How can I reduce the weed seed bank on my farm?

Managing croplands according to nature’s principles will reduce weed problems on horticultural crops, or row crops, in small and medium-sized operations. The following three proactive strategies can be effective in reducing the weed seed bank.

Frequent mowing of farm margins and any areas of the farm not in cultivation can prevent weeds from producing seeds. This helps prevent the addition of new seeds blowing or being carried in to the seed bank. In combination with other practices, mowing can be effective in significantly reducing the seed bank over time.

Soil Solarization
Soil solarization heats the top layer of soil to temperatures high enough to kill many weed seeds, as well as soil pathogens. In this process, moist soil is covered by clear-plastic film for a period of several weeks during summer. A two- to four-week period during which the soil temperature is at 99°F is effective in killing the seeds of many types of annual weeds. However, heating the soil to this degree affects many other soil factors, including microbial life and nutrient availability, so further research should be conducted before implementing this process and planting into solarized soils.

Stale Seedbed Preparation
In this management strategy, after seedbed tillage is completed, weed seeds, mostly in the upper two inches of the soil, are allowed to sprout. Assuming adequate moisture and a minimum soil temperature of 50ºF (to a depth of two inches), this should occur within two weeks. A fi ne to slightly compacted seedbed will germinate a much larger number of weeds. The weeds are then “seared” with a flamer, or burned down with a broad-spectrum herbicide, preferably when the population is between the first and fifth true-leaf stages, a time when they are most susceptible. The crop should then be seeded as soon as possible, and with minimal soil disturbance to avoid bringing new seed to the surface. For the same reason, subsequent cultivations should be shallow (less than two inches deep).

The ATTRA publication Sustainable Weed Management for Small and Medium-Scale Farms goes into much more detail on weed-management strategies, both active and reactive, including mulching, competition, crop rotations, and low-toxicity control alternatives.

Be sure to also check out the ATTRA tipsheet Soil Solarization and Biosolarization, which offers step-by-step instructions and discusses advantages and drawbacks.

Finally, tune into ATTRA podcast Episode 174. Solarization, Biosolarization, and the Pandemic to hear a conversation between NCAT’s Martin Guerena and Alex Ellison of Castlerock Farm in Walnut Creek, California. The conversation includes a description of the Castlerock Farm operation, as well as the initial results and challenges of Castlerock’s trials in solarization and biosolarization.