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• • Russian Knapweed Biology & Management Through Integrated Methods K. George Beck Associate Professor of Weed Science Colorado State University Russian knapweed (Acroptilort repens) is a creeping, herbaceous perennial member of the sunflower family. It is characterized by vertical and horizontal roots that have a brown to black scaly appearance around the crown that may extend down onto the horizontal components of the root system. Roots may grow 20 feet deep, but more commonly glow at least ]0 feet deep. The deep root s}~stem allows Russian knapweed to grow in xeric environments because it can extract available soil moisture from deeper depths than most rangeland grasses. The deep root system also allows Russian knapweed to recover from control endeavors because new shoots may develop at any point along the root system. Research at Colorado State University characterized the root bud development over time. Russian knapweed was sampled at weekly intervals throughout the year from two Front Range locations. Twenty samples were collected each week and segmented into 10 cm sections. Buds were counted along each section and their lengths measured. In late August to early September, root buds begin to develop and reach a maximum number within 2 to 3 weeks. Bud numbers fluctuate v.~ith soil temperature; for example, when soil temperatures fall below 0 C, buds are lost, but replaced immediately with new buds. This was determined by measuring bud lengths at each collection. When bud numbers decreased because of frozen soil, bud numbers decreased but rapidly increased to the average level and they were shorter than measurements taken the previous week. Buds began to elongate in mid-7,4arch and shoots emerge in early April at which time bud numbers began to decrease to zero by May. No buds were found along the roots or crowns from May until late August. Typically, shoots emerge in April and grow as a rosette for approximately 1 month then shoot elongation begins. The first flower buds usually develop in late May to early July and the plant flowers throughout the summer. Seeds that develop in flower heads are not dispersed from the plant by wind but maybe dispersed by small rodents that eat the seeds. Where rodents or other animals do not disperse the seeds, Russian knapweed seedheads from the previous 1 to 3 years can be found on the soil surface. Russian knapweed seedlings (those derived from seeds) have not been found in any ofthe field plots conducted by Colorado State University. It is probable that seeds simply serve to begin new Russian knapweed infestations some distance from the parent infestation and the encroachment of an existing infestation is primarily or exclusively from vegetative propagation from the root system. The variation in Russian knapweed grot~nh and seed production was evaluated at eight Colorado locations. Site conditions were rangeland or roadsides; elevation varied from 4900 to 7600 feet; and precipitation among sites varied from 7 to 18 inches annually. Russian knapweed average shoot height varied from 45 to 70 cm, average shoot weight from 3.6 to 12.6 g/shoot, average canopy diameter from 12.6 to 29 cm, and average shoot -4- C 1