Production of Drought-adapted Intermountain Native Plants Through Low-cost, In-containers for Emerging Western Markets

Final Report for SW01-020

Project Type: Research and Education
Funds awarded in 2001: $71,686.00
Projected End Date: 12/31/2005
Matching Non-Federal Funds: $23,344.00
Region: Western
State: Utah
Principal Investigator:
Roger Kjelgren
Utah State University
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Project Information


Results from this project show that pot-in-pot (PIP) nursery production moderates root zone temperatures of Intermountain West (IMW) native trees, shrubs, and perennials and can accelerate growth of these plants and protect against winter damage. A nursery specializing in production of IMW native plants can use the PIP system to be economically viable by improving plant growth and production. Sustainable short-term, intermediate, and long-term cash flow can be achieved by IMW nurseries with a species mix of native perennial wildflowers, shrubs, and trees by using pot-in-pot production.

Project Objectives:

The overall goal of this study was to develop a model system with economic analysis using alternative in-ground container nursery production systems for drought-adapted native woody and herbaceous perennial species in the rural IMW to encourage adoption by small entrepreneurs. The specific objectives of the project are:
--Compare growth of above-ground container versus PIP production of IMW native perennial wildflowers.
--Conduct a controlled study and cost analysis comparing production time using expensive, high-end artificial media versus local materials (shredded bark, compost, field soil) for native wildflowers and shrubs;
--Apply a scaled-up PIP system to a wholesale nursery that grows IMW native plants, using local materials for artificial media;
--Conduct a cost analyses based on the results of Objective #2.


Irrigated landscapes in the Western U.S. region consume large amounts, up to 70% (Kjelgren et al, 2000), of treated culinary water. Increased water use during the growing season, spring, summer and fall, compared to winter months, that tracks increased evapotranspiration rates can be attributed to water applied to landscapes (Kjelgren et al, 2000). In many rapidly urbanizing areas of the West, constrained urban water supplies will not be sufficient to meet increased demand from population growth. Increasing supply is becoming increasingly difficult if not impossible for many western cities due to economic, environmental, and social constraints. Urban water conservation is so critical in California that a separate organization, the California Urban Water Conservation Council, has been formed to address the need for conserving urban water, particularly in landscapes. Even in the wetter Pacific Northwest, Portland and Seattle have active water conservation programs. In addition, the federal government issued a presidential executive order #12902 (9) mandating resource conservation, specifically including water, on all federal facilities and the use of native plant material where possible. This order is being applied to all projects receiving federal support funds.
Nearly all western cities facing water shortages are encouraging water conservation through low-water-use landscaping, or xeriscaping. Low- water-use landscaping consists of using plants sufficiently adapted -- typically native -- to local soil moisture and rainfall conditions to grow well enough to meet end-user expectations. However, the supply of native, drought-adapted plants, in sizes suitable for landscapes, is limited, particularly in the Intermountain West (IMW) where some of the fastest growing cities in the country are located. Larger landscape sizes are particularly difficult to find because very few nurseries outside the region grow IMW native plants, and these plants are difficult to grow under conventional nursery production systems. Conventional field nursery production works best for larger woody plants, thereby excluding herbaceous perennial species that can be rapidly grown and sold. Also, harvesting field-grown plants requires expensive hand digging or tree spades that results in loss of most of the root system, a method that can damage deep-rooted IMW native woody species. Conventional above-ground container production is very expensive, and extreme summer and winter temperatures, as well as a short growing season, limits production of IMW native plants.
The pot-in-pot (PIP) production system is a hybrid between conventional field and above-ground container production that may be well suited to production of native plants in the IMW region. The PIP production system consists of a holder or socket container permanently installed in field soil into which a second container that contains artificial media and the plant is inserted (Ruter, 1993). To prevent expansion of roots through the drainage holes into ambient soil, a square of weed fabric, similar to that used with in-ground fabric containers, is inserted between the holder and the production pot. The PIP system has several significant advantages over conventional above-ground container or field production systems. Foremost is moderation of root-zone temperatures that keeps roots healthier: cooler in summer and sufficiently warm in winter that additional protection is not needed (Hight and Bilderback, 1993; Ruter, 1998). PIP-produced plants result in more rapid establishment when planted in the landscape. Cooler temperatures in the summer can reduce fertilizer requirements because of reduced microbial breakdown, and warmer temperatures in late winter can stimulate earlier root growth in pot-in-pot systems. Some evidence indicates that media type can affect production of plants in a PIP system. When growing drought-adapted IMW plants that do not tolerate high-water-holding-capacity soils, an extremely well-drained media may be necessary.
For a PIP system to work successfully for small entrepreneurs in producing IMW native plants, low costs must be balanced with a reliable cash flow. A cost comparison of conventional above-ground container, conventional field, and PIP systems showed that the PIP system had higher initial costs due to installation expenses, but over time the cost per plant was lower than the other two methods due to lower overwintering and irrigation costs (Adrian et al, 1998). This study was conducted in Alabama, thus costs may be even lower in the high desert region of the IMW. Using a suitable and affordable media for growing plants may be a challenge. A standard, purchased high-organic-matter media affords excellent drainage and is proven suitable for plant production, but can be expensive. A constructed media from local materials, such as field soil and compost or shredded bark, would be cheaper, but a grower would have to test the medium to determine if it was suitable for nursery production, a costly and time consuming process. Finally, PIP systems have only been investigated for larger woody plant materials that have a production time of 2-4 years, thus cash flow may be a problem for the first several years, particularly with the high installation costs of a PIP system. Many native perennial wildflowers grow rapidly and can be grown to a salable size in 6 months to a year in #1-sized (1 gallon) containers, but growing such small plant material in a PIP system has not been tried before.


Materials and methods:

Study 1: This study, although conducted chronologically last, is logically the first because it compares conventional above-ground to the PIP system in producing IMW native perennials. Two different production systems (treatments) were compared: pot-in-pot (PIP) and traditional above-ground containers. The systems were installed at the Greenville Agricultural Experiment Station in North Logan, Utah, in 2003. This two-year study had two production cycles each of four weeks during 2003 and 2004, beginning approximately at the first of June and ending in the middle of August. The experiment was arranged as a randomized complete block split-plot design. Two production system main plots were considered a block, and each plot consisted of a 7 x7 (49) pots matrix of #1 (1 gallon) pots, with the two rows on N, E, and W sides considered to be border rows. Within each main plot, three native species varying in adaptation were randomly assigned as sub plots to the three interior north-south-direction rows. The middle three positions in each species treatment row were considered sub samples. Each complete block was replicated six times. The three IMW perennial species used in 2003 were Mirabilis multiflora, Aquilegia caurelea, and Penstemon palmeri, and in 2004 Polemonium foliosissimun, Penstemon strictus, and Sphaeralcea grossularifolia were used. Aquilegia and Polemonium are high-elevation, cool-habitat species, Mirabilis, P, palmeri, and Sphaeralcea are low-elevation hot-habitat species, and P. strictus is a mid-elevation species.
Plants were grown in 1-gallon containers filled with a medium consisting of 50% composted bark, 40% screened pumice, and 10% peat moss. A 1 g of actual nitrogen as slow-release granular fertilizer (Osmocote 14-14-14) was applied to each plant immediately after planting. Plants used for the PIP treatment were grown in-ground with a 1-gallon socket pot buried to the rim and the production container containing the plant inserted into the socket pot. A piece of weed fabric was placed between the socket pot and the production container to prevent the plants from rooting into the surrounding soil. All plants were spaced pot-to-pot and sprinkler irrigated. Irrigation took place during mornings for 10 minutes (24mm of water).
Pot weight was assessed predawn during three consecutive days once or twice each production cycle. During the weighing period plants were only watered before the first day’s weighing and immediately after the last day’s weighing. Chromel Constantin thermocouples were used to measure weekly root zone temperature. Two measurements were made, one at the outer edge between the inner pot surface and the growing media, and the second measurement in the middle of the pot under the plant about 3 inches bellow the media surface. The stomatal conductance as a measure of plant stress was also measured using a porometer (model LI-1600 Steady State LiCor; Lincoln, NE). Stomatal measurements were recorded under full sun around solar noon between 12 to 3 p.m. The growth index [(height (cm) + width (cm))/2] was determined weekly. At the end of the experiment, plants were harvested, shoots and roots separated, washed, and dried, and then shoot and root dry weight were placed in paper bags, oven dried for 48 h at 60ºC, then weighed.
Study 2. This study compared growth, production time, and costs of a mixture of IMW native perennial and shrub species in a PIP production system with different irrigation systems and growing media over two years, 2001 and 2002. This experiment was conducted at the Utah State University Research Farm located in North Logan and consisted of two concurrent experiments based on plant size, small herbaceous perennials/ground covers and medium-sized shrubs planted in spring 2001. In both experiments, socket pots were installed flush with the surrounding field soil, and a weed fabric liner was placed between the inside of the socket pot and the inner growing pot. The plants were grown in one of two media: a commercial organic artificial media called Ecomix costing approximately $50 yd3, and a lower cost mix made from local materials. Ecomix consists of 40% composted bark, 30% peat moss, and 30% fired clay or pumice. In 2001 the local mix consisted of 33% sandy loam field soil, 33% general compost from the local landfill, and 33% partially composted fine wood chips. In 2002 the local media consisted of 50% fired clay from a local source called Utelite and 50% composted bark. Plants in both experiments were grown until they reach a harvestable size.
The first experiment was production of herbaceous perennial and ground cover species in #1 (1 gallon) containers using eight species native to IMW or adjacent semi-arid regions. These plants were laid out in replications consisting of a single row that contained these eight species grown in the commercial and again in the local artificial medium, for 16 plants per row spaced 0.3 meters (1 foot) apart. This study had seven replicated rows spaced 0.6 m (2 feet) apart irrigated with overhead sprinklers irrigated every other day at 50% of the local evapotranspiration rate as calculated from an on-farm weather station. The second experiment consisted of the same procedure except for using IMW native shrubs. The shrubs will be grown in #3 (3 gallon) containers spaced 0.6 m apart within a row containing plants in both types of artificial media. The seven shrub row replications will be spaced 0.9 m apart and will be irrigated using low-volume sprinkler irrigation with one emitter per pot.
Perennial plants in study #1 were harvested one month after planting, then a second crop was planted both years. For the shrub study, height growth at the end of the two growing seasons was measured. Height growth will be compared again with an analysis of variance. Cost information gained from this study will be recorded. This includes the total cost of installing both the irrigation system and containers in the ground. The costs of starting a new crop along with the cost of maintaining and harvesting the crop will also be recorded. A per plant estimate of costs was figured for both the variable costs (including planting, maintenance, and harvesting costs) and direct fixed costs (irrigation installation and burying the outside pots in the ground). No attempt was made to apply indirect fixed costs (marketing, secretarial costs, etc.) to each plant.
Study 3: This study was conducted at Chelsea Nursery, 3347 G Road, Clifton, CO, Stacey Stecher proprietor, immediately east of Grand Junction, a wholesale nursery specializing in the production of IMW trees, shrubs, and herbaceous perennials. The study was supervised by Matt Rogoyski, researcher and extension specialist with the Colorado State University Grand Junction field station. This study focused on tree production since Chelsea Nursery had the most difficult time producing larger woody plants, particularly in regards to anecdotal comparison to CAG production and circling roots, thus was not interested in studying smaller plant sizes. Given these constraints, three treatments were chosen: PIP with the production pot painted with a copper compound to cause root tip abortion; the production pot lined with weed fabric that also causes root tip abortion within the fabric weave; and an untreated control. This study was installed in spring 2002 using the locally mixed medium historically used by Chelsea nursery, and the trees were followed for two years. Three tree species were selected, single leaf ash (Fraxinus anomala), big tooth maple (Acer grandidentatum), and mountain mahogany (Cercocarpus intricatus), and planted into #10 (10 gallon) production pots from #1 (1 gallon) liners. The trees were irrigated with a low-volume sprinkler system and fertilized as per normal practices for Chelsea Nursery. The personnel of Chelsea nursery maintained the PIP production area for the duration of the study. In the spring of 2003 stomatal conductance was measured with a steady state porometer (model 1600, LiCor, Lincoln NE) on five leaves per tree. Growth measurements were taken in both spring of 2003 and 2004 on the terminal leads for big tooth maple and single leaf ash, or the five highest shoots for mountain mahogany.

Research results and discussion:

Study 1. Producing smaller herbaceous perennial plants in a pot-in-pot system does have a beneficial impact on growth and plant production as compared to conventional above-ground production (CAG). Being exposed to more sunlight in the CAG system resulted in root zone temperatures that were approximately 5o C warmer, particularly at the edge of the pot, than those growing in the PIP system (Table 1). These differences were consistent with PIP and CAG root zone temperatures observed elsewhere (Hight and Bilderback, 1993; Martin et al, 1999). Higher root zone temperatures lead to higher water loss from the plant-pot system (Table 2). On four dates in 2003 water loss of the CAG plants was significantly higher (P<0.10) by 7-22 grams as compared to the PIP plants, particularly on hot and dry days with high evapotranspiration rates. In 2004, out of four dates where water loss was measured, only on the first two in June was CAG water loss significantly higher (P<0.12) than that for the PIP plants. However, water loss measured during the second production cycle did not see higher water loss in the CAG plants.
Higher root zone temperature also affected gas exchange (Table 3). In 2003 stomatal conductance on three dates was significantly higher in the PIP plants than the CAG plants, and in 2004 the stomatal conductance was significantly higher during the first production cycle but not the second. Lower stomatal conductance rates indicate that photosynthesis, and thus growth, would also be lower. We indeed observed that in terms of shoot and root biomass (Table 4). Shoot biomass for PIP plants was clearly greater (P<0.01) for the second production cycle in 2003 and the first production cycle in 2004. Even for the other two production cycles, PIP plants exhibited a clear trend toward more biomass (P<0.13-0.15). Root growth was less affected by high root zone temperatures, where the PIP plants had significantly greater root biomass in only the first production cycle of 2004. The interaction term was also highly significant for both root and shoot biomass in the second production cycle of 2003. For Aquilegia, a high-elevation species, root and shoot biomass was significantly lower in the CAG plants compared to the PIP plants, while Mirabilis and Penstemon, both lower-elevation drought-adapted species, were not. Finally, mature P. strictus, Mirabilis multiflora, and Aquilegia caerulea were held in the CAG and PIP positions over the winter of 2004-2005 to evaluate potential susceptibility of plants in each system to winter damage (data not shown). CAG and PIP P. strictus experienced only 8% winter mortality. CAG Aguilegia had 26% mortality while PIP was only 2%. The biggest impact of CAG winter exposure was on Mirabilis, where mortality was 80% versus 28% for PIP plants.
Overall, these results suggest that the insulating properties of a PIP system improve production of #1 IMW native perennial wildflowers by increasing growth and reducing winter damage. These benefits from PIP production have been observed elsewhere in the county (Ruter 1993, 1998). However, these results were not definitive, as the negative impact of high root zone temperatures was muted in 2004, where lower temperature differences between CAG and PIP plants were reflected in more similar water use and growth. The data further suggest differences in species response to CAG versus PIP systems. As might be expected, Aquilegia, a shade-tolerant species found largely in high-elevation under-story habitats, was most affected by root zone temperatures in the PIP system. Polemonium is another higher-elevation species that exhibited a trend toward reduced growth in the CAG. At the other end of the adaptation spectrum, Mirabilis and P. palmeri displayed minimal negative impacts from being produced above ground. Anecdotal observation indicated that aboveground growth of these two lower-elevation species in the CAG system were just delayed in growth and not of lower visual quality, and would eventually reach the same size as PIP plants.
Study 2. This study compared irrigation type and growing media for pot-in-pot production. For both perennials and shrubs there were no significant growth differences between plants grown under the drip irrigation or the sprinkler irrigation system during both 2001 and 2002 (Tables 5 and 6). Although perennial and shrub species had higher heights, widths, and ratings under the drip system, these differences were minimal and both systems produced healthy plants. Therefore, the most economical and less labor intensive choice for watering a small-scale PIP nursery would be overhead sprinkler irrigation. The more costly and labor intensive, but more water efficient choice would be drip irrigation.
There were large growth differences between plants grown in Ecomix, the landfill medium, and the Utelite medium. The landfill media used in 2001 was not successful (Table 5), and in fact mortality amongst the shrub species was so high we were not able to conduct statistical analyses. Plants performed poorly in this media because it had an excessively high electrical conductivity (EC) and marginally high pH. The EC of the landfill media was 11 and the pH was 7.5, whereas Ecomix had an EC of 1.9 and a pH of 6.1. It is assumed that these characteristics of the landfill mix originated from the compost component of the media. Only 4.8% of perennial species planted in the landfill medium had a seasonal average rating of four or above (good to excellent health) compared to 71.4% in Ecomix. A grower could not sell a plant for full price that is rated less than 4.0. Keeping this in mind and that plants grown in Ecomix were larger taller, wider, and had higher ratings than those in the landfill medium, Ecomix is the most economical option to achieve the highest quality product.
Plants grown in the Utelite media in 2002 received average ratings, but they did not perform as well as they did when planted in Ecomix (Table 6). The Utelite media had an EC of 1.9 and a pH of 7.1, which is similar to the properties of Ecomix. We suspected that the Utelite media had low water and nutrient holding capacities, but probably more importantly Utelite is a lacustrine sediment such that salts would be continually weathered from the material and present in the root zone and affecting plants. Only 56.1% of the perennial species and 70% of the shrub species planted in the Utelite medium had a seasonal average rating of 4.0 or above compared to 92.7% and 90.0% of perennial species and shrub species in Ecomix. This information, coupled with the fact that most plants grown in Ecomix had larger heights, widths, and ratings than those in the Utelite medium, suggests that Ecomix, or some other type of organic medium, is still the most economical option for growing the highest quality plant material. A locally produced medium may have economic potential if the materials are lower cost than a commercial mix, but the components used in such a medium would have to be carefully screened for salinity.
Overall, perennial plants in 2001 had a higher incidence of mortality than 2002. Mortality for 2001 was 23.8% compared to 0.8% in 2002. The primary reason for the high mortality in 2001 was the stress caused by high salinity in the landfill media, particularly the shrubs, and that all plants were left in the production system longer than they needed to be. Measurement collection was on a two-week basis, so most plants were left in the ground for one to two months. As a result, plants became root bound and water stressed. After learning from the results in 2001, measurements were collected weekly in 2002 and plants were pulled from the production system when their roots had filled out the pot but before they were root bound. The average time in the production system was four weeks. Many plants were ready to pull within three weeks, but were left in for four weeks for data collection purposes.
Study 3. This study was conducted at the industry cooperator in this project, Chelsea Nursery in Grand Junction, Colorado, where PIP production was scaled up to a commercial level. Of the three treatments—control, copper-painted pots, and fabric pot liners—there was a miscommunication with the cooperator such that the control pots were painted with the copper compound, thus eliminating that treatment. Both growth and stomatal conductance measurements showed that the trees in the copper-painted pots performed significantly better that those in the fabric-lined containers (anecdotal observation; data currently possessed by Matt Rogoyski). The owners of Chelsea Nursery were very pleased with the results, and wrote the following testimonial:

July 10, 2005.
Chelsea Nursery is glad to have been a part of the Pot in Pot study. In our opinion, P&P is a viable production method. It uses less water than conventional overhead irrigation, the plants don't blow over in windy weather, and being in ground, plants don't need special overwintering protection. We plan to expand this production method with #10 and larger pots. We may even do some #5 pots for particular plants.
Thanks, Tony and Stacey

Literature Cited
Adrian, J.L., C.C. Montgomery, B.K. Behe, P.A. Duffy, and K.M. Tilt. 1998. Cost comparisons for infield, above ground container, and pot-in-pot production systems. J. Environ. Hort. 16:65-68.
Hight, A. and T.E. Bilderback. 1993. Substrate temperatures in above and below-ground containers in a pot-in-pot system. Proc. SNA Research Conference. North Carolina 39:113-115.
Martin, C.A., L.B. McDowell, and S. Bhattacharya. 1999. Bellow ground pot-in-pot effects on growth of two Southwest landscape trees was related to root membrane thermostability. J. Environ. Hort. 17:63-68.
Ruter, J.M. 1998. Effects of pot-in-pot production system on plant growth. American Nursery-man 189:66-69.
Ruter, J.M. 1993. Growth and landscape performance of three landscape plants produced in conventional and PIP production systems. J. Environ. Hort. 11:124-27.
Young, R.E. and G. R. Bachman. 1996. Temperature distribution in large, pot-in-pot nursery containers. J. Environ. Hort. 14:170-6.

Research conclusions:

The critical scientific impact of the results from these two studies was that PIP production improves plant growth when grown in a quality medium under overhead irrigation. The social impact of these studies is analyzed by how many nurseries adopt the method. The very positive testimonial from Chelsea Nursery regarding their production of trees PIP echoes the spread of PIP production of woody plants elsewhere in the country because of reduced water loss, winter protection, and generally better growth (Ruter, 1998; Ruter, 1993). These benefits are more pronounced in the IMW because of our seasonal temperature extremes. In addition to Chelsea Nursery, a nursery in Utah (Wildland Nursery, Joseph UT, Janett Warner proprietor) has also adopted the PIP system in the production of woody plants after abandoning root control bags that were too difficult to manage. More specifically we have also received inquiries from a nursery in Boise that is planning to adopt PIP production for IMW perennial wildflowers.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

2003 Amy Croft, MS. The production of native and adapted plants for the Intermountain West using the pot-in-pot production system.
2005 Guillermo Cardoso, MS. Optimizing pH, fertilization, and field production requirements of native herbaceous perennials.
Peer reviewed publications in preparation
Cardoso, G., R. Kjelgren, T. Cerny-Koenig, R. Koenig, and K. Kopp. 2005. Production of Intermountain West native perennial wildflowers using pot-in-pot production versus conventional above-ground production. To be submitted to the Journal of Environmental Horticulture
Croft, A., R. Kjelgren, R. Ward, and K. Kopp. 2005. An economic analysis of the production of native and drought-adapted plants for the Intermountain West using the pot-in-pot nursery production system. To be submitted to the Journal of Environmental Horticulture
In 2001, 2003, and upcoming 2005 I help organize the landscape horticulture field day and native plant symposium where the pot-in-pot plots can be inspected by landscape and nursery industry personnel and I can present research results.

Education and Outreach Outcomes

Recommendations for education and outreach:

Areas needing additional study

The original funding for this project was leveraged by additional federal and state funds that supported the above-ground versus PIP study, thus providing a more complete picture of PIP production of #1 IMW perennial wildflowers. These two studies I have reported on here are part of a longer term strategy to develop a complete nursery system for cost effective local production of Intermountain West native trees, shrub, and perennials. The next step in this strategy is compare above-ground to PIP production of 5-gallon native shrubs. This study is being installed during the summer of 2005, and I expect that the production cycle for three species, single leaf ash (Fraxinus anomala), Utah serviceberry (Amelanchier utahensis) and greenleaf manzanita (Arctostaphylos patula ‘Ponchito’), will take two years to the fall 2007. At that time a second set of three species will be planted.

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.