The aim of this study was to investigate the effect of controlled soil water deficits on sprouting and shoot growth of Canada thistle, coltsfoots and quackgrass. A gradient of soil water contents was created by establishing different densities of barley. The plants were harvested 14 days after watering was stopped. On Canada thistle and coltsfoots, relative water content (RWC) in leaves was measured prior to harvest and biomass of all weed shoots were recorded at harvest. In terms of shoot biomass and leaf RWC quackgrass was drought tolerant while coltsfoot was drought sensitive and Canada thistle was between the two. The barley cover crop could have had a competitive effect upon the growth of the weeds; the effect, however, was not detrimental compared to the drought effect, because relationships between initial height and the final height of coltsfoot and Canada thistle were not different among barley densities. The results suggest that the shooting from subterranean parts of broadleaf perennial weeds can to some extent be impeded by reducing soil water availability. However, the use of reduced soil water content can be challenging in fields in humid temperate regions
