Weed Management

This section contains general information on organic weed management.

See the Resources section for links to additional information.

Traditional weed management often relies on synthetic pesticides. Organic production, however, requires a different perspective, as synthetic herbicides are prohibited in organic systems. Weed management for organic production relies primarily on non-chemical approaches that are tailored to the specific weeds and crops.

Identification is the first step in any weed management plan. Knowing a weed’s name gives you crucial information about its preferred growing conditions and life cycle. This knowledge allows you to develop planting strategies, modify growing conditions, and plan management tactics to take advantage of vulnerable stages in the weed’s development, thus gaining maximum control with the lowest possible input of time, labor, and money.

Organic weed management and herbicides

A wide array of tactics are used to achieve maximum weed suppression, including seed bed preparation and maintenance, crop competition and rotation, cover crops and intercropping, grazing by livestock, cultivation and irrigation techniques, and soil management strategies. A large amount of weed management literature is available in print and online. A complete review is outside the scope of this article; however, links to some resources are included at the end of this module.

A key goal in weed management is preventing reproduction. Every mature plant that is allowed to go to seed can deposit hundreds or even thousands of seeds in the soil.

There are few herbicides approved for organic use; the ones that are currently available tend to be expensive and have limited efficacy. For these reasons, herbicides do not play a large role in organic weed management. However, some products are approved for use in organic systems. These include materials such as acetic acid, plant-derived oils, and soaps (e.g. potassium laurate and sodium lauryl sulfate). These are nonselective contact herbicides, most useful for control of very young seedlings or as spot treatments. Corn gluten meal, a soil-applied material which can inhibit root growth of germinating seeds, has shown highly variable efficacy against weeds depending on root system maturity, soil moisture, and microbial activity. None of these organic herbicides provide control of established perennial weeds.

While the few weed control materials approved for organic production are typically considered less hazardous than conventional herbicides, there are still some risks associated with their use. Always read and follow label instructions, take care to minimize exposure and contact, and use appropriate personal protective equipment when using any pesticide.

Summary of best practices for non-chemical weed management:

  • Use seed bank depletion strategies (such as stale seedbeds) to reduce the number of viable seeds in the soil prior to planting a site.
  • Rotate crops on a given site. Crop rotation allows you to change planting times and crop growth habits, thus management timing for problem weeds can be adjusted.
  • Minimize bare ground by using cover crops and intercropping. Some cover crops also have all elopathic effects that inhibit weed seed germination.
  • Plant healthy transplants or vigorously growing varieties to help establish a dense, early canopy of crop plants. Adjust spacing of crops (both in rows and between rows) to minimize open ground for weed establishment.
  • Tillage and cultivation are excellent weed management strategies for some weed species, but soil structure may be damaged by repeated use and erosion must be prevented on sloped land. Low-till and no-till techniques can be effective, but may actually increase occurrence of some pest species, such as cutworms and wireworms.
  • Thermal weed management using flaming or steam can be effective, especially on young broadleaf weed seedlings. Opportunities for thermal weed control may be somewhat limited as crop plants can also be damaged.
  • In some regions, solarization is effective for killing weed seeds and seedlings in the upper layer of the soil. Keep in mind that this technique also impacts other soil flora and fauna, including beneficial microorganisms.
  • Mulches are effective in preventing weed seed germination. Inorganic (plastic) mulches must be removed at the end of the growing season.
  • Maintain crop health with proper water and fertility management. Both can be targeted specifically toward crop plants, leaving non-crop areas less favorable for weed growth.

 

Initial compilation courtesy of Lenora Jones

Washington State University Extension