Crop Rotation on Organic Farms
Management of Weeds with Crop Rotation
Ideally, weed management in an organic cropping system involves the integration of a broad range of cultural practices. Although cultivation after planting is usually a key component, a variety of other factors make important contributions to weed control on organic farms. All of these practices occur within the context of a sequence of crops that are planted on a field—the crop rotation. Crop rotation breaks the life cycles of weeds adapted to the narrow ecological niche imposed by continuous cropping. In general, crop sequences that take advantage of multiple opportunities to suppress and remove weeds from the field will improve weed management on the farm. Success in an organic cropping system depends on the use of crop sequences that employ varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to suppress and remove weeds from the field. A really good weed-suppressive crop rotation creates complex and unpredictable patterns of disturbance that keep the weeds guessing, reduce opportunities for dominant weeds to continue exploiting resources, and even “trap” weeds into life-cycle dead ends that curtail reproduction.
Plants with Allelopathic Properties
Many crop plants can also create what is called allelopathic interference. These plants release chemicals either while they are growing or decomposing that prevent the germination and growth of other plants. Plants differ in their allelopathic properties and in their susceptibility to allelopathic chemicals produced by other crops.
Crop Rotations Principles for Controlling Weeds
In designing a crop rotation for weed control, the overall key to success is diversity. The following principles are commonly incorporated into successful crop rotations designed to suppress weeds.
Follow Poor Competitor Crops with Strong Competitor Crops
Growing poor competitor crops in a special crop rotation in which they alternate only with strong competitor crops (e.g., soybeans, potatoes) may prove worthwhile if weeds become a serious barrier to the production of non-competitive crops.
Rotate Between Crops Having Different Characteristics
Include as wide a variety of crops and crop types as possible having different characteristics such as specific rooting habits, competitive abilities, nutrient, and moisture requirements. Many weed species are adapted to specific environments, so rotating between different groups of crops can be very effective at reducing weed problems.
Rotate Between Crops with Planting Different Seasons
Rotate between crops that are planted in different planting seasons given that weed species have characteristic times of the year during which they emerge. Common ragweed emerges most readily in early spring and is often a problem in spring-sown organic small grains like barley and oat. In contrast, henbit and shepherd’s purse are typically fall-germinating species and are likely to be found in winter wheat and spelt.
Adopt Different Cropping Strategies
Follow weed-prone crops with successive plantings of short-season crops, short-cycle cover crops alternating with clean fallow periods, a competitive cover crop that smothers out weeds, or crops grown with weed-suppressing mulch. Weed control is often more difficult in direct-seeded vegetables than transplanted vegetables because the direct-seeded crops take longer to establish and consequently are more prone to weed infestations.
Include Fallow Periods in the Rotation
Perennial Weeds. Deep moldboard plowing is typically used for those perennials with a deep tap root in order to completely bury the weed’s crown (See Figure 8.1). This is usually done at the weakest point in their life cycle, that is, when these perennials are approaching maturity but before they set viable seed. For perennial weeds that reproduce from rhizomes (a continuously growing horizontal underground stem) such as quackgrass, shallow plowing or chiseling is preferable to deep moldboard plowing.
Annual Weeds. The strategy for controlling annual weeds involves setting aside a few weeks or months to stimulate weed seeds to germinate. When the weeds are still tiny, follow up the flush of weeds with shallow cultivation (or flaming). In especially problematic areas repeat this practice a few times.
Rotate between Annual Crops and Perennial Sod Crops
Although weed seed populations decline more rapidly when the soil is tilled, substantial decreases in populations of most annual weeds occur when sod crops are left in the ground for a few years. This occurs by natural die-off of seeds in the soil and because annuals that germinate in a perennial sod are competitively suppressed by the already well-established perennial legumes and grasses.
Work Cover Crops into Crop Rotations
Work cover crops into the rotation between cash crops at times when the soil would otherwise be bare. Weeds establish most easily when the ground is bare. Plant canopies suppress seed germination of many weed species by reducing the amount of light. In addition, cover crops compete with any weeds that do emerge for nutrients and water.
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Topics Within This Chapter:
- Introduction Crop Rotation on Organic Farms
- National Organic Program Standards
- Benefits of Crop Rotation
- Management of Diseases with Crop Rotation
- Management of Insect Pests with Crop Rotation
- Mangaement of Nematodes with Crop Rotation
- Management of Weeds with Crop Rotation
- Crop Rotation Considerations