Michigan State University Extension
Ornamental Plants plus Version 2.0 - 00001564
01/01/98
Weeds inevitably become a problem, especially on low quality lawns. Controlling weeds involves more than spraying a weed killer or herbicide. There are several types of weeds and several types of herbicides.
Weeds can be either broad-leaved or grasses. Within each of these two types, weeds may be annual or perennial.
Herbicide are post-emergent, pre-emergent or total vegetation killer. The post-emergent herbicides kill growing weeds and the most common example is 2,4-D. Pre-emergent herbicides like Dacthal kill germinating seeds so the plant never emerges from the ground. Total vegetation killers kill any type of vegetation they land on.
Perennial broad-leaved weeds are controlled with post-emergent herbicides. When the weeds are young and actively growing they absorb the herbicide readily, giving good control. Post-emergent herbicides also control annual broad-leaved weeds that were not controlled with pre-emergent herbicides.
Pre-emergent herbicides kill annual broad-leaved weeds and annual grasses, if they are applied before the seedlings get established. Such herbicides must be applied before seeds germinate.
Perennial grasses are controlled with total vegetation killers. This type of herbicide is also used to chemically edge lawns and to kill weeds in drives and walks. Clumps of perennial grasses in lawns can be killed with these chemicals but surrounding lawn grasses will be killed as well. These chemicals vary in their toxicity so read the label thoroughly before the chemical is purchased. Total vegetation killers are often used prior to lawn establishment.
Most lawn weedkillers contain 2,4-D so do a good job on easily killed weeds such as dandelion, buckhorn and plantain. When the lawn is infested with more difficult-to-control weeds such as chickweed, ground ivy or creeping charlie, use a product that contains 2,4-D plus either Dicamba or MCPP. Some weedkillers contain all three chemicals but will only control actively growing broad-leaved weeds. Combination fertilizer\herbicide products will control the easily controlled weeds but a liquid will give better results when difficult-to-control weeds are present.
The most common use of pre-emergent weedkillers is for crabgrass control. These must be applied in April or early May before the blossoms fall off the forsythia bushes. These weedkillers are sometimes combined with fertilizers for convenient application.
Total vegetation killers are available in squirt bottles for spot treatment or they can be applied to larger areas.
Use one sprayer for herbicides and another for other pesticides. A herbicide residue may linger in the sprayer and injure plants during a subsequent spraying if only one sprayer is used.
Do not spray on windy days. The small droplets will be blown onto desirable plants and may cause injury. Use a low sprayer pressure to increase droplet size and reduce the possibility of drifting.
Do not use the sprayed clippings as a mulch for at least 4 weeks after applying a herbicide.