Sunflowers (Helianthus annuus) are Allelopathic to Weeds

Laboratory, greenhouse, and field studies were conducted to determine the allelopathic potential of sunflower (Helianthus annuusL.) cultivars to suppress weed germination and growth. Germination of wild mustard [Brassica kaber(DC.) L.C. Wheeler var.pinnatifida(Stokes) L.C. Wheeler] seeds at 25 C in undiluted aqueous extracts of sunflower leaf tissue was inhibited 75%, but was stimulated by up to 150% at 10- and 100-fold dilutions. Stem-tissue extracts at all concentrations stimulated wild mustard seed germination. The germination response of other weed species varied with the sunflower cultivar and concentration of tissue extract. In sand culture, leachates of dried sunflower leaf and stem tissue inhibited broadleaf - weed seedling growth, but had little or no effect on the growth of grass weeds. Sunflower root exudates inhibited seedling growth, but were less effective than leaf and stem tissue leachates. Germination of weed seeds was unaffected by root exudates. Over a 5 -yr period, weed density and percent ground cover increased less in field plots of sunflower than in control plots.