VI. CONTROL OF HERBACEOUS PERENNIAL WEEDS 

 IN CROPLAND 



Herbaceous perennial weeds often have deep and extensive root systems 

 and are difficult to kill with chemicals or other methods. On cultivated 

 land, control of such weeds may be most effectively and economically 

 achieved by intensive cultivation in combination with suitable competitive 

 crops and selective herbicides or temporary soil sterilant chemicals. 

 Usually, different chemicals must be used for killing perennial weed 

 grasses, such as Johnsongrass and quackgrass, than for killing broad- 

 leaved perennial weeds like bindweed, Canada thistle, and leafy spurge. 

 However, a few chemicals are effective on both types. 



BROADLEAVED PERENNIALS 

 The Phenoxy Compounds 



(1) For control of field bindweed, whitetop (hoary cress), sowthistle, 

 and Canada thistle: Repeated treatments of 2,4-D at 1/2 to 2 pounds per 

 acre. 



(2) For control of bur ragweed, dogbane, leafy spurge, and Russian 

 knapweed: 2,4-D at 1 to 4 pounds per acre. 



(3) For control of weeds in the horsenettle group: 2,4, 5-T at 1 to 4 

 pounds per acre. 



Comments and precautions .- -2,4-D, MCPA, and 2,4,5-T are effective 

 on many broadleaved species and usually provide the most economical 

 means of control, although eradication is not often attained. Some of 

 these weeds require repeated annual chemical treatments for 2 or more 

 years for satisfactory control. 



Where possible, the use of phenoxy compounds on weeds should be 

 combined with the growing of competitive crops of cereals, corn, or 

 perennial grasses for effective weed control. Sometimes cereals and 

 corn are damaged by the relatively high rates of chemical required for 

 these weeds. Under these circumstances, treatment should be made 

 before the crop is planted or after it is harvested. In dryland areas it 

 is often advantageous to precede chemical treatment with 1 to 3 months 

 of intensive cultivation to reduce the root reserves and to provide 

 optimum growing conditions for weeds. 



A single heavy application of 25 to 80 pounds per acre of amine or 

 ester formulations of 2,4-D sometimes will control Canada thistle, 

 Russian knapweed, leafy spurge, and some other broadleaved perennial 

 weeds. Application should be made in early spring or fall. Effects of 

 the chemical remain in the soil for only a few weeks after treatment. 



Chlorinated Benzoic Acids u 



(1) Recent experimental trials with these acids have produced excellent 

 results. 



"Chlorinated benzoic acids cannot be recommended (as of April 1, 1958) for use on broadleaved perennials on 

 cropland since evidence has not been developed to prove they will leave no residue or/and tolerances have not 

 been set under Public Law 518. 



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