During winter, lawns can become infested with numerous weeds which can be just as troublesome as warm weather weeds. Fall is the best time to take pre-emptive action to control them and reduce their numbers.
Winter annual weeds, such as annual bluegrass, henbit, common chickweed and others, begin to emerge from seed as the temperatures begin to cool. During winter, they have a slow rate of growth. As the temperatures begin to rise in February and March, the winter weeds start growing and developing rapidly.
Winter weeds can cause damage to the turf in addition to destroying the beauty of the lawn. They compete with the turf for available sunlight, soil moisture and plant nutrients. Even though the grass appears dormant, grass roots continue to grow and weeds interfere with this growth. As the weather begins to warm, large mats of weeds begin to die. This can have a detrimental impact to warm season grasses, such as bemudagrass and zoysiagrass, by disrupting the spring green-up process. The turfgrass can be severely stunted or have large dead areas that can easily be infested by summer annuals, such as crabgrass, in addition to being more susceptible to insects, diseases and environmental stresses.
Keeping the lawn healthy and properly maintained will help reduce the weed population. Make sure it receives appropriate amounts of fertilizer and water. Mow at the correct height for the turf species and remove no more than one-third of the leaf blade at each cutting. These practices will increase the effectiveness of using chemical herbicides for weed control.
The use of pre-emergent herbicides, which prevent weeds from germinating, are applied in the early fall prior to the germination of winter weeds to reduce their populations. Post-emergent herbicides, which control weeds after germinating from seed, are used on an as needed basis.
If you are over-seeding fescue lawns, the pre-emergent herbicides cannot be used since they will also inhibit the germination of the grass seeds. Control options are limited, so the best way to deal with weeds is to apply post emergent weed killers to control broadleaf weeds.
Use a fertilizer spreader to distribute the pre-emergent herbicide granules uniformly, and follow the recommended labeled rate. Make sure the application of the pre-emergent herbicide is made before rain is expected or water it thoroughly. Poor control of weeds often results from the lack of rain or supplemental watering within 7 days of the pre-emergent application. Do not mow or disturb the soil in any way during or following the first week after the application. Doing so will reduce its effectiveness.
The control of winter weeds is best accomplished by applying pre-emergent herbicides combined with appropriate cultural practices. This will help prevent them from becoming established in the lawn. Remember, always read pesticide labels and follow all safety precautions carefully.
Timothy Daly, MS is an Agricultural and Natural Resource agent with the Gwinnett County Extension Service. He can be reached at 678-377-4010 or firstname.lastname@example.org.