The effect of shade on the growth of variegated thistle ( Silybum marianum L.) and cotton thistle ( Onopordum sp.) *

Summary The effects of shading on growth of the annual, variegated thistle ( Silybum marianum L.) and the biennial, cotton thistle ( Onopordum sp.*) were studied in an unheated glasshouse during winter. The response to relative level of irradiance (r.l.i.) of both thistles was similar, seedling growth being systematically reduced with increased shading. Relative growth rates of seedlings of both thistles were virtually identical at all levels of irradiance up to 0.68 r.l.i. Light compensation points for growth were close to 0.024 r.l.i., equivalent to c. 2 W m −2 photosynthelically active radiation under the winter conditions. Increased shading progressively delayed time of initiation of leaves and decreased their total number at harvest. Apart from reduction in plant size, increased shading had only minor effects on gross seedling morphology until irradiance was reduced to less than 0.20 r.l.i. Although the thistle seedlings are quite shade tolerant, it is suggested that heavy shading by persistent pasture species in cool seasons could contribute to the control of thistle infestations in subhumid tableland environments of southeastern Australia.