DESCRIPTIVE MANUAL 



381 



Figure 223A 



Fig. 223-A. 



Figure 223B 



Trichome or plant hair from leaf of Canada Thistle. 

 (Drawing- by Charlotte M. King.) 

 Fig. 223-B. Distribution of Canada Thistle. 



plowing, the soil should be dragged and the roots exposed to the 

 sun and removed, when possible. It may be necessary to run 

 over the field with a hoe to cut off the stray plants which appear. 

 This method was tried on a patch several years ago and no 

 Canada thistles have since made their appearance in this place. 

 Various crops, such as clover and sorghum, are said to be ef- 

 fective in subduing the thistles. 



Of the various chemicals which have been used to exterminate 

 Canada thistles none are more effective than sodium arsenite. It 

 is applied at the rate of 1^-2 pounds to 52 gallows of water. 



Carbolic acid at the rate of one part to one part water destroys 

 the root when it comes in contact with the mixture, and for a little 

 distance beyond. This is not an effective method, as the roots 

 sprout out from below. 



In response to circulars of inquiry sent out by the Iowa Experi- 

 ment Station, the majority of correspondents recommend shallow 

 plowing, disking, and harrowing; and continuing cultivation and 

 hoeing as long as the thistles make their appearance. Some report 

 successful treatment with salt when scattered thickly about the 

 thistles, especially if cattle or sheep are given access to it. Some 

 report success with carbolic acid when it is applied directly to the 

 stem. Tarred paper in a few cases gave success, as did also the 

 method where the thistle was covered thickly with straw or ma- 

 nure. The depth of covering was not, however, given. 



