COMPOSITE FAMILY 



COMPOSITAE 



CANADA THISTLE 



Cirsium arvense (L.) Scop, 



This is another immigrant from Europe which has become a 

 pernicious weed in this country from Newfoundland to Virginia, 

 Utah, Nebraska and British Columbia. It is probably the most 



hated of all Thistles if not of all 

 weeds. 



The plant is readily identified 

 by its jointed horizontal root- 

 stocks, which other Thistles do 

 not have. It is perennial by 

 these extensively creeping under- 

 ground stems, which make it 

 very difficult to exterminate. It 

 grows 1-3 feet high and the 

 sessile, slightly clasping leaves 

 are very prickly. The 5-8-inch 

 basal leaves are sometimes 

 petioled. 



A patch of these Thistles, in 

 full bloom from June to Septem- 

 ber, is a colorful display. The 

 heads are numerous and the 

 tubuhar flowers are purple or 

 rarely whitish. They are imper- 

 fect and the staminate and pis- 

 tillate heads are on separate 

 plants. This has led some farmers 

 to believe that the Canada Thistle produces no seed, but oii the 

 contrary there are smooth oblong akenes with a conspicuous 

 fine white pappus. Outer bracts of the head are ovate or ovate 

 lanceolate, appressed and with short prickly points. Inner 

 bracts of the pistillate heads are linear and long. 



The Tall Thistle, Cirsium altissimum (L.) Spreng., is common 

 along the borders of woods and thickets. It grows 3-10 feet tall. 

 The leaves are densely clothed on the lower surface with white 

 wool. They may be entire, toothed or lobed, and are armed with 

 rather weak prickles. The outer bracts of the involucre are marked 

 by a dark line on the back and are tipped by a prickle, but the inner 

 bracts are unarmed. The flowers are light purple. 



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