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WEED FLORA OP IOWA 



Fig. 18S-A. Distribution of Horseweed. 



Great Ragweed (Ambrosia trifida L.). 



Description. — A stout, scabrous, hispid or nearly glabrous an- 

 nual, 3-12 ft. high ; leaves all opposite and petioled, 3-nerved, deeply 

 3-5-lobed ; lobes ovate-lanceolate and serrate, upper leaf sometimes 

 ovate and undivided ; flowers monoecious, staminate, borne in spikes 

 surrounded by the larger bract-like leaves; involucre turbinate to 

 obovoid, 5-7-ribbed, beaked, each rib bearing a tubercle near the 

 summit ; involucre enclosing a single oily seed. 



Distribution. — This North American weed is most abundant in 

 the Mississippi valley from Texas to Minnesota and in the Da- 

 kotas; however, it also occurs east from New England to Quebec. 

 It is abundant in every part of Iowa, especially along highways, in 

 grain fields and corn fields. 



Extermination. — It is certain that cultivating the young plants 

 followed by three or four other similar treatments will remove the 

 weed in a single season. 



