Landscape and Nursery - Winter Annual Weeds

The following is some information on winter annual weeds that will be resuming growth as the weather warms up. Some control recommendations are also given.

Over the past several years we have noticed that winter annual weeds have become increasingly widespread and problematic in turfgrass and landscape beds. Winter annuals and some perennial weeds germinate in the late summer/fall, produce vegetative growth, and then will go dormant in winter. As temperatures warm in March and April these weeds will undergo explosive vegetative growth followed by a reproductive phase in late April/May. The increasing pervasiveness of these weeds has been due to moderate fall seasons followed by high amounts of soil moisture as these weeds break dormancy in March/April.

Although these weeds will generally mow out by late May/early June, in many turfgrass sites weed infestations may be so heavy that an application of broadleaf weed herbicide may be warranted to reduce competition with desired turfgrass. If a decision is made to apply a broadleaf weed herbicide, keep in mind that many winter annuals such as chickweed and henbit are tolerant to 2,4-D so be sure the herbicide mixture you choose contains herbicides other than 2,4-D such as dicamba, triclopyr, 2,4- DP (dichloroprop), MCPA/MCPP, or fluroxypyr. For optimum weed control with broadleaf weed herbicides daytime highs should at least be in the 60’s. In addition, there is a greater risk of injury to desired turfgrass if these herbicides are applied under current cool weather conditions. We recommend that broadleaf weed herbicide applications be delayed until sustained warmer temperatures occur.

Wild garlic can be especially problematic. For wild garlic control 2,4-D is a good choice but the ester formulation should be used in place of the amine formulation. Due to the orientation of the leaves of wild garlic, the addition of a surfactant, preferably a high quality non-ionic surfactant with 80 or 90% active ingredients, may increase the retention of the spray solution on the leaves and improve control.

One winter annual weed that has become increasing problematic over the past through years in turfgrass and landscape beds is hairy bittercress. This weed grows in a rosette and is generally not noticed in turfgrass sites until it produces a flower stalk in April/May. Fall or Spring applications of 2.4-D will control emerged bittercress but will not prevent subsequent germination from the soil seed bank. Although an expensive proposition, September applications of Gallery will provide residual control of this weed as well as other winter annuals but not wild Garlic. In landscape beds an early spring application (prior to bittercress flowering) of organic mulch applied at 3 to 4 inches will smother and provide suppression of this weed. More effective control can be obtained with September applications of herbicides such as Snapshot, OH2, or Rout. Always consult the herbicide label to see if all desired plants within the bed are tolerant to these herbicides.

All winter annuals can be effectively controlled with non-selective herbicides such as Roundup, but make sure the spray does not contact desired plants.

Modified from "Get Ready! The Weeds Are Coming" by Stephen E. Hart, Ph.D., Specialist in Weed Science and Patrick McCullough, Program Associate in Weed Science, Rutgers University in the Plant and Pest Advisory Newsletter http://njaes.rutgers.edu/pubs/plantandpestadvisory/2008/ln0313.pdf