CHAPTER 17 

 WEEDS AND WEED CONTROL 



Definition. "A weed is a plant out of place." This is a short con- 

 cise definition, easily remembered by students. "Any useless, or trouble- 

 some plant" is another definition, which has been given and in addition 

 longer definitions have been formulated, as follows: "Every plant which 

 grows in a field other than that of which the seed has been (intentionally) 

 sown by the husbandman is a weed." "Any plant which obtrusively 

 occupies cultivated, or dressed ground, to the exclusion or injury of some 

 particular crop intended to be grown" is another statement. "Thus, 

 even the most useful plants may become weeds, if they appear out of their 

 proper place. The term is sometimes applied to any insignificant looking 

 or unprofitable plants which grow profusely in a state of nature, also to 

 any noxious, or useless plant." "Weeds aie plants which tend to take 

 prevalent possession of soil used for man's purposes, irrespective of his 

 will; and, in accordance with usage we may restrict the term \o herbs." 



Absolute and Relative Weeds. Weeds may be divided into two classes; 

 absolute weeds and relative weeds. An absolute weed is one which has 

 no recognized use, as the horse nettle (Solanum carolinense) . A relative 

 weed is one which may be extremely useful to man, but becomes a weed 

 when out of its proper place. The Johnson grass of the south is an 

 example. It is a very nutritious and valuable grass, if kept under con- 

 trol, but if allowed to seed, it spreads rapidly into new ground and becomes 

 extremely troublesome and difficult to eradicate. 



Injurious Nature of Weeds. Weeds are injurious to man for the fol- 

 lowing reasons. 



1. They crowd other plants. Two plants cannot occupy the same 

 ground at the same time, and if weeds are abundant, they occupy the soil 

 to the exclusion of the cultivated plants. 



2. They rob the soil of moisture. This may not be harmful, if the 

 soil water is abundant, but when the supplies of water are reduced during 

 dry weather, the weeds transpire through their leaves undue amounts of 

 the precious liquid. Cultivation of the soil under such conditions is 



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