Identifying the weed and trying to determine why it has invaded is the first step in managing broadleaf weeds in lawns. Weeds can be indicators of underlying problems. For example, ground ivy invades lawns in shade, while knotweed may indicate soil compaction. Assorted weeds may indicate overall poor conditions for lawn grasses and/or poor management.
After identifying the weeds present, step two for controlling broadleaf weeds should be to review lawn care practices and make adjustments as needed to assure a good stand of grass. Sound lawn care practices should promote a healthy, vigorous turf able to prevent and compete with weed invasions. These practices include proper selection and establishment, fertilization, watering, mowing, thatch management, and related practices. Alter the environment that may be favoring weeds, such as reducing shade or improving poor soil conditions.
The third step is removal of existing broadleaf weeds. Pulling by hand is one option; be sure to get as much of the root system as possible. There are a number of broadleaf weed herbicides (weed killers) available for use on lawns. Only apply to actively growing weeds. Choices found in garden centers typically include 2,4-D (2,4-dichlorophenoxyacetic acid); mecoprop or MCPP (2-(2-methyl-4-chlorophenoxy) propionic acid); or dicamba (3,6-dichloro-o-anisic acid); with two and three-way combinations available. Additional herbicides are available for commercial landscape care services for use on lawns. The Illinois Commercial Landscape and Turfgrass Pest Management Handbook and Illinois Homeowners Guide to Pest Management (order through local Extension office) offer the latest information on various weeds and herbicides to control them.
Thoroughly read, understand, and follow all information on herbicide labels. There are general guidelines for using broadleaf herbicides on lawns. Avoid windy days, as these materials can damage many landscape and garden plants if they drift (spray droplets land off the lawn). Also avoid hot days (over 85 degrees F). It's best to have adequate soil moisture, but no rain for 24 hours after application. Don't mow for few days before and after application. Consider spot treating weeds rather than broadcasting weed killer over the entire area. Use caution on newly seeded areas; wait 4 mowings before treating a newly seeded lawn and wait 30 days before seeding an area treated with broadleaf herbicides. Refer to the label regarding any potential hazards when used on lawns over the root zone of trees (such as with dicamba).
Early to mid fall can be a good time to control perennial broadleaf weeds, such as dandelions. Control may be good as weeds prepare for winter dormancy and lawns fill-in bare areas created by the weed dying readily in the cooler weather of fall. Spring and early summer applications may not provide as good of control as fall. In addition, warmer temperatures increase the chance of lawn injury. Regardless the time of the year, weeds need to be actively growing for herbicides to work.