Annual grassy weeds sprout from seed in the spring, grow, produce new seed for next year, and die all in one growing season. Crabgrass is the most well-known of these weeds. It can be identified by its flat, spreading habit. The use of a pre-emergent herbicide in the spring is the most effective chemical control. Pre-emergents do not allow the germinating weed seeds to grow. Timing is critical. If applied too early, the chemical will dissipate before weeds germinate; if applied too late, the seeds will already have germinated. Annual grassy weeds usually germinate when the ground temperature reaches about 50 degrees. This corresponds to the time that the lilacs are beginning to bloom. In the Twin Cities, this is typically around May 1-10, but will vary from year to year, given actual weather conditions. Pre-emergent herbicides are either packaged by themselves or are combined with a lawn fertilizer. Do not use pre- emergents if you will be seeding your lawn at the same time unless
Perennial grassy weeds die down to the ground in the winter but come back from the same roots in the spring. Quackgrass is the most common perennial grassy weed. It is light green and grows 2-3 times faster than lawn grass. Control of perennial grassy weeds is quite difficult because herbicides cannot distinguish between the “bad” weeds and the “good” lawn grass. A product like Round-Up will control quackgrass, but needs to be used with extreme caution as it will kill all vegetation it contacts. You can either spray the entire weedy area and start over, or you can carefully spray just the weeds and fill in those spots. Since the quackgrass is usually taller than the lawn grass, you can use a paintbrush to paint the weedkiller onto the weeds and miss most of the lawn grass. Never use these chemicals on a windy day and do not apply when rain is forecast in 24-48 hours. Reseeding can be done 7-10 days after herbicide application.
Broadleaf weeds can be either annuals or perennials. Annual broadleaf weeds are controlled the same way annual grassy weeds are. Most broadleaf weeds, however, are perennial - dandelions being the most well known. Dandelions are best controlled when they are actively growing in the spring, or especially in late summer/early fall. Use either a granular product (frequently mixed with lawn fertilizer) or a spray such as Trimec. If using a granular herbicide, apply to a damp lawn with a drop spreader. Apply broadleaf weed killers when rain is not forecast for 24-48 hours and do not apply on windy days. Air temperatures should be above 60 degrees for best control. Use caution around desirable plants and in the root zone area of trees.
Physical methods of weed control include mulching and hand weeding. Mulch should be at least 4” thick to be especially effective. When pulling weeds, it is important to get the entire root out. Rain helps loosen the soil and will make hand weeding easier.
Weeds invade weak areas in a lawn. Grass that is underfed, mowed too closely, or watered too shallowly is prone to weed infestation. See some of our other fact sheets for proper lawn care instructions.
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