164 FARM WEEDS OF CANADA 



CANADA THISTLE (Cirsium arvense (L.) Scop.) 



Other English names: Creeping Thistle, Soft Field Thistle. 



Other Latin names: Carduus arvensis (L.) Robs.; Cnicus 

 arvensis Hoffm. 



Introduced from Europe. Perennial with deep running 

 rootstocks. Stems erect, 2 to 4 feet high, marked with fine 

 longitudinal lines. Leaves variable in shape, deeply pinnatifid, 

 waved and crested, very prickly, in some plants much less so 

 than in others, somewhat downy, particularly beneath. Flower 

 heads numerous, in a large loose corymb at the top of the stems. 

 Flowers variable in colour, ranging from pale purple through 

 shades of pink to white. Some plants bear male flowers only, 

 which form no seeds, other plants female flowers only, which 

 produce many seeds; the flower heads of male plants are nearly 

 globe-shaped, 1 inch across, those of the female plants only about 

 half as large, oblong, with short florets. Large patches may 

 be found bearing only male flowers, showing that all the plants 

 originated from a single seed. 



The seed (Plate 76, fig. 89) is 1/8 of an inch long, light brown, 

 elongated oblong, smooth, somewhat flattened and curved, 

 more or less bluntly angled, marked with faint longitudinal 

 lines; the top is nearly round, flat, and has a narrow rim with 

 a small cone-shaped point in the centre; pappus copious, white. 

 The pappus easily breaks off by threshing and handling and is 

 absent when the seed is found in commercial grain and seeds. 



Time of flowering: June to August; seed ripe by July. 



Propagation: By seeds and extensive rootstocks, which 

 send up both leafy barren shoots and flowering stems. This 

 well known pest, which, although called Canada Thistle in 

 North America, is really the Field or Creeping Thistle of England, 

 has been introduced into almost all the British colonies and has 

 everywhere proved a troublesome and persistent enemy of farmers. 

 The rootstocks run down into the soil from eight to fifteen inches, 

 and often much deeper; when patches are covered up stems can 

 be thrown up through many feet of soil. 



