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density from 3 to 12 shoots/ft2. Average number of seedheads~shoot varied from 8 to 36, average total seed shoot from 9 to 340 and average total viable seed from 6 to 137/shoot. This study is ongoing and is matched with a nursery study where each of the eight accessions were transplanted and the same data are being collected, however, 1995 was the first year of the nursery study and data are not yet available. The nursery study will help to determine if the variation observed in the field is due to genotype, environmental conditions, or a combination of both. It is apparent that Russian knapweed growth potential varies with location in the state and from a management standpoint, more vigorously growing Russian knapweed may be more difficult to control and/or better equipped to recover from management stresses. The allelopathic potential of Russian knapweed makes it more difficult to manage than some other noxious weeds. It tends to form monocultures in some locations and may be present in a mixed stand in other locations. The mechanism of this is poorly understood but maybe related to allelopathy. A simple experiment was conducted at Colorado State University where a single accession of Russian knapweed was grown in root boxes filled with three different soils; a clay soil, an organic soil, and a sandy soil. Visual observations indicate a difference in root growth among the three soils where Russian knapweed roots branched to form secondary and tertiary roots at much shallower depths in the clay soil compared to the others. Research conducted by USDA-ARS at Albany, CA found that Russian knapweed exudes polyacetylenes from its roots and these are known to inhibit the root grovnh of certain bioassay species by 50% at a concentration of ] 2 ppm. The concentration of polyacetylenes under field conditions v.~as found to vary from 4 to 5 ppm over the growing season and at this concentration, it was calculated that root growth of sensitive plant species would be inhibited by 30%. We hypothesize that the more branched root system observed when Russian knapweed was gown in a clay soil may allow a greater soil volume influenced by polyacetylenes and this may explain our observations that monocultures tend to be found in locations where clay content is higher than where mixed stands occur. However, this still is speculative and research is in progress to test this hypothesis. Regardless, from a management perspective u,~e know through preliminary research conducted by the University of Wyoming that to successfully establish perennial grasses during revegetation of a Russian knapweed infested site, the soil must be tilled to destroy the alielopathic influence. Exactly what is occurring biochemically is unknown. A cooperative project bet.veen Colorado State University and the University of Wyoming was initiated in 1993 to test the combined effects of various suppression treatments combined with reseeding of perennial grasses on Russian knapweed control and site revegetation. The experiment was conducted a[ two ranseland sites;l~lead, CO and Boysen Reservoir, WY. Our hypothesis vas that the combination of suppression treatments and seeding v,~ith perennial grasses would control Russian knapweed more thoroughly and more successfully revegetate the site than suppression treatments or reseeding alone. -5-