A weed might have the simple definition of being a plant out of place. But winter annual weeds can also be considered parasites. They take nutrients out of your soil. They remove moisture that may be needed for spring crop germination. They will clog up tillage equipment. But they do provide one benefit. They serve as the landing spot for moths to lay eggs that will turn into black cutworms. So they can’t be all bad. Right? Yeah, right. So how do you get rid of winter annuals, or should the question be, what should you not do in an attempt to control winter annuals?
There is a good chance your harvested fields are green, but maybe most of that are corn and soybean seedlings that have germinated after being victim to shatter loss from the combine header. No need to worry about those, since the frost will soon take care of that volunteer crop. However, they will soon be followed by such friends as common chickweed, henbit, and various mustard species, says University of Illinois weed specialist Aaron Hager. One of his recent newsletters indicates that early control is better than waiting until the onset of winter. The latter could turn out to be a waste of money.
Your first chore is to identify the types of weeds you have in your field, and then determine if a treatment is needed this fall. Some fields may be clean, but others may be lushly vegetated. And your weed crop may be a function of how much precipitation you have had recently. As you assess your problem, consider which herbicide you may want to use, but your favorite spring herbicide may not be a candidate for fall. For example, Hager says Atrazine is not labeled for fall application, which indicates you should check the label of the herbicide of choice.
Another consideration is the calendar, since Dual II Magnum is restricted to fall applications prior to October 31. It also has a temperature restriction, since it cannot be applied at more than 55º and the temperature must be falling.
As you move into late October and there has been sufficient time for winter annuals to emerge, their vegetative state could be sufficient for treatment with glyphosate or 2,4-D which have little soil activity. But if you want to apply an herbicide with more soil activity, then the earlier in October the better.
Hager says combinations of herbicides can also broaden your spectrum of weed control. That could be a combination of a contact herbicide plus an herbicide that has more soil residual activity.
The label of your herbicide may also restrict the geography for which it can be applied. Operations in the more southern reaches of the Cornbelt may be able to apply later in the season, since weed growth will be occurring later in the year, and begin again earlier the following year.
When considering the time of application of an herbicide with soil residual activity, keep in mind that its activity and weed growth are on pause during the coldest months, and it may not have time to control your weed crop by the time you want to get into the field in the spring. Hager says that if winter annuals are controlled so their presence is minimal early in the spring, there will be earlier germination for typical summer weeds.
Just like a nitrogen application to corn, it is better when herbicides are applied at the time when weeds are actively growing, so Hager says fall herbicides will not have much impact on summer weeds.
Some weeds that are biennials, such as thistle, wild carrot, poison hemlock, and others, can be more effectively controlled in the fall than in the spring, particularly after they have produced a stalk that will produce seeds. The initial phase that forms a rosette close to the ground will be the target for the herbicides that are most effective. Hager says the best control may be glyphosate or 2,4-D.
Perennials which store food in their root system which make them more immune to herbicides that kill the vegetative part of the plant, since they have the ability to survive while growing new leaves. Hager says the best control of those will depend on the time of year the herbicide is applied, and spring may not be that time of year. Better control may be in the fall when energy is moving from leaves down to the roots, which could be the time when the plant flowers in the fall prior to seed production.
While you may have a great plan to control such weeds, it can be fouled by the frost. There is no point on applying a contact herbicide to a leafy plant the day before a frost will kill the leaves and they drop to the ground with your herbicide investment.
Control of winter annuals can save money that is lost from their use of nutrients and moisture needed for the spring crop, but strategy has to be used. Type of herbicides must be matched with the type of weeds along with the timing of the application. Different weeds will exhibit different life cycles in the fall, and may be good candidates for control either early or late in the season, or maybe not at all. Some weeds may be easily controlled with a contact herbicide, but others need a good soil residual herbicide, and the herbicide label will not only be a guide, but keep you within the limits of the law.