Abstract
Recently weed scientist are more interested in weed management by allelopathy . The main objective of the study was to evaluate allelopathic potential of sunflower on redroot pigweed and common lambsquarter. this study was conducted in two series of laboratory and greenhouse experiments. Under laboratory conditions, leaf, stem and root aqueous extracts of sunflower at 0%, 2.5%, 5%, 7.5 % and 10% (m/v) concentrations were applied to determine their effect on redroot pigweed and common lambsquarter seed germination and initial growth under laboratory conditions. The results indicated that germination and mean daily germination was reduced by this extracts by 43% and 50% respectly. The effects of leaf and stem extracts on germination were more than root extracts of sunflower. The percentage and quality of germination was decreased by increasing the concentration of extracts. The root length and shoot length were reduced by 80% following application of sunflower extracts. In greenhouse experiments, to study the role of decomposition plant debris on release of allelochemicals, stem, root and leaf residues of sunflower were incorporated with pot soil by 5% (w/w) and weed seeds were sown and samples were taken in five stages of growth(2, 4, 6, 8 leaf and mature plant). Dry weight, leaf area and height of weeds were determined. The results of greenhouse experiments indicated that incorporation of sunflower residues in the soil (fresh or decomposed residues) reduced growth of redroot pigweed and common lambsquarter. The difference between fresh and decomposed residues was not significant. This study indicated that, fresh stem residues of sunflower are able to apply, its harmful effects on dry weight, leaf area and height of both weeds species.

Key words: Allelppathy, Aqueous extract. Sunflower, Redroot pigweed, Common lambsquarter.