454 THE SMALL GRAINS 



In general, clean cultivation, which will prevent seed- 

 ing of the plants, is the preventive. Plants already 

 mature should be gathered and burned. 



496. Other annual weeds that occur frequently in 

 grain fields, and the seeds of which are found as impurities 

 in grain, are lamb's quarters, false flax, spurrey, stickseed 

 or bluebur, Pennsylvania smartweed, peppergrass, darnel, 

 wild radish, hare's ear mustard, barnyard grass, ball 

 mustard, black mustard, alligator's head or button weed, 

 dragon head, fleabane or horse weed, marsh elder, yellow 

 foxtail or pigeon grass, green foxtail, pigweed, prickly 

 lettuce, shepherd's purse, morning glory, Indian pink, 

 and tansy mustard. Some of these are of considerable 

 importance locally and some are bad in certain seasons, 

 but apparently no one of them causes great damage to 

 grain the country over. No doubt there are still other 

 annuals that are of purely local interest in grain fields. 



497. Biennial weeds. The seeds of biennial weeds 

 germinate in the spring, and produce a tap root below the 

 ground and only a rosette of leaves above ground the first 

 year. The second year these plants start growth early 

 from the stored food in the tap root, and send up stems that 

 produce flowers and seeds, and then die. Examples are 

 the bull thistle and common evening primrose. Biennials 

 are rare in intertilled crops, as the young plants are de- 

 stroyed in cultivation. 



498. Bull thistle (Cirsium lanceolatum, Hill.). The 

 common thistle, because of its habits of growth, is not 

 usually a serious crop weed, but it is often sufficiently 

 abundant in winter grain to be very annoying in harvest- 

 ing, and wasteful of time and labor. It is troublesome in 

 the winter wheat of the middle Great Plains, and likely 

 to be abundant on summer fallow not well cultivated. 



