Spotted knapweed is an important weed of rangeland in the northwestern United States. A study was conducted near Corvallis, MT, during 1992 to 1994 in order to assess the relationship among the growth attributes of spotted knapweed to identify a minimum set of measurable plant characteristics that are representative of spotted knapweed vigor. Spotted knapweed growth attributes that were examined included plant age, root diameter, plant height, number of stems per plant, aboveground biomass, number of capitula (seed heads) per plant, and number of capitula per stem. Spotted knapweed age was positively correlated with root diameter, number of stems per plant, aboveground biomass, and proportion of bolted plants. Most spotted knapweed plants did not bolt until the third or fourth year. Although plant age is not measured easily in the field, it may be useful as a covariate in an analysis of experiments involving plant competition or nonlethal biological control agents. Root diameter can be used as a nondestructive measure of approximate plant age, especially for the first 5 yr of growth. Root diameter was also highly correlated with many growth measurements, including number of capitula per plant and aboveground biomass, which are most relevant to assessing overall plant vigor. Plant height was positively correlated with aboveground biomass, number of capitula per plant, and mean number of capitula per stem. Number of stems per plant was positively correlated with plant height, aboveground biomass, and number of capitula per plant. Aboveground biomass was positively correlated to number of capitula per plant and mean number of capitula per stem. Measurements of root diameter, plant height, and number of stems are easy to perform and should provide a good indication of plant vigor.
Nomenclature: Spotted knapweed, Centaurea maculosa Lam. #3 CENMA.
Additional index words: ANCOVA, analysis of covariance; growth–attribute relationships.