eOrganic author: Mark Schonbeck, Virginia Association for Biological Farming

Introduction

A growing cover crop can suppress weeds in several ways:

  • Direct competition
  • Allelopathy—the release of plant growth–inhibiting substances
  • Blocking stimuli for weed seed germination
  • Altering soil microbial communities to put certain weeds at a disadvantage

After a cover crop is tilled in, mowed, rolled, or otherwise terminated, its residues can prolong weed suppression by:

  • Physically hindering seedling emergence (if residues are left on the surface as mulch)
  • Releasing allelopathic substances during decomposition
  • Promoting fungi that are pathogenic to weed seedlings
  • Tying up nitrogen (N) (when low-N residues are incorporated into soil)

Competition

A vigorous, fast-growing cover crop competes strongly with weeds for space, light, nutrients, and moisture, and can thereby reduce weed growth by 80–100% for the duration of the cover crop’s life cycle. Timely cover crop plantings occupy the empty niches that occur in vegetable production systems:

  • After vegetable harvest
  • Over winter
  • Before planting a late-spring or summer vegetable
  • Between wide-spaced rows of an established crop

Buckwheat (Fig. 1, left), soybean, and cowpea planted in warm soil can cover the ground within two or three weeks. This “canopy closure” puts tiny, emerging weeds in the shade and hinders their growth. Summer or winter annual grasses like sorghum–sudangrass, various millets (Fig. 1, right), oats, rye, and wheat form dense, fibrous root systems that appropriate soil moisture and nutrients, leaving less for the weeds. Combining a grass with a legume or other broadleaf crop is often more effective than growing either alone (Fig. 2).

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