Chapter 14

Weed Management for Organic Crops

Biorational Control of Weeds

An increasing number of herbicides are permitted for use in organic agriculture. These materials are based on naturally occurring compounds such as plant oils, corn gluten meal, fatty acids, acetic acid, and biological materials. Most products are categorized as plant oil extracts and act as a nonselective contact on weeds. High spray volumes must be used to ensure adequate coverage of the target weeds. These products are not recommended as a sole management tactic and are best utilized as a supplement to an established organic integrated weed control strategy. They generally provide no systemic control, which can lead to inconsistent activity and poor control of perennial weeds. In addition, repeat applications may be needed to get adequate and consistent control of late weed flushes and suppress perennials. If needed, repeat applications should be done shortly after the initial application so weeds not completely killed are not allowed to recover. Certain plant families, such as grasses, and leaf characteristics, such as a waxy surface, are usually more tolerant of natural products.

NOP Requirements for Using Herbicide Products

Natural products may be used if the requirements of National Organic Program (NOP) § 205.206(e) are met. The rule states that if preventive, mechanical, physical, or other weed management practices do not provide adequate control certain substances can be used as long as they are documented and approved in an organic system plan. In order to legally use any biorational for weed control, the product must be labeled as an herbicide or qualify for exemption under the Federal Insecticide, Fungicide, and Rodenticide Act. Adjuvants, including surfactants and wetting agents, may be needed to improve herbicide activity.

Types of Biorationals Used for Weed Control

Several OMRI-certified contact herbicides are available. The active ingredients of these herbicides include citric acid, garlic, thyme and clove oils, and acetic acid (vinegar). Many different brands of natural product herbicides are marketed for use in organic production systems for weed control. Manufacturers of natural product herbicides frequently modify their product formulations, and market availability and active ingredient concentrations will differ over time.

Acetic Acid (Vinegar)

Acetic acid is an ingredient found in several products that is on the Organic Materials Review Institute approved list as a non-synthetic pesticide. Acetic acid, commonly known as vinegar, but also known as ethanoic acid, affects the cell membranes of a plant, causing rapid breakdown/desiccation of foliage tissue on contact. Herbicidal vinegar is stronger than household vinegar: the acetic acid concentration for herbicidal use is 10 to 20 percent, compared to five percent acetic acid for household use.

Corn Gluten Meal

Corn gluten meal (CGM) is a granular product that must be applied prior to weed germination to provide control of germinating seedlings. CGM is a by-product of corn milling that has been sold as an organic fertilizer and animal feed. As a non-selective pre-emergence herbicide CGM inhibits root development, decrease shoot length, and reduce plant survival of weed and crop seedlings.

Cinnamon Oil

Essential oils include substances that collect in plant cells and have been found to have herbicidal characteristic.

Citrus Oils and Extracts

Citrus oil extracted from citrus fruit rinds, provides broad-spectrum control of various grass and broadleaf weeds.

Clove Oil

Clove oil is the active ingredient in a number of organically approved post-emergent non-selective herbicides. Clove oil is believed to kill plants by decreasing cell membrane integrity. Weeds found to be tolerant to clove oil applications are thought to have increased leaf waxes that may hinder activity. Clove oil is most effective when weeds are green and actively growing.

Lemongrass Oil

Essential oils from lemongrass have been used in products including perfumes, cosmetics, and soaps.

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