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It's a Good Time to Think About Controlling Perennial Weed Species

October 6, 2000
Perennial weed species often become established in no-till production fields and can sometimes cause a great deal of frustration regarding how best to control or eradicate them. Without the option of mechanical control (in other words, tillage), perennial weed species are generally best controlled by postemergence translocated herbicides. Selection of which translocated herbicide to use as well as when to make the application can impact the level of success achieved.

Perennial weed species are frequently difficult to control because they store a large amount of food reserves in their root systems. Controlling the aboveground part of perennial species is usually not enough to achieve satisfactory, long-term control; the root system must be controlled as well. Translocated herbicides (those that can move into the roots) are usually the most effective chemical options to control perennial weed species, but the time of year these herbicides are applied is important. In the spring, perennial species rely on stored food reserves to initiate new growth, so most of the food at this time of year is moving upward from the roots to support new vegetative development. Because of the upward direction of food movement within the perennial plant, it's often difficult to get a sufficient amount of herbicide into the root when applications (burndowns, for example) are made in the spring. Good control of perennial broadleaf species can be achieved when applications of postemergence translocated herbicides are made when the perennial broadleaf species have begun to flower. Although the time has obviously already passed for this year, another good time to treat perennial weed species is fall. During the fall, as day length becomes shorter and temperatures become cooler, perennial plant species begin to move food back into their roots. Because food reserves are moving downward in the plant during the fall, more translocated herbicide is moved into the root of perennial species, and control is generally much greater than can be achieved during the spring.

Dandelions are often very common in no-till production systems and frequently escape spring burndown applications of translocated herbicides. This fall, as harvest progresses at a good pace, there will likely be a very good opportunity to work on dandelions and other perennial weed species once the crops are removed. Food reserves are being moved to the roots, and good herbicide translocation can occur, resulting in more complete control of the roots. Additionally, higher rates of certain translocated herbicides frequently can be used in the fall compared to spring. For example, 2,4-D is commonly used as a burndown herbicide in the spring but usually at only 1 pint per acre because of increased potential for soybean injury and a longer interval prior to planting at rates greater than 1 pint. Higher rates of 2,4-D can be used in the fall, and control of perennial weed species such as dandelion will usually be greater than following spring applications. Keep in mind that fall applications should be made before many hard frosts occur, as leaf tissue damage caused by hard freezes usually decreases herbicide absorption.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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