Weeds
Organic Crop Production
Pest Management in Organic Systems
Weeds
Managing weeds in organic systems uses many of the same techniques as conventional systems, but relies much more on nonchemical control strategies. To plan an effective weed management program in organic systems, historical pest problems, soil management, crop rotation, machinery, markets, weather, time and labor should be considered. Adjusting weed control strategies based on these factors and observing and avoiding potential problems is a big part of staying ahead of weed problems. The primary methods for managing weeds in organic systems are the use of cultural and mechanical weed control strategies, which primarily focus on prevention, crop rotation, crop competition, and cultivation.
Prevention
The primary focus of prevention in organic systems is keeping new weeds out and preventing further spread of weed seed or perennial plant parts. In particular, minimizing the addition or introduction of weed seed to the soil can be critical for successful weed management. Understanding weed biology is an important component in developing a preventive approach. Weed species have strengths and weaknesses and are vulnerable or resilient at different stages in their life cycle. Disking or field cultivating a creeping perennial such as quackgrass or hedge bindweed in the spring may make the problem worse by spreading underground rhizomes or other vegetative structures. Therefore, proper identification and knowledge of weed lifecycle, reproduction, and spread are important factors for developing management strategies.
Some preventive tactics might be classified as sanitation—removing or destroying weeds in fields or near fields prior to flowering and the release of weed seed. Weed seeds can live for a number of years, depending on the species and whether it is exposed or buried beneath the soil surface. This may mean removing by hand if necessary those weeds from the field before they produce seed. Weeds can also be introduced into fields via manure, compost, hay, straw, animal feed, contaminated crop seed, or other materials. Whenever you plant, apply, or drive something in a field, make an effort to learn whether weed seeds are present and evaluate the benefits versus potential risks.
Cultural Weed Control
Some cultural practices—in particular, crop rotation and altering the planting date—can be key components for managing weeds in organic systems. Date of planting will influence the type and number of weeds present. Organic growers should plan rotations so that weed species favored in one year or season will not be favored in another year or cropping sequence. This generally means mixing summer annual crops with fall seeded species or even perennials that allow different weed management strategies. Delaying planting of some spring-seeded crops is common among organic producers. Although some yield potential may be sacrificed with this planting delay, higher soil temperatures will help the crop emerge more quickly, and weeds that emerge earlier in the season can be killed prior to planting to reduce the potential weed seedbank. A stale seedbed is a technique sometimes used in vegetable production systems that can also be used in agronomic crops. A seedbed is tilled several weeks prior to planting. The weeds are allowed to emerge and then they are killed while still small by shallow cultivation, flame weeding, or other nonselective methods. Depending on the length of time before planting, one or more flushes of weeds may emerge and be killed between seedbed preparation and planting. The success of a stale seedbed depends on the weed spectrum and the time of planting. Delayed or later-planted crops are generally more successful. Late-emerging weeds will still be a potential problem. Any tactic that makes the crop more competitive against weeds is considered cultural management and is critical for successful organic production systems.
Crop competition is an important component of cultural weed control and an effective way to control weed growth. Soil open to sunlight is available for weed growth and competition. A vigorous growing crop is less likely to be adversely affected by weeds. Tactics that allow the crop to establish quickly and dominate will help reduce the impact from weeds. Use high-quality vigorous seed, adapted varieties, uniform proper placement of the crop seed, optimal soil fertility, and plant populations that lead to vigorous crop growth and canopy closure.
Mulches and Cover Crops
Mulches are used in some organic production systems to help manage weeds. The mulch provides a physical barrier on the soil surface and must block nearly all light reaching the surface so that the weeds that emerge beneath the mulch do not have sufficient light to survive. Plastic mulches are acceptable in some organic programs, but are generally not practical for lower valued large-scale field crops. Mulches of organic material such as straw or newspaper or killed cover crop residue left on the surface can also effectively block sunlight and are more commonly used in organic row crop production systems.
Cover cropping can help in the management of weeds in several ways. Cover crops can provide an opportunity for crop rotation and rapid turnover of weed seedbanks. In addition, cover crops can provide some weed control by competing with weeds for light, moisture, nutrients, and space. This can be particularly helpful for suppressing winter annual weed growth or certain cool-season perennials. In addition, cover crops and their residues can act as mulches or physical barriers by smothering weeds, suppressing weed seed germination and growth, and lowering soil temperatures. In general, the larger the cover crop and greater the biomass or dry matter production, the greater the impact on weeds. Cover crops may contain allelopathic compounds, which are released from living or decaying plant tissue that chemically interfere with weed growth. However, these qualities can vary depending on the type and quantity of cover crop and environmental conditions during the growing season. Despite these potential benefits, physical and chemical effects from cover crops may not be to be a major factor for weed control. Mechanical control tactics as well as cultural controls should still be used to compliment cover crops for weed management.
Mechanical Weed Control
Mechanical weed control is generally considered critical for managing weeds in organic systems. In organic row crops such as corn or soybean, mechanical cultivation is generally necessary for adequate weed control. Mechanical weed control includes the use of preplant tillage such as plowing, disking, and field cultivating. These primary and secondary tillages can help reduce the rate and spread of certain perennial weeds and can also kill emerged weed seedlings and bury weed seeds below the germination zone. Most organic corn and soybean producers prepare a conventionally tilled seedbed prior to planting their spring crop. Cultivation should generally begin a few days after planting. To control weed seedlings that are just beneath the soil surface or barely emerged, implements such as a rotary hoe, chain-link harrow, or tine weeder are dragged over the field for control of these very small weed seedlings. These implements will displace small seedling weeds and expose them to the drying effects of the wind and sun. Rotary hoes or similar implements are the best method for controlling weeds in the crop row. Operate a rotary hoe at 10 to 12 miles per hour with enough drag to stir the soil and displace the small seedlings .Continue to use a rotary hoe or similar implement about every 5 to 7 days, as long as the weeds are germinating or until the crop is too big. Do not rotary hoe soybeans when the soybeans are in the “hook” stage (the stem is exposed and the cotyledons have not yet opened above the ground). Also, use rotary hoes or similar implements in the afternoon when turgor pressure is less and soybeans and corn are more flexible. In general, up to three rotary hoeings may be performed within 2 to 3 weeks after planting.
Crop rows planted 30 inches or more apart allow for row cultivation. Once soybeans have three trifoliate leaves and corn is beyond the two leaf stage (V2) and 8 to 10 inches tall, use a row cultivator to control small weed seedlings. Shallow cultivation at 1 to 2 inches deep will avoid harming crop roots. Continue to cultivate at 7- to 10-day intervals until the corn is too tall and the soybean canopy closes the rows. Organic corn and soybeans generally require between one and three cultivations, depending on weed species, severity, and rainfall. Cultivation works best when performed during the heat of the day in bright sunlight; weeds quickly desiccate and die under these conditions. Rainfall shortly after cultivation or wet cloddy soils at or following cultivation may allow weeds to recover and survive. Hand-pulling escaped weeds will help assure maximum crop yield and prevent weed seed production, which can affect future weed problems.
Mowing may also play a critical role in managing weeds in forage crops or noncrop areas. Repeated mowing reduces weed competitive ability, depletes carbohydrate reserves in the roots, and prevents seed production. Some weeds, mowed when they are young, are readily consumed by livestock. Mowing can kill or suppress annual and biennial weeds. Mowing can also suppress perennials and help restrict their spread. A single mowing will not satisfactorily control most weeds. However, mowing three or four times per year over several years can greatly reduce and occasionally eliminate certain weeds including Canada thistle. Also, mow along fences and borders to help prevent the introduction of new weed seeds. Regular mowing helps prevent weeds from establishing, spreading, and competing with desirable forage crops, and also can keep them from spreading into tilled fields.
Herbicides
Chemical weed control is generally not allowed in organic crop production systems. The USDA National Organic Program rule (NOP) does allow certain nonsynthetic soap-based herbicides for use in farmstead maintenance (roadways, ditches, right of ways, building perimeters) and in ornamental crops. In addition, several products that contain natural or nonsynthetic ingredients are classified as approved by the Organic Materials Review Institute (OMRI). The OMRI listing does not imply product approval by any federal or state government agency. It is the user’s responsibility to determine the compliance of a particular product.
Corn gluten meal is sold as an organic fertilizer that has some preemergence herbicide activity on certain small-seeded annual weeds. Because of the volume of product necessary and the associated cost, corn gluten meal is generally not practical for agronomic crop production systems but may be suitable for smaller areas such as home gardens and lawns. In addition, the need for and use of corn gluten for weed control must be explained in the Organic System Plan and the product must not be derived from genetically engineered sources. To learn more about corn gluten, visit the corn gluten meal research page at Iowa State University (www.gluten.iastate.edu).
Nonsynthetic (natural) postemergence herbicides contain plant-based ingredients including eugenol (clove oil), cinnamon oil, citrus oil, lemongrass oil, and others, and act as nonselective contact type herbicides. They will injure or kill all vegetation they come in contact with, including desirable vegetation. Weed species, its size or age, and environmental conditions at application time are important for optimum performance. As a general rule, these products are only effective on small seedling weeds and activity increases with air temperature, humidity, and sunlight. Seedling broadleaves are most susceptible, while grasses are harder to control. We suggest a minimum air temperature of 75°F, 50 percent relative humidity, and sunny conditions. Cool temperatures, low humidity, and cloudy conditions will reduce performance. The effectiveness of a clove oil product (Matran II) on several annual weeds is provided in Table 1-11.2.
The need for the use of herbicides derived from plant or animal sources should be explained in the grower’s Organic System Plan and the farmer must obtain permission from their organic certifying agencies to use these materials. Acetic acid or vinegar is an ingredient in a number of products, and distilled white vinegar is on the OMRI approved list but not as an herbicide. The performance of vinegar is similar or less effective than that of clove oil (Table 1-11.2). As a general rule, a 20 percent concentration (200 grain) is necessary applied at a minimum of 60 gallons per acre. Individual certifying agencies can help decide the merits and need for acetic acid as an herbicide. Several products are labeled as organic herbicides, and additional products and ingredients are currently under review.
Remember that an adjuvant (i.e., surfactants and wetting agents) is often necessary to enhance herbicide performance. All synthetic adjuvants are prohibited, which includes most adjuvant products in the market. A number of plant-based adjuvants are available and are often derivatives of pine resin (Nu-Film P) or yucca (Natural Wet) or other plant-based substances. Some products contain acidi- fying agents as well as other ingredients touted to enhance pesticide or nutrient uptake. Check with your organic certifier concerning the allowance of these additives. Table 1-11.3 contains some herbicides listed by OMRI or others at the time of printing. Some of these products already include surfactant-type adjuvants in their formulation. Penn State does not ensure the effectiveness or allowance of any of these products.
