Abstract : The successful spread of the weeds Acroptilon repens (Russian knapweed, RK) and Centaurea maculosa (Spotted knapweed, SK) has been attributed to the exudation of allelopathic chemicals by their roots. RK releases 7,8-benzoflavone and SK releases (+/-)- catechin. Understanding the conditions resulting in optimum sorption of these allelochemicals could provide a means to arrest the spread of the weeds that exude these allelochemicals. Our objective was to determine which soil constituents and conditions provide maximum sorption of the allelochemicals (+/-)- catechin and 7,8-benzoflavone and determine if the optimum soil sorption conditions diminish the phytotoxicity of these allelochemicals. We found that 1) sorption of (+/-)- catechin and 7,8-benzoflavone onto soil with or without carbon was 100%. This rendered (+/-)- catechin and 7,8-benzoflavone unavailable, and 2) carbon significantly inhibited the growth of RK while promoting the growth of bluebunch wheatgrass (BBWG). Thus, application of carbon to RK infested sites may limit its spread on military bases.