Weed Management in Organic Seed Production

eOrganic authors:

Micaela Colley, Organic Seed Alliance

Alex Stone, Oregon State University

Linda Brewer, Oregon State University

This is an Organic Seed Resource Guide article.

Cultivated carrot seed field

Cultivated carrot seed field. Photo Credit: Micaela Colley, Organic Seed Alliance


Weed management in seed production is important for two primary reasons:

  • Weed competition can result in reduced seed yields and seed quality, and in some cases increased disease risk
  • Weed seeds can contaminate harvested crop seeds

Weed management in vegetable seed crops can be more difficult than in vegetable crops because:

  • The crop is in the field longer, exposing the field to more wind-disseminated weed seeds, more weed seed germination windows, and more time for weeds to mature and set seed
  • Some seed crops remain for long periods at a height at which they can't be easily mechanically cultivated

Management of Weeds during Production of your Organic Seed Crop

Organic seed growers should develop a multiple-tactic systems approach to weed management that results in fewer weed seeds in the soil and fewer opportunities for their germination, growth, and seed production.

Below are listed the steps involved in a systems approach to weed control; this list is taken from the eOrganic article Twelve Steps Toward Ecological Weed Management in Organic Vegetables, authored by Mark Schonbeck.  For comprehensive information on how to design a systems weed management plan for your farm, read the more in-depth articles on each weed management step that are linked from the title of the step.

Planning for Ecological Weed Management in Organic Vegetables

Cultural Practices for Preventing Weed Problems

Weed Control Practices

Management of Weed Seeds in your Harvested Seed Lot

Weed seed contamination in harvested seed is a problem that affects seed quality and marketabilty.  The first steps in avoiding weed seed contamination are 1) managing weeds in the field, and 2) avoiding weed seed contamination during the harvesting process.  Once harvested, crop seed is contaminated by weed seed, which must be removed in the seed cleaning process.  Weed seed mixed in with the harvested seed is difficult to remove, particularly if the weed seed is of a similar size, weight, shape or color as the crop seed.

If the seed is even a slightly different size or weight, it may be removed with standard seed cleaning practices such as screening or fanning. Some similar-sized crop seed may be lost in the process. The related eOrganic publication Organic Seed Processing: Threshing, Cleaning, and Storage provides information on seed cleaning practices such as screening and winnowing useful in removing weed seed. 

Seed of some weed species is regulated by state governments and classified as noxious or prohibited. Seed for sale in quantities over one pound must be tested for purity and labeled accordingly. Seed sold in quantities lower than one pound must follow the same regulations, but labeling is not required. Purity tests measure the amount of inert material in the seed crop, the level of noxious weed seeds, and the presence of prohibited weed seed. Seed lots that contain a restricted weed seed count above a specified count per volume cannot be legally sold; prohibited weed seeds have a zero tolerance.  Find and read your state's seed regulations, as these regulations vary by species and by state. In organic production, weed seed contamination can be even more detrimental as organic producers have fewer weed control options than conventional growers.

The eOrganic resources linked below are useful for the identification and management of weed seeds in seed crops and for general organic weed managment practices.

Weed Seed Contamination Concerns and Management
Situation Management Recommendations Consequences of weed seed contamination
Noxious or prohibited weed seed. Similar size and/or weight as crop seed. Focus efforts on weed management and contamination prevention. Unmarketable seed or significant yield loss in cleaning process.
Not noxious or prohibited weed seed. Similar size and/or weight as crop seed. Focus efforts on weed management and contamination prevention. Marketable, but lower quality seed crop if contaminated with weed seed.
Noxious or prohibited weed seed. Distinctly different size and/or weight from crop seed. Manage weeds and prevent contamination, but if contaminated, remove in cleaning process. Extra work to remove weed seed in cleaning process. Must take extra care to ensure 100% of prohibited seed removed and noxious weed seed below allowable level.
Not noxious or prohibited weed seed. Similar size and/or weight as crop seed. Manage weeds and prevent contamination, but if contaminated, remove in cleaning process. Extra work to remove weed seed in cleaning process, but not as critical as if noxious or prohibited.

Seed-Specific Resources

Noxious Weeds

  • State noxious-weed seed requirements recognized in the administration of the Federal Seed Act [Online]. United States Department of Agriculture. Agricultural Marketing Service. Seed Regulatory and Testing Service. Available at: http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELPRD3317318 (verified 11 Mar 2010).
    This publication contains information about the various state labeling requirements and prohibitions of noxious-weed seeds and shows the botanical names and common names according to the law and regulations of the particular state in which the seed is noxious.  It is the responsibility of seed dealers to know the requirements of state laws and the changes in those laws.

Weed Identification Resources

General Weed Management Resources

Web resources

Weed fact sheets

A series of fact sheets highlighting more than 60 specific weeds and their characteristics. These publications cover basic plant biology and offer both cultural and chemical control recommendations. See the “Weeds” section of the OSU Extension Publications and Videos catalog at https://extension.oregonstate.edu/topic/pests-weeds-diseases/weeds/resou... (verified 8 Mar 2024).

Print resources

  • Agricultural weed management principles. C. Boerboom et al. 1993. WAEM 0167. Washington State University Cooperative Extension. 48 pp. Order from 509-335-2857 or 800-723-1763 or https://pubs.extension.wsu.edu/agricultural-weed-management-principles (verified 8 March 2024).
  • Grower’s weed identification handbook. W. Fischer. 1998. University of California-Davis Cooperative Extension. Out of print.
  • Managing cover crops profitably. A. Clark (ed). 2007. 3rd Edition. Sustainable Agriculture Network (now SARE Outreach). Available at: http://sare.org/publications/covercrops.htm (verified 11 Mar 2010).
  • Steel in the field: a farmer's guide to weed management tools. Sustainable Agriculture Network (now SARE Outreach). Available at: http://sare.org/publications/weeds.htm (verified 11 Mar 2010).
  • Weed science: Principles and practices, 4th ed. T.J. Monaco, S.C. Weller, and F.M. Ashton. 2002. Wiley, New York, NYp
  • Weeds of the west. T.D. Whitson et al. 2000. Western Society of Weed Science. Available at: https://wyoextension.org/publications/Search_Details.php?pubid=696 (verified 8 March 2024).


Published January 23, 2009

This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.