PULLMAN, Wash. — Fall is an excellent time to control winter annual, biennial, and perennial weeds because many of these weeds are vulnerable at this time of year.
Moreover, cold and moisture stress during the winter may kill weeds not completely controlled by herbicides, tillage, or other measures.
And there are some side benefits. Fall herbicide applications after deciduous trees and shrubs have shed their leaves will also lessen potential injury to these desired plants.
Controlling weeds in the fall also can allow desired plant species in pastures, lawns, roadsides, crops, and other areas to fill in areas previously occupied by the weeds.
This creates a more competitive environment against weeds attempting to establish later in the fall or the following spring. Minimizing weeds assures that water, fertilizer, and other resources benefit desired, rather than undesired vegetation.
Winter annual weeds germinate in the late summer or fall and will be present as seedlings during the fall. Control of these weeds with herbicides can be done either before or after they emerge.
To be effective, pre-emergence herbicides must be applied before weed seeds germinate. Some herbicides must be applied to the soil and incorporated with a disk, cultivator, harrow, or other tillage equipment in crops or when establishing pastures, turf, or other perennial plants.
Fall-applied post-emergence herbicides can be very effective against winter annual weeds, such as downy brome, jointed goatgrass, or henbit.
Post-emergence herbicides are most effective when used after most weeds have emerged but before the go into winter dormancy. Research has found that November applications give best control, however, results may vary depending upon weed species and environmental conditions. If late fall conditions become dry, control may decrease because the stress caused by lack of moisture reduces herbicide efficiency.
December applications sometimes work but weeds are normally hardened off by then, reducing your odds of success.
Spring herbicide applications can be as effective as fall applications in controlling winter annual weeds. However, since weeds are allowed to compete with crops for many more months, gaps left by dying weeds may be filled in by summer annual weeds.
Cultural methods of controlling winter annual weeds can also be effective during the fall. Early planting of winter wheat or other winter annual crops can give those crops a head start on weeds when they emerge later on.
Tillage also can control flushes of winter annuals following late summer or fall rains. However, delaying planting can cut yields and tillage can reduce soil moisture.
Biennial weeds can be a problem in no-till cropping systems as well as in pastures, turf, and non-crop areas. Biennials, including many species of thistle or common mullein, usually germinate during the summer and are present in the fall as rosettes.
Take your time in scouting for biennial weeds in the fall. They may be difficult to see within crops and other desired vegetation. Often, these plants are not noticed until spring when they develop large, vigorous flowering stalks.
By the spring, biennials that have been growing for several months are more difficult to control and they leave bare spots when they die. Fall tillage prior to crop, forage, or other plant establishment should control most of the earlier germinating biennial weeds.
Typically, fall herbicide applications are more effective against biennials than spring applications and desired plants are given a chance to fill in bare spots before other weeds.
Perennial weeds, such as Canada thistle, field bindweed, or quackgrass, are most effectively controlled with herbicides when they are fall applied.
Applications should be made to field bindweed in the late summer or early fall. Other weeds, such as quackgrass, are more effectively controlled with later applications of herbicides. As plants harden off for winter, they move more of their stored nutrients into underground roots, rhizomes, or tubers to assure regeneration the following year.
Many herbicides applied during or just before the weeds harden off, will move with these nutrients and kill underground buds, preventing re-establishment the following year.
Tillage is also effective in controlling most perennial weeds. Deep tillage, such as moldboard or chisel plowing, can bury perennial weeds deeply enough in the soil to prevent re-establishment. However, shallow tillage, such as disking or cultivating, may not bury perennial weeds deeply enough and can serve to spread or intensify infestations.
A healthy crop, forage, turf, or other desired plants established in the fall will provide strong competition against perennial weeds trying to re-establish following tillage.
Systemic herbicides and tillage used in combination can prove very effective in controlling perennial weeds. Be sure to allow time between herbicide application and tillage to ensure that the chemical has time to move throughout the plant. Delaying tillage 10 to 14 days after herbicide application is usually adequate for systemic herbicides to translocate throughout the plant.
Again, perennial weeds don’t necessarily need to be totally controlled with herbicides or tillage applications. Often, causing severe injury to these weeds can ensure they won’t survive the winter or will not be able to compete against desired plants.
Fall control also provides the opportunity for desired plants to fill in bare areas left by dying weeds and be more competitive against any weeds that try to establish the following spring or summer.
It is important to find and map weed infestations. Scouting and proper identification of weeds will allow you to select the best method of control. Typically, the same species of weeds will infest the same area year after year. You should watch these areas closely and intensify control measures as needed.
Whatever you do, try to minimize environmental impact. Make sure that soil erosion is controlled by establishing plants that will cover the soil surface or leave adequate surface residue.
Also, read and follow all herbicide label directions.
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