CEREAL PESTS WEEDS 455 



The bull thistle grows 3 to 4 feet high, is branching, tomentose, 

 becoming dark green and villous or hirsute with age, branchlets 

 bearing large heads ; leaves lanceolate, decurrent on the stems with 

 prickly wings, deeply pinnatifid, the lobes having rigid prickly points, 

 upper surface roughened with short hairs, lower surface with a cot- 

 tony tomentum ; heads if to 2 inches high, involucre bracts lanceo- 

 late, rigid when young, flexible with age, with long alternate prickly 

 pointed spreading tips; flowers hermaphrodite; achenes bare, \\ 

 inches long, pappus of numerous plumose bristles (Fig. 141 j). 



In eradication, the important thing is to prevent seed 

 from forming. Only a flattened mass of leaves is produced 

 the first year. All plants that have escaped destruction 

 that year should be cut off early the following spring. 

 The flowers begin to appear early in August. In a good 

 rotation the thistle cannot survive. Summer fallow should 

 be cultivated until late in the fall. 



A more common species in parts of the Great Plains 

 and in Montana than the one here described is the field 

 thistle (Cirsium undulatum, Spreng). In the western 

 Great Plains still a third species (Cirsium ochrocentrum, 

 Gray) is the most frequent. 



499. Green tansy mustard (Sisymbrium incisum var.. 

 filipes, Gray). This mustard is native, and occurs com- 

 monly in grain crops in the Prairie Provinces of Canada 

 and in British Columbia. 



The first season this weed appears as a flat rosette of finely di- 

 vided leaves. The second year it produces stems 3 to 4 feet high, 

 erect, widely branching at the top, and bearing an enormous num- 

 ber of narrow, bare, slightly curved pods from i to f inch long, on 

 slender spreading pedicles ; whole plant bright green and somewhat 

 glandular; leaves twice pinnatifid; flowers yellow, f inch across 

 in an elongated raceme ; seeds ^5 inch long, oblong, sometimes com- 

 pressed at the scar end, reddish brown, minutely roughened with 

 mucilaginous hairs (Fig. 142j). 



