Water infiltration rates were less on spotted knapweed sites than on bunchgrass sites. Biology and Ecology Spotted knapweed is a perennial that lives up to nine years and is capable of producing seeds each year. Seed production of spotted knapweed ranges from 5,000 to 40,000 seeds per square meter per year. Site conditions and precipitation during the growing season have the greatest effect on the number of seeds produced per year. More seeds are produced during wet years.
Seeds germinate in the fall and early spring when moisture and temperature are suitable.
Fall and early spring germinating seedlings are capable of maturing into seed-producing adults in one year. If rosettes do not bolt, they die back to the root crown to overwinter. Root crowns form rosettes in the early spring and bolt in early June and flowering occurs from July through September. Mature seeds are formed by mid-August. Bracts of the flowerheads open when dehydrated, two to three weeks after maturity, and wind or passing animals can flick the loosely held seeds up to one yard from the parent plant.