CHAPTER XXV 

 WEEDS 



430. What is a weed ? It is not possible to put into a short 

 sentence a complete statement of what is meant by a weed. 

 It is often said that a weed is a plant that is not wanted. 

 Perhaps a better definition, from the farmer's point of view, 

 would be : A weed is a plant which interferes with some crop. 

 The word crop must, in this case, be taken in a very gen- 

 eral sense. The dandelions which interfere with the growth 

 of grass on a lawn, or the raspberry bushes which spring up 

 in burnt-over clearings in white-pine woods and crowd out 

 young tree seedlings, must be reckoned as truly weeds as 

 the ragweeds and the pigweeds 1 that are so troublesome in 

 cornfields. 



Cultivated plants may become very injurious weeds. Horse- 

 radish, and Johnson grass, 2 which is valued in the South as a 

 hay plant, are good instances of this. 



In the ordinary sense the term weed is applied only to 

 flowering plants or to the larger representatives of the lower 

 groups, such as ferns and horsetails. The many bacteria and 

 higher fungi which do so much harm in the farm and garden 

 are never spoken of as weeds. 



431. Classes of weeds. Weeds may be classified in many 

 ways, according to the kinds of resemblances and differences 

 taken into account in grouping them. 3 The kind of classifica- 

 tion which would first suggest itself to most botanists is that 

 into families, such as the Grass family, the Nettle family, and 

 the Buckwheat family. Another kind of division would be 

 into annual, biennial, and perennial plants ; still another into 



1 Amaranthus. 2 Sorghum halepense. 



8 See Percival, Agricultural Botany, Part V. Henry Holt & Co., New York. 



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