CHAPTER IX 

 WEEDS 



A WEED is a plant troublesome to man. In the more 

 restricted sense it is an herbaceous plant that becomes 

 aggressively troublesome in cultivated fields. Plants 

 may become weeds because of their intrusiveness, as 

 bindweed (Convolvulus arvensis L.) and Canada thistle 

 (Cirsium arvense (L.) Scop.) or they may be able quickly 

 to occupy waste land, dooryards, or roadsides, as knot- 

 weed (Polygonum aviculare L.), horseweed (Erigeron 

 canadensis L.) and ragweed (Ambrosia trifida L.). Others 

 may become weeds in pastures because, not being eaten 

 by stock, they thrive at the expense of the palatable 

 species. 



101. Classes of weeds. — Weeds may be divided into 

 three classes according to method of growth. These are 

 annuals and biennials, perennials with crowns, and peren- 

 nials with rhizomes or creeping roots. Methods of eradi- 

 cating depend upon the group to which the weed belongs. 

 Annual weeds may become abundant, but are not difficult 

 to eradicate as they may be pulled up or removed by 

 cultivation. Perennial weeds with crowns are usually not 

 aggressive but may become conspicuous in pastures, 

 especially those that are over-grazed. Certain kinds of 

 plants usually classed with this group differ in having an 

 upright fleshy caudex capable of producing adventitious 

 buds and shoots. These may become troublesome weeds 

 in pastures or lawns. The dandelion (Taraxacum Taraxa- 



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