Chapter 14

Weed Management for Organic Crops

Cultural Weed Control

Any tactic that makes the crop more competitive against weeds is considered cultural control. Some cultural practices—in particular, crop rotation and altering planting dates—can be critical components of weed management in organic production systems. Cultural methods are the first line of defense in weed management. Following are several crop management practices that can enhance crop competitiveness toward weeds.

Managing Weeds with Cover Crops

Cover crops offer many benefits to an organic farming system, including protection against soil erosion, improvement of soil structure, soil fertility enhancement, and weed suppression. Cover crops can be used in a variety of ways to suppress weeds (See Figure 14.1). First, cover crops occupy the space and limit weed growth through direct competition for light, nutrients, and moisture. Second, many cover crops and their residues release allelochemicals into the soil that prevent or hinder weed seedling growth. Third, a vigorous cover crop can change the environment for weed seeds on and in the soil—inhibiting germination due to a heavy crop canopy. Finally cover crops can enhance the vigor, and therefore weedcompetitiveness of the following cash crop, by providing nitrogen (e.g., legumes), enhancing availability of other nutrients, or improving soil conditions.

Cover Crops as Living Mulches

Winter cover crops can occupy the niche that exists after a summer crop is harvested and before the next season's crop is planted.

Allelopathic Cover Crops

Many cover crop species produce allelochemicals as they grow and during decomposition, meaning that both living cover crops and decaying residue (incorporated or on the surface) can help to suppress weeds. Commonly used cover crops known to produce allelochemicals and effectively suppress weeds are listed in Table 14.1.

Role of Crop Rotation in Weed Management

Crop rotation provides the foundation for long-term weed management. Planting a wide variety of crops with varied characteristics reduces the likelihood that specific weed species will become adapted to the system and become problematic. In designing a crop rotation for weed control, the overall key to success is diversity. The following principles or practices are commonly incorporated into successful rotations.

Selecting Crop Varieties that Better Compete with Weeds

Organic systems need varieties that actively suppress weeds. Quick germination and establishment, rapid early and vigorous growth, and the ability to rapidly cover the soil and shade it (prostrate or tall varieties) in order to out-compete weeds at an early a stage in the crop cycle, are all desirable traits that will potentially shade out weeds. Crop varieties with a larger seed size have also been shown to exhibit greater initial vigor of emergence and growth, which may subsequently provide extra competitive ability. Tall grain crops are generally more competitive with weeds because they intercept light.

Adjust Crop Planting to Disrupt Pest Habitat

Crop planting can be adjusted both in space and time to reduce the development of large pest populations.

Altering the Planting Dates to Control Weeds

Altering planting dates to disrupt weed life cycles can be timed to limit competition from potentially troublesome weed populations. In some instances, it is wise to seed or transplant a cash crop early to get canopy closure as soon as possible.

Adjusting Seeding Rate to Control Weeds

Increasing the seeding rate is another common strategy for organic growers to control weeds. The theory is that the greater amount of space taken up by the crop in the rows, the less space there is available for the weeds to invade. There is also an allowance for potentially lower germination rates and loss of crop by mechanical weeders.

Mulches for Weed Control

Mulch is any kind of material applied to the soil surface for protection or improvement of the area covered. Mulches contribute to weed management in organic crops by reducing weed seed germination, blocking weed growth, and favoring the crop by conserving soil moisture and sometimes by moderating soil temperature. Where a mulch layer is sufficiently deep, few weeds will grow. There are various types of mulches available and may be divided into synthetic and organic mulch.

Advantages and Disadvantages of Mulching

Advantages and disadvantages in using mulches for weed control are as follows:

Synthetic Mulches

Synthetic mulches like black polyethylene film (the most widely used plastic mulch) are laid on a prepared seedbed just before transplanting or seeding a vegetable crop through holes or slits cut into the mulch. Mechanization, with equipment such as tractor-drawn bed shapers, mulch layers, and planters, allows the farmer to mulch and plant a field (See Figure 14.2). Black plastic, other opaque materials and infrared-transmitting (IRT) mulch effectively block weed emergence, and promote soil warming and early crop growth.

Organic Mulches

Organic mulches cover the soil and provide many of the same benefits as cover crops, especially the prevention of soil erosion. Many organic materials, such as straw, leaves, pine needles, and wood chips, can be effective mulches. Straw and other materials that are easily decomposed are applied to strawberries and vegetables during the growing season. The mulch can be tilled in at the end of the season, where it will quickly decompose.

NOP Guidelines for Mulches and Weed Barriers

NOP has developed guidelines to help producers understand organic standards as they relate to the use of mulches and weed barriers. USDA Organic Regulations define mulch as any material that serves to suppress weed growth, moderate soil temperature, or conserve soil moisture. Mulches and weed barriers are production inputs on organic farms. All mulches and weed barriers, synthetic or non-synthetic, must be included in the producer’s annual Materials Inventory. Guidelines for acceptable and unacceptable mulches are listed as follows:

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