CHAPTER XXVII 

 WEEDS 



676. Definition. A weed is any plant which is growing 

 where it is not wanted ; that is, a plant out of place. A stalk 

 of corn in an oat field is just as much a weed as is a thistle, 

 though it may do less damage and in its place be a very use- 

 ful plant. A plant may thus be a weed under some condi- 

 tions, while it is not under others. Many of the wild plants 

 of our native meadows and pastures must now be classed as 

 weeds, though before the land was put to use they could 

 hardly have been so regarded. The smaller plants in a 

 forest are not weeds, for they are of use to shade the ground, 

 prevent washing, and protect the young tree seedlings. 



677. Need for a Study of Weeds. A study of weeds is a 

 very useful and necessary part of a study of field crops. In 

 the production of every crop weeds must be considered. A 

 method of treatment that is efficient in destroying one weed 

 or class of weeds may furnish a means for the spread of some 

 other, and it is, therefore, necessary to be able to recognize 

 the principal weed pests and to know how to deal with them. 

 The seeds of some of the most troublesome weeds frequently 

 occur in grain or grass seed; hence it is important to be able 

 to recognize them and to avoid sowing them with useful crops. 



' CLASSES OF WEEDS 



678. Basis of Classification. Weeds are classified accord- 

 ing to the length of time they live, as annuals, biennials, and 

 perennials. It is desirable to know to which class any weed 

 belongs, because the methods of combating it depend very 

 largely on whether it lives one, two, or several year?,. 



493 



