliminate one miticide spray, they will save $15 to $100 in chemical costs in one growing season.
• Reduction of weeds:
  • Established cover crops resulted in reduced ground cover from weeds. Some cover crop species, such as barley, are allelopathic and exhibit herbicidal activity against weeds.
  • Discing also results in significant weed cover reduction in hop interrows; however, dust can be a larger problem due to its effect on spider mites. Between spider mites and weeds, spider mites are a much more severe and economically impactful pest to manage in hop yards with fewer chemical control options.
• Marketing
  • Several hop farms are Global G.A.P. certified or participate in other sustainable farming or good agricultural practice programs. These programs often suggest or require a farm to implement dust control strategies in hops.
  • Brewers are the main customers of hop growers, and they are an involved customer: hop harvest and the month beforehand is a common time for brewers to visit, check the hops they’ve purchased, and select others that they haven’t yet purchased. The appearance of the hop yard is thus an important marketing tool. Discing interrows has been effective at removing weeds and is a preferable appearance compared to a weed cover, however the fine, talcum-like dust produced from this practice can be irritating to visitors in the slightest wind. Cover crops can be a great alternative for both weeds and dust, and grasses look comparably nice when planted thick and mowed before harvest.
• Soil qualities
  • The significantly greater organic matter of cover cropped interrows may not directly influence hops since they are grown outside of the cover cropped swath; however the gain in soil organic matter in interrows would rapidly benefit any new hop plantings that would follow their removal and subsequent mixing or cultivating of the soil.
  • Options to increase organic matter in Yakima Valley hops include compost, partially composted manure, hop harvest waste, and even humic acids and other organic acids applied through the drip system. An average composted manure in our area would contain approximately 225 lbs of carbon per dry ton of material, and typically costs $20 to $25 per ton applied. A 5-ton increase in soil organic matter would therefore require more than 44 tons of this composted manure. Such a rate would be detrimental due to excessive phosphorus, potassium and salts. If the goal is to increase organic matter of the overall hop yard, a cover crop is one of most economical ways to do so. Additionally, organic matter from root exudates is already incorporated throughout a 1-ft depth of soil, whereas most other inputs are surface-applied. This being said, there benefit from a diverse source of inputs in hops to achieve various physical (i.e. drainage), chemical (i.e. minerals or pH) and biological goals for the soil.

The mineral differences resulting in this study (i.e. potassium, boron, zinc, etc.) can be considered beneficial depending on what a specific hop yard needs. For example, potassium was already present at adequate or high levels in our test hop yard, so the increase in potassium in the top foot of soil was not needed. It was likely collected from deeper feeder roots from leached/excess potassium applied to hops over the years. Boron and zinc are almost always deficient in hop yard soils in the Yakima Valley. Direct mineral benefits would vary most and would be difficult to attribute value to outside of the context of a specific hop yard.