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 (EHrigeron 
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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