Spotted knapweed germination response to stratification, temperature, and water stress

Spotted knapweed (Centaurea maculosa Lam.) is an introduced noxious weed on grasslands in the northwestern United States and adjoining Canadian provinces. This research examined germination responses of spotted knapweed to stratification, temperature, and water stress under controlled conditions. Immediately after harvest, germination was sharply reduced by sub- and supra-optimal temperatures and water stress; as seeds aged, germination increased at sub- and supra-optimal temperatures. Total germination was enhanced and days to 50% of final germination (D50) was reduced by cool–moist stratification. Under water stress, germination of unstratified seeds was limited at suboptimal temperatures, but seeds pretreated with cool–moist stratification germinated more rapidly and to a higher percentage over a broader range of temperatures and osmotic potentials.