Optimum germination of Canada thistle [Cirsium arvense (L.) Scop.] occurred at alternating temperatures of 20 to 30 C and 30 to 40 C and at a constant temperature of 30 C. Osmotic pressures of 7 bars and greater reduced germination, but 2% of the seeds germinated at 15 bars. Canada thistle seeds (14%) were able to germinate in NaCl concentrations of 20,000 ppmw. Light was important in seed germination, however, its effect could be overcome by adding GA3 (gibberellic acid) to the germination medium. Optimum pH for germination was between 5.8 and 7.0. Depth of planting influenced emergence; the highest percentage of seedlings emerged from depths of 0.5 to 1.5 cm. Canada thistle seedlings tolerated average soil moisture tensions of 1.3 bars and continued to grow. Canada thistle seedlings 19 days old and with two true leaves were able to resprout after top-growth removal. Regrowth was evident on 8% of the clipped plants 4 days after initial top-growth removal.
