Analyzed the relationship between shoot vegetative (v) and reproductive (r) mass in five experiments on Solidago altissima from an invading population in Switzerland. There was large environmentally-induced and genetic variation in r and v. A large amount of variation in r could be explained by variation in v, using the simple linear model. There was a minimum size for sexual reproduction, and above this size, shoots devoted a relatively constant proportion (c 1/3 of their biomass to reproductive structures. There was significant genetic variation for both the minimum size and the slope of the r-v relationship, but there was no evidence for an hypothesized trade-offs between minimum size and slope. There was significant genotype-environment interaction for the slope of the r-v relationship. -from Authors