Seed germination of Italian thistle (Carduus pycnocephalus L.) and slenderflower thistle (Carduus tenuiflorus Curt.) was rapid and high, with no strong requirements for afterripening. Both species germinated at a wide range of alternating temperatures from −2 to 40 C. Their germination-temperature profiles were similar, with Italian thistle germinating over a slightly wider range of temperatures. The achenes of both species have abundant mucilage. This mucilage, by improving water retention, tended to increase germination on the surface of clay soil.
