Cover Crops for Weed Management in Organic and Transition Systems

Cover crops are a locally grown source of organic matter and nitrogen and a weed management tool. Despite these benefits, many organic farmers have not been able to work cover crops into their management systems.

We are evaluating different cover cropping systems, to help organic farmers determine how to best use cover crops on their farm. Relay planting is a system where farmers plant cover crops into standing cash crops, allowing early establishment of the cover crop. We have compared several types of legume cover crops relay-planted into corn (a tall crop) and beans (a short but leafy crop) to see if the cover crops can survive in the shade of the cash crops, produce good ground cover in the winter, and supply nitrogen-rich organic matter in the spring. Hairy vetch and red clover are promising relay cover crops, and we are now gathering long term information on how they affect weed and nutrient management.

We are comparing different blends of rye (nitrogen poor) and hairy vetch (nitrogen rich) as fall planted cover crops to determine how the blends affect weed management and nutrient availability for the next crop. Hairy vetch-rye blends have not effected weed pressure compared with pure rye, while they have provided substantial plant-available nitrogen. Planting date had a large effect on cover crop biomass, while harvest date had a large affect on biomass and quality. Based on two years’ data, blends ranging from 50 to 75% vetch in the seed mix and planted by early October will yield significant biomass and nitrogen benefits to farmers.

Finally, we are assessing cover crops as a tool in low-input transition to organic production. Sudangrass was the most promising low-input summer cover, but our low-input system was not satisfactory for organic transition.

Grant Information

  • Project ID: 011
  • Project Status: Complete


  • Principal Investigator(s): Bary, A., Cogger, C.
  • Grant Amount: $14,319


  • Principal Investigator(s): Bary, A., Cogger, C.
  • Grant Amount: $14,536



  • Principal Investigator(s): Cogger, C.
  • Grant Amount: $9,949


Lawson, A. Cover crops to enhance soil productivity in organic vegetable cropping systems. MS Thesis. Dept. of Crop and Soil Sciences. 2010.

Lawson, A., A. Fortuna, C. Cogger, A. Bary, and T. Stubs. Nitrogen contribution of rye-hairy vetch cover crop blends to organically grown sweet corn. IN INTERNAL REVIEW for submission to Agronomy Journal, April 2011.

Lawson, A., C. Cogger, A. Fortuna, A. Bary, and T. Stubs. Evaluating fallcover crop blends for biomass production, residue quality, and weed suppression. IN PREPARATION for submission to J Sustainable Ag.

Website: Cover crops.

Websites from 2010 winter cover crops school. (These will be organized in to a single site, but are currently separate).


Additional Funds Leveraged

Cogger, C.G., A. Fortuna, A. Kennedy, A. Bary, K. Painter, and B. Cha. Cover crops to enhance soil productivity and nitrogen management in organic and transition vegetable production systems. WSU Emerging Research Issues. $74,528. 2007-2009. This funded a continuation of the CSANR project.

Cogger, C.G., M. Ostrom, R. Alldredge, A. Fortuna, A. Kennedy, and K. Painter. Designing production strategies for stewardship and profits on fresh-market organic farms. USDA Integrated Organic Agriculture program. $644,232. 2008-2012. CSANR grant was one of several that positioned us to receive this larger grant.

Collins, D. A, Corbin, C. Cogger, C. Benedict, and A. Bary. Selecting management practices and cover crops for reducing tillage, enhancing soil quality, and managing weeds in western Washington organic vegetable farms. $196,624. 2011-2014. CSANR grant laid the groundwork for our move into cover crops reduced tillage systems, and this is the first major grant we had funded for that work.

Burrows, C. D. Collins, A. Corbin. No-till organic vegetable production in western Washington: A Planning Proposal. USDA/NIFA Organic Research and Extension Initiative, $46,794.


We developed recommendations for seeding ratios (1:1 cereal rye: hairy vetch) and planting (mid-September) and termination (late April) dates to achieve weed management and nitrogen fertilization benefits from fall-planted cover crops under western Washington conditions. We estimated N replacement value at 50 lb N/acre under recommended management, which is less than achieved in parts of the country with an earlier onset of warm spring weather. This project allowed us to lay the groundwork to move into new research on reduced tillage cover crop systems.

Evaluation data collected from extension classes showed increased knowledge and increased interest in the use of cover crops. Specifically, cover crop workshops held in 2010 showed that one workshop was effective at increasing knowledge (>95%), and that the use of online education can be effective for both farmers (ave: 3.59 out of 5) and non-farmers (ave: 4.25 out of 5). In an evaluation of a second workshop 90% of participants indicated that they greatly increased their knowledge of summer cover crops during the workshop. Ninety-two percent indicated that they somewhat or greatly increased their skills and ability to incorporate summer cover crops. The vast majority (93%) indicated that they were very likely to use or encourage the use of cover crops as a result of the workshop.