Weed Management in Organic Vegetable Production
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A3845 Weed management in organic vegetable production A PRACTICAL REVIEW OF APPLIED RESEARCH JED COLQUHOUN University of Wisconsin-Extension I Cooperative Extension Contents Foreword . 1 Problem avoidance: Preventive strategies . 2 Managing the weed seedbank . 4 Weed suppression with mulches . 6 Cover crops and allelopathy . 8 Reduced tillage and dark cultivation . 10 Thermal weed management . 12 Alternative herbicides . 14 Crop competition for weed control . 16 In-season cultivation . 18 Integrated weed management . 21 Foreword he intention of this manual is to Few things are harder to put provide an objective analysis of Torganic weed management up with than the annoyance strategies. One that is based on peer- reviewed and published applied of a good example. research. The strategies used in organic weed control have been Mark Twain around, in most cases, for centuries, and therefore current peer-reviewed research is often not discussed when weighing management options. With this in mind, the research is first objec - tively summarized for those who are interested in“just the facts,” the back - ground information is presented in more detail, and finally the research that serves as the basis for the sum - mary is reviewed. The strategies approved for use in organic production by the United States Department of Agriculture (USDA) National Organic Program, local organic grower organizations, and end-users change periodically. Consult the appropriate organizations for approved strategies and products prior to any use, as well as labels and registration status for products used as herbicides. How to use this manual: I Interested in just the facts? Focus on the key points and background information for practical guidance and an overview of weed manage - ment options. I Interested in the details and eval - uation of options? Consider also reading the research sections for an overview of recent peer-reviewed studies conducted on vegetables. 1 KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION I It’s often easier to prevent the introduction of a new weed species than to eradi - Problem avoidance: cate it once established. Preventive strategies I Consider potential sources y far, the best management strat - should be cleaned of soil and vegeta - of new weeds—as contami - egy for any weed is to prevent tion before being moved between nants in crop seed, manure, Bintroduction and dispersal. There fields. are several routes for weed introduc - Historically, many weed species have or mulches; on equipment; tion, and the majority involve inadver - been introduced as a seed contami - and in open irrigation tent grower actions. Potential sources of weed seed on the farm include nant in crop seed. This source of water. compost, manure, mulches such as weeds is particularly prevalent when straw, equipment, open irrigation “saved” seed is replanted in the follow - I Continue to manage weeds water, and contaminants in crop seed ing year or on another farm without after harvest and in field and transplant containers. cleaning or analysis by a seed certifi - cation agency. margins to minimize weed Occasionally, new invasive weeds are intentionally planted, as is the case seed production through Prevention is also important in mini - with kudzu, which was first marketed mizing the spread of weed species mowing or planting of for forage and erosion control in the already found on the farm. competitive, non-invasive southeastern United States, and vel - Management practices that minimize vetleaf, which was initially cultivated species in boundary strips. weed seed production are important as a source of fiber for cloth and rope. in fields after crop harvest and in sur - Consider all potential entry routes for rounding field margins, such as: new weed species and methods to I Till or mow after crop harvest but reduce weed seed and vegetative before weeds go to seed. propagule introduction and viability. I Identify weeds prior to using tillage Weed seed in compost, for example, for weed management—perennial can often be destroyed by prolonged species can be spread by cutting heat and adequate moisture in the below-ground vegetative tissue, pile. Some weed seeds such as curly such as rhizomes and roots. dock can pass through the digestive tracts of cows and other farm animals. I In field margins, consider planting and managing a boundary strip Consider the source of all compost, that is kept free of weed seed pro - manure, and straw, and question duction. The planted species could providers about weed contamination include a mix of those that are com - prior to purchasing. For example, was petitive with weeds and support a hay field weedy, and if so, when was beneficial wildlife or predators of the field harvested relative to weed other agricultural pests. Field mar - flowering? Were perennial weeds gin plantings might also be har - present that could be dispersed vested for hay. through root cuttings? Cultivators, I Prevent weed seed production harvesters, and other field equipment along irrigation canals, reservoirs, are sources of vegetative root tissue and other open water sources. and seed for many weed species and 2 PROBLEM AVOIDANCE: PREVENTIVE STRATEGIES Research Open irrigation water Planted field margin boundary as a source of weed seed strips for weed control Manure composting to Source: Catalan, B., J. Aibar, and C. Source: West, T. M., E. J. P. Marshall, and reduce weed seed viability Zaragoza. 1997. Weed seed dispersal G. M. Arnold. 1997. Can sown field Source: Tompkins, D. K., D. Chaw, and through irrigation channels. Proc. boundary strips reduce the ingress of A. T. Abiola. 1998. Effect of windrow 1997 Congress Spanish Weed Science aggressive field margin weeds? Proc. composting on weed seed germina - Society, pp. 187 –193. Brighton Crop Protection Conference, tion and viability. Compost Science and pp. 985-990. Utilization 6:30–34. Summary: Irrigation channels were sampled at three depths and weed Summary: Boundary strips planted Summary: Manure was composted in seed germination was tested in con - with grass and wildflower seed mix - a windrow with a core temperature of trolled conditions. Seed from 23 fami - tures were compared to unplanted 130°–150°F. After 2 weeks of compost - lies and 63 species germinated, strips surrounding agricultural fields. ing, the high temperatures killed primarily from the Asteraceae and The planted strips limited movement 100% of the seed of green foxtail, Poaceae families. Common species of weeds including Canada thistle, catchweed bedstraw, kochia, wild included Conyza spp., annual sowthis - catchweed bedstraw, poverty brome, buckwheat, field pennycress, and tle, smooth pigweed, Bromus spp., and and quackgrass into the fields. pineappleweed, and all but 3.5% of annual bluegrass. the seed of redroot pigweed. No seeds survived after 4 weeks of composting. This study demonstrated the effective - ness of even short periods of com - posting at eliminating the distribution Table 1. Germination of selected of viable weed seeds. weed species after 17 years of burial in Nebraska. Weed seed longevity Species Germination, % Source: Burnside, O. C., R. G. Wilson, Annual grasses S. Weisberg, and K. G. Hubbard. 1996. Barnyardgrass 0 Seed longevity of 41 weed species Green foxtail 0 buried 17 years in eastern and west - Longspine sandbur 0 ern Nebraska. Weed Science 44:74–86. Yellow foxtail 0 Summary: This study examined the Annual broadleaves long-term survival of weed seeds and Common lambsquarters 7 reveals the importance of preventing Hairy nightshade 65 weeds from becoming established. Jimsonweed 90 Weed seeds were buried and then Redroot pigweed 1 dug up and tested annually to see if Velvetleaf 35 they would germinate. Following Biennial/perennial weeds 17 years of deep burial without tillage, Canada thistle 7 seed of 4% of annual grasses, 11% of Common mullein 95 annual broadleaf weeds, 30% of bien - Curly dock 61 nial weeds, and 8% of perennial weeds Dandelion 0 MAKING THE CASE germinated (table 1). Common Source: Adapted from Burnside et al. mullein proved to have superior 1996. FOR PREVENTION : longevity, with 95% of the seed germi - After 17 years underground, nating at the end of the study. 95% of the seeds of common 3 mullein still germinated. KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION I The soil contains a seed - bank—a vast supply of weed seeds—of potential Managing the weed seedbank future weed problems. he soil contains millions of weed additional tillage into a seedbed that seeds and serves as a seedbank or has been depleted of germinable I Two techniques for Treservoir for future weed infesta - weeds. This technique is particularly depleting weed seeds: tions. Many of these seeds are dor - useful with small-seeded or slow- mant, and will not germinate until growing crops that are not very com - Stale seedbed technique, triggered by environmental or physio - petitive with weeds, such as onions where soil is tilled 20 to logical signals. Although seeds can and carrots. remain dormant for several years, The second technique, , 30 days before crop plant - weed pressure during the cropping solarization uses sheets of clear plastic mulch to ing. Weeds that germinate season can be reduced through prac - cover the soil and trap solar energy tices that deplete those seeds that will during the delay period are near the soil surface. Intense heat and readily germinate before crop planting . killed shortly before seed - sunlight are required for effective There are two techniques for deplet - solarization, thus limiting the use of ing using flaming, mowing, ing weed seeds. In the first, known as this strategy in more temperate pro - or shallow tillage. the stale seedbed technique, the duction areas. Moist soil conducts seedbed is tilled and prepared for heat better than dry soil and promotes Solarization is the practice crop planting, then allowed to rest microbial activity. After about 4 weeks of covering soil with plastic while non-dormant weeds germinate of solarization, the plastic mulch is and emerge.