VI. CONTROL OF HERBACEOUS PERENNIAL WEEDS 



Herbaceous perennial weeds which die back to the ground each fall and develop new 

 growth from crowns or roots the following spring usually have deep and extensive root 

 systems and are difficult to kill with chemicals or other methods. On cxoltivated 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. On noncultivated land herbicides alone or in 

 combination with tolerant grass crops must be used. Usually, different chemicals must 

 be used for killing perennial grass weeds such as Johnson grass and quackgrass than 

 for killing broadleaved perennial weeds like bindweed, Canada thistle, and leafy 

 spurge. However, a few chemicals are effective on both types. 



BROADLEAVED PERENNIALS 



The phenoxy compounds 2,4-D, MCPA, and 2,4,5-T, are effective on many broad - 

 leaved species and usually provide the most economical means of control. Some of 

 these weeds require repeated chemical treatments for 2 or more years for satisfactory 

 control. 



The broadleaved species which usually can be controlled by repeated treatments at 

 1/2 to 2 pounds of 2,4-D per acre are field bindweed, whitetop (hoary cress), sow- 

 thistle, and Canada thistle. More resistant species that require 1 to 4 pounds per acre 

 of 2,4-D are bur ragw^eed, dogbane, leafy spurge, and Russian knapweed. Weeds of the 

 horsenettle group should be treated with 2,4,5-T at 1 to 4 pounds per acre. 



Where possible, the phenoxy compounds should be used in combination with com- 

 petitive crops of cereals, corn, or perennial grasses. Sometimes cereals and corn are 

 damaged by the relatively high rates of chemical required for these weeds so, when 

 possible, the treatment should be made before the crop is planted or after it is 

 harvested. In dryland areas it is often advantageous to precede chennical treatment with 

 1 to 3 months of intensive cultivation to provide optimum growing conditions for the 

 v/eed. 



Canada thistle, Russian knapweed, leafy spurge, and some other broadleaved peren- 

 nial weeds can sometimes be controlled with a single heavy application of amine or 

 ester formulations of 2,4-D at 25 to 80 pounds per acre. The application should be 

 made in the fall or early spring. The effects of the chemical remain in the soil for 

 only a few weeks after the treatment. 



Soil sterilants such as sodium chlorate, borax, or combinations of the two are 

 effective and suitable for controlling deep-rooted broadleaved species on noncultivated 

 land or small patches in cultivated fields. Chlorate should be used at 3 to 10 pounds 

 per square rod, borax at 20 to 40 pounds, and mixtures at 6 to 20 pounds. Surviving 

 plants should be re-treated as necessary In subsequentyears. The arsenlcals generally 

 are much less effective on deep-rooted broadleaved species than on the shallower 

 rooted grasses and sedges. 



PERENNIAL GRASSES AND SEDGES 



Johnson grass, Bermuda grass, and quackgrass In cultivated fields can be most 

 easily and economically controlled by frequent cultivation- -every 2 or 3 weeks during 

 one growing season. Frequently the amount of cultivation necessary can be greatly 

 reduced by applying TCA at 25 to 50 pounds or dalapon at 10 to 20 pounds per acre as 

 a foliage and soil surface treatment before plowing or disking In late fall or early 

 spring. Intertilled row crops should be grown the first year following such treatments 

 to provide frequent tillage, competition for light, and permit spot cultivation or 



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