Chapter 13

Weed Management for Organic Crops

Chemical Weed Control

Weed control options for organic growers are limited because use of synthetic herbicides is restricted. However, products derived from plant-based materials as well as some synthetically derived soap-based products are available for potential use in organic crop production systems. These chemicals, often referred to as “natural products,” are formulations that contain herbicide active ingredients and include acids, soaps, and oils, as well as some other materials.

NOP Requirements for Using Herbicide Products

Natural products may be used if the requirements of 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 chemical for weed control, the product must be labeled as an herbicide or qualify for exemption under the Federal Insecticide, Fungicide, and Rodenticide Act.

Application Techniques

With post-emergence foliar-applied products, effective weed control is heavily dependent on air temperature, sunlight, and humidity at time of application. A minimum air temperature of 75 degrees F (30°C) combined with 50 percent relative humidity and clear, sunny conditions for maximum activity is recommended for effective application. Some product formulations are applied with a carrier, such as water, to form a spray solution, while others may be directly applied. Natural products are fast acting and the majority of weed injury will occur in the first few hours after application.

Types of Herbicides

Many different brands of natural product herbicides are marketed for use in organic production systems. 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.

Adjuvants Approved for Organic Weed Control

Adjuvants, including surfactants and wetting agents, may be needed to improve herbicide activity. Clear recommendations for adjuvants are not given on the herbicide label for several of the commercially available products and research regarding natural product adjuvants has been limited.

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