VI. CONTROL OF HERBACEOUS PERENNIAL WEEDS 



ON CROPLAND 



Herbaceous perennial weeds often have deep and extensive root systems 

 and are difficult to kill with herbicides 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, dif- 

 ferent herbicides must be used for killing perennial weed grasses, such as 

 Johnsongrass and quackgrass, than for killing broadleaved perennial weeds 

 like bindweed, Canada thistle, and leafy spurge. However, a few herbicides 

 are effective on both types. 



Comments and precautions . --Spot sprays and preplanting application 

 of all herbicides on herbaceous perennial weeds in roNv crops must be 

 used in accordance with the restrictions on crop use stated on the manu- 

 facturer's label. 



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 l/2 to 2 pounds per acre, 



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

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



(3) For control of weeds in the horsenettle group: Repeated treatments 

 of 2,4,5-T at 1 to 4 pounds per acre. 



(4) For control of white bedstraw: Apply silvex at 3 pounds per acre in 

 late fall or early spring. 



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

 on many broadleaved species and usually provide an 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 

 grow^ing conditions for weeds. 



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

 ester formulations of 2,4-D sometimes w^ill control Canada thistle, 

 Russian knapw^eed, leafy spurge, and some other broadleaved perennial 



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