CEREAL PESTS WEEDS 451 



Fall plowing at once after harvest will kill many of 

 the plants before they mature seed, but further cultiva- 

 tion, afterward, before winter, may be necessary. The 

 grain crop should be followed by corn or some other inter- 

 tilled crop. 



In the western Great Plains this sunflower is replaced 

 by another ( H. petiolarus) ; while in Manitoba the 

 perennial species (H. scaberrimus, Ell., H. Nuttallii, Torr. 

 and Gray, and H. Maximiliamis, Schroed.) are common in 

 grain fields, though said to be not very serious weeds, in 

 the Prairie Provinces. 



491. Common vetch (Vicia sativa, Linn.) occurs fre- 

 quently in grain fields of the Northwest, and the seeds are 

 often found in wheat screenings. It is glabrous or slightly 

 pubescent, 1 to 2| feet high, with a simple stem ; leaflets 

 5-7 pairs, obovate-oblong to linear, notched or mucro- 

 nate at the tip ; the 1 or 2 nearly sessile flowers borne in 

 the axils of the leaves, corolla violet purple ; pod linear, 

 several seeded, seeds black (Fig. 142 /). 



Clean seed sown in clean soil is the means of extermina- 

 tion. Crop rotation, as always, should be practiced. 



Clark (1911, p. 52) mentions another species (V. 

 angustifolia, Reichardt), the seeds of which " are a com- 

 mon impurity in grain grown in the Maritime Provinces, 

 Quebec, and parts of Ontario," and that the seeds " are 

 especially objectionable in oats required for milling." 



492. The Russian thistle (Salsola pestifer, A. Nelson). 

 - The bulk of the wheat crop of this country and Can- 

 ada, and much of the production of other small cereals, 

 are derived from seed originating in the plains of Russia. 

 It is natural, therefore, that one of our worst weeds in 

 grain fields, the Russian thistle, should also have come, as 

 it did, from the same plains, and should have first spread 



