No. 24 Article 5/November 3, 2006

Controlling Perennial Weed Species

Perennial weed species often become established in no-till production fields and can cause great frustration with respect to how best to control or eradicate them. Without the option of mechanical control (i.e., 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 often hard to control because they store large amounts of food reserves in their root systems. Controlling the aboveground parts 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, 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 that time of year is moving upward from the roots to support new vegetative development. Because of that upward food movement, it's often difficult to get sufficient 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 about the time perennial broadleaf species begin to flower. Since this time has (obviously) already passed for this year, another good time to treat perennial weed species is fall.

As day length becomes shorter and temperatures become cooler, perennial plant species begin to move food back into their roots. Since 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 in spring.

Dandelions are often very common in no-till production systems, and they 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 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 with spring.

For example, 2,4-D is used as a burndown herbicide in the spring, but usually at only 1 pint or less per acre due to 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 in fall than in spring. 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. If possible, try to make applications on days when air temperatures range into the 50s and sunshine is abundant.

What about other perennial species? While the general principles ascribed to fall timing hold true for most perennial species, not all the ones common to agronomic crops have sufficient leaf surface remaining (considering the calendar has turned to November) to absorb a foliar-applied herbicide. Take the time to scout target fields to determine if the perennial species are still healthy and actively growing. If frost has caused the perennial plants to drop leaves and stop growing, the benefits of a fall-applied herbicide likely will be reduced.--Aaron Hager and Dawn Nordby

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