Italian thistle is an aggressive annual often confused with plumeless thistle (Carduus acanthoides) and slender flowered thistle (Carduus tenuiflorus). C. acanthoides, however, is a biennial, and C. tenuiflorus usually has a greater number of heads per stem. The large photo on page 268 of the California Growers Guide is misidentified; it is actually Canada thistle. The insert in the lower right is correct. See Weeds of the West page 78.