Landscape - Winter Annual Weed Control in Landscape Beds

It is time to apply premergence herbicides to control winter annual weeds in landscape beds (before the end of August). The following is some information.

Winter annual weeds start to germinate in late summer when soil temperatures have cooled down. Soil temperatures are usually cool enough by the end of August to get some germination, especially in irrigated beds. Common winter annuals include chickweed, henbit, purple deadnettle, field violet, annual speedwells and mustard family weeds including bittercress. Winter annual grasses include annual bluegrass, annual ryegrass, and downy brome. Several short lived perennials and biennials will also be germinating at this time such as mouseear chickweed and marestail (horseweed). To prevent winter annual weed germination, apply preemergence herbicides to landscape beds before the end of August. OH2 and Rout are examples of preemergence materials highly effect in landscapes on mustard family weeds, speedwells, Lamiums (henbit and purple deadnettle), horseweed, and annual bluegrass. They are weak however on chickweed which is better controlled with herbicides containing isoxaben (Snapshot, Gallery), pendamethalin (Pendulum, others), trifluralin (Snapshot, others), or prodiamine (Barricade). Preemergence herbicides can be effective in reducing late summer germinating winter annuals; however, many of these weeds germinate from fall through spring requiring long residual materials or repeat applications. Check with each product label for compatibility with specific landscape plants and efficacy on specific winter annual weeds. Most of these herbicides will require some irrigation to activate them.

Gordon Johnson, Extension Horticulture Agent, UD, Kent County.