CEREAL PESTS WEEDS 447 



glabrous above, downy hairy beneath ; flowers like those of 

 kinghead but smaller ; seeds T V to | of an inch long, light- 

 straw color to dark brown, pear-shaped (Fig. 142 h). 

 The pollen from the upper flowers fertilizes the lower 

 flowers. At flowering time, in late July and August, the 

 plants become quite yellow with pollen, which is sup- 

 posed to induce hay fever. The seeds in wheat injure its 

 flour-making quality. 



As ragweed matures its seeds usually after harvest, 

 plowing or disking immediately after cutting the crop, or 

 an intertilled crop following the grain, will eradicate the 

 weed. 



Kinghead or great ragweed (Ambrosia trifida, Linn.) 

 occurs commonly along roadsides and edges of fields, but 

 is also reported to be frequent in grain fields in the north- 

 ern states. The leaves and stem are very rough, lower 

 leaves deeply trifid, upper leaves entire. The upper 

 flowers pollinate the lower flowers as in common ragweed. 

 The seeds are usually inclosed in a hull J inch long, hav- 

 ing a crown and tip of short pointed teeth. They are 

 more difficult to separate from cereal kernels than those of 

 common ragweed. 



486. Penny cress or French weed (Thlaspi arvense, 

 Linn.). This winter annual is considered one of the 

 worst weeds in some districts in the East. Because of 

 its disagreeable odor, it is often called " stink weed/' 

 It grows from the seed in the spring, from 6 inches to 2 

 feet high, produces white flowers, inch across, and 

 then seeds before the end of the season. The pods are 

 flattened, f inch wide, and light yellow in color when 

 mature. The leaves at the base are petioled, but on the 

 branches they clasp the stem. Seeds reddish-brown, oval, 

 with concentric rings on the surface (Fig. 141 g}. 



