382 WEED FLORA OF IOWA 



Clark and Fletcher recommend the following treatment : 

 "The chief safeguard against the Canada thistle and all similar 

 deep-rooted perennials is undoubtedly a regular short rotation. A 

 three-year rotation, including two cuttings of early red clover for 

 the first year, followed by a deep fall plowing for hoed crops with 

 clean cultivation, and a cereal crop for the third year, will sup- 

 press it." 



A correspondent in The Prairie Farmer on the resisting quali- 

 ties of Canada thistle says : "I have been debating in my mind 

 and endeavoring to determine whether or not root-stocks or por- 

 tions thereof have to some extent the hibernating qualities of the 

 seed. For instance, take two seeds of the cocklebur — one hiber- 

 nates for one season and the other for two seasons, with growth 

 always present. Now the question is may not a Canada thistle 

 with its root-stock or a portion of it lie dormant for a season or 

 more? If this be true, then some of the best known methods of 

 extermination are useless." 



To this Prof. R. A. Moore says: "In regard to Canada thistle 

 roots hibernating and retaining their vitality for several years, 

 will say, that I think your correspondent's version seems reason- 

 able, and it is quite conclusive from this fact that the eradication 

 of the pest is all the more difficult. It seems that many of the 

 seeds and plants are given this power of remaining dormant when 

 subjected to adverse conditions. In the lower order of plants, 

 many of the species of bacteria when subjected to adverse environ- 

 ments are transformed into resistant spores and will not vegetate 

 until the conditions are favorable." 



Wallace's Farmer says concerning its eradication: "We 

 believe if we had a quarter of an acre of Canada thistles 

 we would let them alone until August, when the thistle will put 

 forth its utmost efforts to produce seed. While the thistles are in 

 full blossom we would mow them, rake them up and burn them, 

 and then plow the ground about eight inches deep, throwing the 

 furrow flat. Letting them put forth their full strength to produce 

 seed and thwarting that by mowing and burning would weaken the 

 roots materially. Then by plowing them under eight inches deep, 

 if possible, you would attack them at their weakest point. 



