WEEDS AND POISONOUS PLANTS 335 



lawns disappear in a comparatively short time if the lawns 

 are regularly mowed ; and it is a reason for the mowing of 

 fence rows and for the cutting of weeds that appear in pas- 

 tures. Sometimes it is helpful to burn weeds, especially if 

 they have begun to form their seeds when they are cut. 

 When such precautions as these are taken, and the original 

 weed plants die or are destroyed, there is no chance for the 

 species to appear in the following year. But unfortunately 

 these simple methods are not finally effective against weeds 

 whose seeds are carried long distances by the wind. No 

 matter how carefully thistles may be eliminated from a field, 

 or dandelions from a lawn, the owner is at the mercy of his 

 neighbor ; and if the neighbor persists in raising a good crop 

 of thistles or dandelions each year, only a continuous warfare 

 will keep them from making serious trouble on another's 

 premises. This is the reason that lies back of the enforce- 

 ment of laws against noxious weeds ; one person cannot 

 make much headway in the fight without help from the 

 community. 



Each of the different ways in which weed seeds may be 

 introduced requires to be guarded against by special pre- 

 cautions. It is important to use in every case only the clean- 

 est seed that can be obtained seed, that is, which is free 

 from weed seeds. It is commonly advised that a hay crop 

 be cut early ; one advantage of this practice is that weeds 

 growing among the crop are prevented from going to seed and 

 thus having their seeds distributed with the hay. Most 

 of the weed seeds in a manure pile rot rather quickly ; the 

 danger from manure can, therefore, be largely met by allow- 

 ing it to rot thoroughly. Of the tumbleweeds, probably the 

 worst is the Russian thistle, which is one of the most danger- 

 ous weeds in the prairie region of the western United States. 

 Its migration is stopped whenever it meets a good fence ; 

 fences, therefore, offer a means of combating the spread of this 

 and similar weeds. Tumbleweeds are of comparatively little 



