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WEED FLORA OF IOWA 



ows, but also in corn and grain fields. It grows in patches like the 

 Canada thistle. These patches increase in size from year to year. 

 It was described as a doubtful perennial by early botanical writers 

 and so far as I have been able to determine, it is a perennial. It 

 may be exterminated by thorough cultivation, plowing well and 

 then following with the cultivator. 



Prairie Thistle (Cirsium discolor (Muhl.) Spreng.). 



Description. — Tall, branching, leafy biennial, 5-7 ft. high, with 

 heads larger than in Canada thistle; stem striate, slightly hirsute; 

 leaves radical, 12-14 in. long, deeply pinnatifid, the divisions fre- 

 quently divided, prickly-toothed, the upper surface smoothish, and 

 the lower white ; woolly single heads terminating the branches, with 



Pig. 221. Prairie or Wood Thistle (Cirsium discolor). Common border of 



woods, etc. 

 ( Photographed by Colburn.) 



