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 I-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 
tubular 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 on 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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