456 THE SMALL GRAINS 



The seeds occur in poorly cleaned western grain. It is 

 stated that the prevalence of the biennial mustard in 

 grain fields of the Prairie Provinces of Canada " is largely 

 due to the practice of raising cereal grains on stubble 

 lands, with only surface cultivation in the fall or spring 

 sufficient to produce a suitable tilth for a seed-bed, with- 

 out first destroying the weed growth by plowing, thor- 

 ough disking, or the use of the broad-sheared cultivator " 

 (Clark, 1911, p. 48). Summer fallow should be clean 

 cultivated all summer. 



500. Evening primrose (CKnothera biennis, Linn.). 

 This biennial weed is native in this country, and found 

 both in waste places and cultivated fields. It is occa- 

 sionally abundant in winter grain. The flowers appear 

 in June of the second year, and seeds mature near the end 

 of September. The flowers are yellow and very attrac- 

 tive, opening in the evening and closing the next morn- 

 ing, but some of them remain open during the day. The 

 fruit is a long, tapering, 4-celled capsule. On ripening, 

 it breaks open and the small, dark, reddish brown, 4- 

 angled seeds are shaken out by the wind (Fig. 141 e). 

 The seeds are found usually in various clover and grass 

 seeds. 



The evening primrose in grain stubble may be destroyed 

 by fall or spring cultivation. Only clean seed grain should 

 be sown. 



501. Other biennial weeds in grain that are of local 

 interest are willow herb, golden fumitory, tower mustard, 

 hairy tower mustard, cone flower, western wall flower, 

 small wall flower, and biennial wormwood. Some or 

 all of these would probably be of little or no importance 

 except for the practice of growing grain on poorly culti- 

 vated summer fallow or on stubble ground not plowed. 



