Integrated Management of Purple and Yellow Nutsedge in Organic Vegetable Production

Integrated Management of Purple and Yellow Nutsedge in Organic Vegetable Production

Summary

At Gainesville, suppression of purple nutsedge by all summer fallow treatments persisted into the subsequent fall cash crops, but solarization was the least effective. Highest lettuce yields were obtained with incorporated sunn hemp, solarization, and tillage. Highest broccoli yields occurred with flaming, solarization, and tillage. In Tifton, solarization showed potential as an alternative method of weed management in organic crop production. At Clemson, green film, solarization, or solarization following a turnip cover crop suppressed purple nutsedge tuber density before transplanting fall grown pepper, which lowered the shoot density of purple nutsedge at 3 weeks after transplanting pepper compared to no nutsedge management.

Objectives/Performance Targets

Gainesville, Florida

To compare the summer fallow techniques of a summer cover crop, soil solarization, clean fallow with disking, clean fallow with flaming, and a weedy fallow on purple nutsedge population density, tuber number and size distribution, and tuber viability.

To evaluate the persistence of suppression in two subsequent fall cash crops with differing canopy sizes and rates of growth and development.

To compare the effect of clean fallow and an allelopathic winter cover crop on purple nutsedge tuber viability.

To assess the effect of spring crops of differing canopy type and rate of growth and development and weed-suppressive synthetic mulch (IRT – infrared transmitting film).

To identify a combination of treatments applied in sequence that result in the most cost effective and efficacious suppression of purple nutsedge.

Tifton, Georgia

Objectives at Georgia are the same as Florida except for the target weed species, which is yellow nutsedge.

Clemson, South Carolina

To integrate tillage, solarization, an IRT film, a biofumigation/cover crop, mulching, and mowing as multi-facet strategies for suppressing purple nutsedge in organically grown bell pepper.

Economic Analysis

To assess the monetary costs and returns to producers in comparison to conventional (non-organic) production systems, non-market environmental and social benefits associated with reduced consumption of pesticides, and regional economic impacts of expanded local vegetable production.

Accomplishments/Milestones

Gainesville, Florida

Soil was sampled for initial purple nutsedge tuber density in late June, 2002 and an initial assessment of existing weed species and nutsedge shoot density was done on July 1. No difference in viable tubers or nutsedge shoot density was observed. Summer fallow treatments were established on July 2. Clean fallow treatments accomplished with weekly tillage or weekly flaming were conducted for 12 weeks. Two sets of summer cover crop treatments of sunn hemp (Crotalaria juncea) were established by broadcasting 40 lb of seed per acre and were undercut at 13 weeks after seeding. Cover crop residue was either incorporated prior to transplanting or retained on the surface as mulch for the fall crops of lettuce and broccoli. Soil solarization was initiated on July 2 and the transparent solarization film was maintained in place until mid-October. A weedy fallow treatment was included as a control, which was tilled before establishing the fall crops. After fallow, flaming had the highest number of viable tubers, with all other treatments similar to the weedy control. Nutsedge shoot density was suppressed by all fallow treatments to lower levels than with the weedy control, but solarization was the least effective. Leaf-cutting insects eliminated the crops in the sunn hemp mulch treatment within days of being transplanted. Lettuce stands with all other treatments were similar and greater than with the weedy control. Highest broccoli stands were obtained with flaming, solarization, and tillage; but broccoli stand with incorporated sunn hemp was similar to the weedy control. Highest lettuce yields occurred with incorporated sunn hemp, solarization, and weekly tillage. However, lettuce yields with flaming and the weedy control did not differ statistically. Broccoli yields were greatest with flaming, solarization, and tillage. Broccoli development was delayed with the weedy control and incorporated sunn hemp treatments and no significant yield was obtained.

During the winter fallow period rye will be planted (January-February) as a cover crop on half of the plots and the other half will be maintained weed-free using tillage. The effects of these treatments on nutsedge tuber viability will be determined. In March 2006, if sufficient rye biomass is accumulated, it will be undercut and for use as mulch. If insufficient biomass is obtained, rye will be incorporated prior to setting out bell pepper and zucchini squash transplants. The halves of the plots maintained weed-free during winter fallow will be mulched with a dark-green infra-red transmitting (IRT) film. The effect of IRT film on the suppression of purple nutsedge and the growth and yield of the cash crops will be assessed. Beginning in June, the experiment will be repeated in the same location to determine the cumulative effects of treatments over time.

Tifton, Georgia

The same summer fallow treatments used in Gainesville also were conducted in Tifton with yellow nutsedge as the target weed species. Trials were initiated in late June 2005, which was about 1 month later than what was preferred. This particular site has a history of heavy yellow nutsedge infestation, although the weed was present in 2005 at low densities. However, crowfootgrass and southern crabgrass were present at very high densities that responded to treatments. Sunn hemp was a robust cover crop that suppressed weed emergence, but was difficult to shred prior to seeding fall crops. Turnip green stand was reduced in plots where sunn hemp was planted as a cover crop, due to pieces of sunn hemp stalk physically inhibiting turnip seed placement. Solarization with clear plastic mulch suppressed weed emergence during the summer months and this effect was extended to fall crops. Solarization effectively reduces weed populations in the upper portions of the soil profile. A sunn hemp cover crop suppressed weeds, but those benefits were offset by difficulty in establishing the stand of direct-seeded turnip green. The major accomplishment is further validation that summer solarization has potential as an alternative method of weed management in organic crop production. As for Gainesville, spring cash crops will be bell pepper and squash.

Clemson, South Carolina

A field experiment was conducted at Clemson, SC, in 2005 to test integrated strategies for managing purple nutsedge utilizing various combinations of cultural and mechanical control measures. The experiment was organized as a split-plot design with four replications. Main plots consisted of 1) a green film from mid-March through early August, 2) a clear film (solarization) from mid-March through early August, 3) turnip from mid-March to mid-June followed by (fb) a green film through early August, 4) turnip from mid-March to mid-June fb solarization through early August, 5) monthly tillage from mid-March through early-August, and 6) no nutsedge management (weedy control). Subplots were 1) hand hoed (weed free), 2) mulched with wheat straw, and 3) no nutsedge management. All subplot treatments were applied from early August through early November. Measurements taken included tuber viability and size in mid-March, early August, and mid-November; shoot density in mid-June, late July, and August through late October; time of weeding the pepper crop, and marketable fruit yield. Tuber size categories were small (0.1 to 0.25 g), medium (0.26 to 0.5 g), and large (>0.5 g).

Average tuber density over the test site at initiation of the experiment in March was 499, 298, and 110 viable tubers/m2, totaling 907 viable tubers/m2. Green film, solarization, and turnip fb solarization from March through early August significantly reduced purple nutsedge density by 473 to 649 viable tubers/m2 over the initial density in March. From March to early August, tuber density increased by 779 viable tubers/m2 in the ‘no management’ main plots. Main plot treatments, excluding the ‘no management’ treatment, depleted the number of medium and large sized tubers, with no significant impact on small sized tubers. The shoot density of purple nutsedge at 2 weeks after transplanting pepper followed the same trends as viable tuber density sampled in early August. Among subplot treatments, hand hoeing was more effective in managing the viable tuber density compared to mulching, mainly by depleting the number of small sized tubers. In contrast, tuber density in ‘no management’ subplots increased by 1,561 tubers/m2 from August through November, averaged over main plots. The initial depletion of tubers in main plots from March through early August was of little benefit if some form of nutsedge management was not practiced for the remainder of the season (August through November). For instance, in the ‘no management’ subplots, tuber density increased by 968 to 1,230 viable tubers/m2 over the initial density in March for all main plot treatments, excluding the ‘no management’ treatment. Purple nutsedge shoot density at the end of the season in the ‘no management’ subplots did not differ among main plot treatments, averaging 456 shoots/m2, indicating that early season nutsedge management strategies must be supplemented by hand hoeing during the fall cropping season. There was a significant reduction in weeding time in all main plot treatments (averaging 17.9 min/subplot) compared to ‘no management’ (27.5 min/subplot). These results indicate that green film, solarization, or solarization following a turnip cover crop suppress purple nutsedge tuber density before transplanting fall grown pepper, which lowers the shoot density of purple nutsedge at 3 weeks after transplanting pepper compared to no nutsedge management. Research will be continued at this site in the upcoming year to determine the long-term impact of each strategy on purple nutsedge tuber dynamics.

Economic Analysis

Pro-forma budget information on costs of production has been compiled for four selected vegetable crops (broccoli, squash, green peppers, and lettuce or leafy greens) grown in Florida, Georgia, Alabama and South Carolina. These budgets were generally developed by university extension specialists through consensus opinion of growers, allied suppliers and researchers, regarding quantities and prices of inputs used for conventional best management practices. In some cases, separate budgets were available for double cropping practices. Expenses include both direct costs such as seed, fertilizer, chemicals, fuels, equipment repairs, maintenance, cultural and harvest labor, and fixed costs such as rent, asset depreciation and interest. Budgeted expenses vary widely among crops and across states, from under $2,000 per acre to over $5,000 per acre. The expense items for each crop/state budget are being evaluated to determine the items that may change under organic production practices, by substitution of labor and other allowable inputs for non-allowable chemicals and synthetic fertilizers. Expenses for insecticides, herbicides, fungicides, equipment and labor for their application may exceed $200 per acre. Data on labor time and direct expenses for vegetable test plots have been tracked by the horticulturist investigators in the project, and are being evaluated to incorporate into the full budgets. Labor time expended in the summer and fall plots was 98 and 42 hours per acre, respectively, which would represent a cost of $637 and $273 per acre, assuming a wage rate of $6.50 per hour.

In the next year, the budgeting and cost analysis work will be completed. Evaluation of market prices and returns for organic produce will be conducted, for both wholesale market channels and for local/retail markets. Based on the outcome of the cost analysis, the regional economic impacts of organic production will be evaluated by constructing a regional input-output model using the Implan Pro software system and associated state and county datasets.

Impacts and Contributions/Outcomes

In Gainesville a graduate student from the Plant Medicine Program, Jeff Pack, is being trained in the conduct of systems research by participating in the study. An undergraduate intern Clinton Warren was trained in the scientific method for a 6-wk period during summer 2005 through his participation in the summer fallow treatments and scientific report writing based on his involvement with the project. Both students as well as grower and co-PI, Rose Koenig, were coauthors on a presentation given by Carlene Chase in January on the summer and fall 2005 results for Gainesville at the meeting of the Southern Region American Society for Horticultural Science in Orlando, Florida. An abstract is forth-coming in the journal HortScience. Growers have been identified from each state to participate on the project committee. An organic grower workshop is being planned for Gainesville in September 2006 during which Carlene Chase will give an update on the results of her work on weed management in organic vegetables over the past 3 years, including the results of this study. We are planning an associated field visit for Rosie’s Organic Farm (the Gainesville research location) when farmers will be able to see the summer fallow treatments in progress.

Economic Analysis

It is still too early to make any definitive comparison of costs and returns for organic vegetable production. However, it is expected that greater labor costs for hand weeding, scouting inspections and other organic practices will be offset by expenses eliminated for pesticide materials and application. When completed, these results will help vegetable growers to better evaluate tradeoffs for organic vs. conventional production systems.

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