Weeds in winter can be recognized by their lighter color green and thin blades. Many weeds, such as common chickweed, henbit, annual bluegrass and others will emerge from the root in early Spring when temperatures warm up. In most cases these weeds are easy to control because they have not accumulated starch reserves in their roots.
These weeds generally need 2-3 weeks of warm weather (55-60 degrees) before you can successfully control them with a preemergence herbicide. If the ground is still frozen in February, then there is no point in applying an herbicide until it has been warm for a couple of weeks.
Winter weed control
In March, you can control winter weeds with a herbicide containing the active ingredient “ferbam” such as Barricade 4E or Hi-Yield Winter Weed Killer Plus 2. Both products have the active ingredient of Methyl Bromide, which is being phased out by January 2009. During the next several years, there will likely be a different active ingredient used in these products as manufacturers phase out Methyl Bromide due to its harmful effects on the Earth’s ozone layer.
In the meantime, the control from these products is excellent and will keep your lawn weed-free until Spring growth starts. In most cases, these products will control winter weeds before they have a chance to emerge from the root. Generally you do not need to water in these products after application, but if rain is lacking, then you may want to give it an occasional light watering so germination will not be inhibited by drought stress.
If you are using Hi-Yield Winter Weed Killer Plus 2, then you must use this product before the weeds have begun to break ground in Spring. If you apply this product after those weeds have emerged from the root, then those weeds will survive and require a second application to control them.
In April or May, an additional weed control application is required to kill any surviving winter weeds as well as provide residual control of crabgrass, goosegrass and others. A product that contains “quizalofop” or “dithiopyr” will be good for this purpose.
You can also use a pre-emergence weed control such as Dimension 4EW if you want to prevent winter weeds from germinating at all. Dimension 4EW should be applied in early September; however, will not give any control of winter weeds that have already germinated.
Another alternative to weed control is to plant a winter cover crop like rye or oats, which will prevent weeds from germinating throughout the winter months. As the cover crop dies by late March or early April, weed seeds stuck in the dead cover crop will not be able to germinate.
If you decide to plant a winter cover crop, then do not apply herbicides (with the exception of Dimension 4EW) as it can injure those crops and reduce their ability to compete with weeds. Most winter annual grasses such as rye, oats, wheat and ryegrass are effective at suppressing winter annual weeds.