376 General Botany 



How rapidly a weed may spread is illustrated by the history 

 of the Russian thistle. It was introduced into South Dakota 

 in 1874 with imported flaxseed. By 1888 there were enough 

 plants in the Dakotas to have it reported as a weed. In 1893 

 it was abundant around Chicago. In 1898 it was reported in all 

 the states and provinces east of the Rockies, from the Gulf of 

 Mexico to Saskatchewan. 



The control of weeds. The measures taken to control weeds 

 depend first of all upon whether the weed is (i) an annual, like 

 crabgrass, smartweed, ragweed, or foxtail grass; (2) a biennial, 

 like blueweed, bull thistle, or wild carrot; or (3) a perennial, 

 like Johnson grass, Canada thistle, wild onion, or milkweed. 



The first principle to be observed in controlling weeds is to 

 avoid bringing weed seeds to the farm or lawn. All seeds planted 

 should be inspected for weed seeds, and if they are present the 

 seed should be either cleaned or discarded. 



The second rule is that no weeds should be allowed to produce 

 seed. Since annuals and biennials are propagated only by seeds, 

 the strict observance of this rule will ultimately rid an area of 

 these two classes of weeds. 



The third principle of weed control is the prevention of the 

 growth of shoots. Depriving a plant of its photosynthetic tissues 

 leads to starvation of the underground parts. This principle 

 is particularly applicable to perennial weeds with underground 

 stems. The shoots may be destroyed by cutting, by spraying 

 with poisons such as salt, copper sulfate, and petroleum, or by 

 covering the area when small with roofing paper. 



Preventing weeds from producing seed. A single plant of 

 many common weeds will produce hundreds or thousands of 

 seeds. Moreover, not all these seeds may germinate the first 

 year, and seedlings may continue to appear for several years. 

 Harrowing and cultivating farm lands not only improve soil 

 conditions for the growing crop, but they also destroy countless 

 numbers of weed seedlings, which in good soil is far more im- 



