CHEMICAL WEED -KILLERS OR HERBICIDES 



115 



CHEMICAL WEED-KILLERS OR 

 HERBICIDES 



By L. R. Jones 



The use of chemicals as herbicides offers no spe- 

 cific cure-all against weeds. Cultivation, short 

 rotations, watchfulness against the introduction 

 and scattering of weed seeds, are all of more fun- 

 damental importance than chemicals in combating 

 weeds. There are, however, various cases in which 

 chemicals Intelligently used are more expeditious 

 and economical than any other means for weed- 

 killing. A practical difficulty is so to use the her- 

 bicide as to kill the obnoxious plants without 

 working permanent iiljury to the soil or to 

 neighboring cultivated plants. This difficulty 

 limits the chief usefulness of chemicals as 

 weed-killers to the following cases: 



(1) When an especially obnoxious weed, as 

 poison ivy, occurs in a limited locality and is 

 to be destroyed regardless of consequences 

 to soil or neighboring plants. 



(2) When the aim is to render the soil 

 permanently sterile, as in roadways, tennis 

 courts, and the like. 



(3) When the weed plant, as orange hawk- 

 weed and mustard, is much more sensitive 

 than the associated useful plants to the 

 action of some herbicide. 



Chemicals useful as herbicides. 



Any soluble chemical, even including the 

 various commercial fertilizers, if used in 

 sufficient amount, will kill plants. Some act 

 directly and quickly as poisons, e. g., arsenic 

 and carbolic acid; others, such as salt, have little 

 or no direct poisonous effect but kill the plants 

 primarily by drawing the water from the tender 

 foliage, or by holding the moisture of the soil so 

 that it cannot be absorbed by the roots. It is 

 important in this connection to note that in either 

 case the herbicide is most effective on young plants 

 that are in active growth. Effectiveness in one or 

 the other of these ways, together with cheapness 

 and convenience of application, are the things to 

 determine choice among the various compounds 

 available. Without attempting to list all of these, 

 we include those whose worth has been best estab- 

 lished by trial. 



Salt (sodium chlorid) is probably more commonly 

 used than any other compound, chiefly because of 

 cheapness and handiness. Its action depends almost 

 wholly on the withdrawal and retention of moisture 

 from the plant, therefore it should be applied dry 

 or in strong solution; and it is most effective in 

 hot, dry weather. Salt can be used in any weed- 

 killing operation, but it is most valuable on road- 

 ways and like surfaces and for certain lawn 

 weeds. 



Blue vitriol (copper sulfate). — This is more pow- 

 erful in herbicidal action than salt, but its cost 

 prohibits its general use. For most purposes it is 

 best used in solution, 2 to 10 per cent being effec- 

 tive. It is often used on gravel walks and similar 

 surfaces, but salt will generally be found cheaper 



and arsenical poisons more effective. Its chief value 

 is against charlock, as noted on page 117. 



Kerosene. — This and other coal-oil products will 

 kill plants. Because of handiness it is frequently 

 used, but it is weak in efficiency, and relatively 

 more costly than any other chemical here listed. 

 A pint of crude carbolic acid will do better service 

 than two gallons of kerosene, and costs much less. 

 When crude petroleum is available at very low 

 price it is commended. 



Carbolic acid. — This is one of the quickest and 

 most valuable herbicides. The crude acid is rela- 

 tively cheap. It is not quite equal to the arsenical 

 poisons for penetrating the soil or in lasting 



effects, but it is often 

 preferable because of 

 cost or convenience. 

 It does not corrode 

 metals, hence, may be 

 applied with any can 

 or pump. An effective 

 method is to squirt the 

 strong acid from an 

 ordinary oil can on the 

 roots or crown of in- 

 dividual weeds. If it 



Fig. 157. 

 Heads of 

 orange hawk- 

 weed. 





The orange hawkweed, or paint-brush {Hieracivm 

 aurantiacum) . This plant originated from the runner 

 shown at the lower right-hand corner. The two young 

 runners at the left have already taken root and will soon 

 give rise in turn to new plants. (Adapted from Vermont 

 Experiment Station.) 



