PLATE 29. 

 CANADA THISTLE, Cntau anemii, Hoffm. 



Other English names: Creeping Thistle, Soft Field Thistle. 



Other Latin names: Carduus arvensis (L.) Eobs. ; C'irsium arvense, Scop. 



(Noxious: Dom., Ont., Man., N.W., B.C.) 



Introduced. Perennial with very deep running rootstocks. Stems erect, 

 2 to 4 feet high, striate. Leaves very variable in shape, deeply pinnatifid, 

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

 somewhat downy, particularly beneath the leaves. Flower heads very numer- 

 ous, in a large loose corymb at the top of the stems, dioecious, some plants 

 bearing male flowers only, which form no seeds, others female flowers only, 

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

 lar, 1 inch across, those of the female plants oblong, with short florets and 

 only about half as large; large patches may be found bearing only male or 

 female flowers, showing that each originated from a single seed. Flowers 

 very variable in colour, ranging from pale purple through shades of pink to 

 white. The seeds are i inch long, elongated oblong, smooth, somewhat 

 flattened and curved, more or less bluntly angled and finely grooved ; the top 

 is squarely rounded and has a narrow rim with a small conical point in the 

 centre, colour light brown; pappus copious, white. [Plate 56, fig. 64 natural 

 size and enlarged 4 times.] 



Time of Flowering -. June to August ; seed ripe July. 



Propagation : By seeds and by extensive underground rootstocks, which 

 send up both leafy barren shoots and flowering stems. This well-known pest, 

 which, althpugh called Canada Thistle in North America, is simply the 

 Field Thistle or Creeping Thistle of England, has now been introduced into 

 almost all of the British colonies, and has everywhere proved a troublesome 

 and persistent enemy of farmers. As an instance of the rapidity with which 

 this plant can spread, a single seed was planted on sandy but fertile land 

 in the spring ; by the first autumn there were two or three shoots showing at 

 a distance of about a foot from the central plant; by the second autumn a 

 patch 20 feet across with numerous stems had developed. The rootstocks 

 naturally run down into the soil to a depth of 8 to 15 inches; but when 

 patches are covered up stems can be thrown up through many feet of soil. 



Occurrence: Abundant everywhere in Eastern Canada and Manitoba, 

 here and there through the North-west Provinces and in British Columbia. 



Injury : A gross feeder and a vigorous grower, which not only crowds 

 crops but is a conspicuous evidence of negligent farming. Seeds frequent in 

 grass and clover seeds, also in seed oats. 



Remedy : Being a deep-rooted perennial, the Canada Thistle should be 

 ploughed deeply in summer just as the flowers open, or the flowering stems 

 may be mowed down and the land ploughed as soon as a new growth has 

 appeared.. As new stems are thrown up, they must be cut down with a broad 

 toothed cultivator at intervals through the autumn. A deep ploughing late 



59 



