CEREAL PESTS WEEDS 441 



are annuals. They usually have small fibrous roots and 

 produce a large quantity of seed. Some of the annual 

 weeds that are pests of grain fields are buffalo bur, chess, 

 mustard, ragweed, Russian thistle, wild oats, penny 

 cress, sunflower, cockle, tumbling mustard, and wheat- 

 thief. Some of these germinate in the fall, live through 

 the winter, and mature their seeds in the spring or early 

 summer, and are therefore to be classed like winter wheat, 

 as winter annuals. Chess, cockle, wheat-thief, and penny 

 cress are of this class. 



480. Dissemination of annual weeds. It is probable 

 that nearly all weeds on any farm have started from seed 

 sown by the farmer himself, which shows the necessity of 

 cleaning seed. Thorough cleaning is difficult, but every 

 effort should be made to accomplish it. One mustard 

 seed in a bushel of wheat sown on an acre of land seems a 

 trifle, but the one weed resulting will produce thousands of 

 seeds for weeds on the same acre the next season. Pull- 

 ing the one weed on each acre would solve the mustard 

 problem for the time, showing the importance of early 

 attention. 



The wind carries seeds such as penny cress or prickly 

 lettuce long distances. Many seeds are no doubt car- 

 ried in the drifting snowstorms of the winter. Various 

 tumble weeds are driven by the wind many miles away. 

 Bolley (1908, pp. 526-527) tested the seed-carrying power 

 of blizzard winds in January and February. Fifteen 

 minutes after exposure to a 15-mile-an-hour wind, many 

 millet seeds (proso) and some heavy-weight oat kernels 

 were found 80 rods distant from the point of dissemination. 

 Some weeds, like the bull thistle and penny cress, have 

 hairy or winged seeds, adapted for wind dissemination. 

 Seeds of ragweed, wild oats, and mustard are carried on 



