Effect of Cropping Systems On Weed Population and Biomass

A field experiment conducted at the South Coast Research and Extension Center in Irvine, California measured weed populations and their biomass in organic broccoli planted after two summer cover crops, cowpea (Vigna unguiculata) and marigold (Tagetes patula). The population density of the dominant weed species,  Portulaca oleracea (common purslane), peaked at 370 plants per m2 just before the first hand weeding in broccoli planted after summer fallow. The fallow treatment had 5x as many purslane as when broccoli followed cowpea and 11x more than the marigold treatment. Weed density wasn't significantly different between the treatments after the first weeding.  However, by harvest, the population of all weed species was 4x higher in broccoli planted on previously fallow plots relative to those in the cowpea and marigold plots. During the same trial year, common purslane had the highest dry biomass for broccoli planted after summer fallow. 

Weed population density prior to first hand weeding followed a similar trend in 2008. Prior to hand weeding, there were  4x as many purslane plants in broccoli planted after the fallow treatment than for  broccoli following either cowpea or marigold. Differences in weed population at broccoli harvest for the 2008 trial were also significant for the cropping systems, with 7x and 11x more common purslane when broccoli followed summer fallow than when broccoli was planted after cowpea or marigold, respectively.  Biomass of all weeds for the 2008 trial was greater in broccoli planted after summer fallow than those that followed summer cowpea or marigold plots. 

Broccoli height and canopy spread were greater following either cowpea, a nitrogen fixing legume, or marigold. Broccoli following cowpea or marigold had higher number and fresh weight of broccoli heads when compared to those grown after summer fallow. Similar trends were observed with the fresh weights of marketable broccoli heads. Broccoli plants grown after summer cover crops matured faster than those that were planted following a summer fallow. The lower densities and biomass of weeds in broccoli plants and the better vegetable growth and marketable yields following summer cover cropping are good indications that summer cover cropping reduces the intensity of weed populations and their competitive ability with subsequent vegetable crop production.

Key words: cover crops, weed population, weed biomass, organic farming