It is early February and already we are experiencing warm, spring temperatures. It is time to apply a pre-emergent herbicide to control crabgrass and goosegrass. Crabgrass begins to germinate when the soil temperatures reach 57 degrees. Since most of us do not track soil temp, a good rule of thumb is to apply pre-emergent herbicide once we receive several 70-degree days. Goosegrass usually begins to germinate a week after crabgrass. Most summer annual weeds begin to germinate later in April.
Pre-emergent herbicides are used to prevent weed seedlings from growing and most provide 60 days of control. There are various pre-emergent herbicides on the market and most them are safe on all warm season grasses. Most pre-emergent herbicides are effective at controlling annual grassy weeds and some broadleaf weeds. The broadleaf weeds controlled by pre-emergent herbicides vary by product, so if you need help determining an effective product for your landscape contact the Clemson Extension Service.
Pre-emergent herbicides will not control any of the weeds you are seeing in the landscape now. A post-emergent product is effective at controlling actively growing weeds. Do not use a weed and feed product, as it is too early to apply fertilizer. Your lawn is not actively growing and the nitrogen in the fertilizer component will leach out of the soil or worse: It can contribute to disease developing in your lawn. It is best to wait until after the grass has greened up completely and nighttime temperatures consistently stay above 70 degrees, or until after the beginning of May.
To achieve the best control with a pre-emergent herbicide application, split the recommended dose in half. Apply the first half to your lawn in a vertical direction. The second half of the product should be applied in a horizontal pattern. This will guarantee complete coverage of the lawn. Pre-emergent herbicides need to be watered in with a half inch of water to allow it to move through the soil and reach an effective depth to stop germinating weed seeds. Products need to be watered in within 7-10 days to prevent sunlight from breaking them down and losing effectiveness.
To calibrate your sprinkler system, take 5-7 straight edge canisters, such as cat food cans or tuna fish cans, and place them out in the irrigation zone. Turn the sprinkler on for fifteen minutes and then measure the water collected in each can. This will help you determine how long your sprinkler system must run to apply a half inch. This calibration hack also works well for oscillating sprinklers.
A note of caution for centipede lawns: Most pre-emergent herbicides belong to the DNA chemical family and stop weeds from growing by preventing cells in the root system from elongating. This causes the roots to remain very small and ineffective at taking up water and nutrients. Centipede stolons or runners have shallow roots and DNA pre-emergent herbicides can prevent centipede grass from successfully rooting into the soil. This has a long-term effect of keeping centipede lawns thin and allowing more weeds to thrive in the lawn. A better approach for centipede lawns is to split the recommend rate of pre-emergent in half. Apply the first half early in the season and the second half of the application sixty days later. This provides effective control of weeds while limiting the effect of clubbed roots on turf.
St. Augustine lawns can also suffer from the effects of clubbed roots but there are limited products available for post-emergent herbicide applications. It is a better practice to limit weeds through pre-emergent applications.
Culturally, there are many things homeowners can do to prevent weeds in their landscape. Keep your lawn cut at the upper end of the recommended mowing heights for your grass species. This helps to shade out weed seeds. Most weed seeds need light to germinate. In fact, crabgrass is an indicator weed and usually signifies that the grass is being mowed too low or that soil is too moist.
A golden rule of lawn maintenance is to never remove more than a third of the blade while mowing. Scalping the lawn stunts the grass and can stop growth for up to forty-eight hours depending on the lawn species. Routine scalping sends the lawn into dormancy earlier and gives weeds the advantage.
Turfgrass only needs an inch of water a week to perform exceptionally well and most warm season turf has excellent drought tolerance. Only watering your lawn as needed encourages the grass to develop a deeper root system, which can better tolerate environmental stresses and limits the amount of moisture in the soil that is required for weed seeds to germinate.
Fertilize your lawn as recommended. Each turfgrass has different nutritional requirements and over or under applying fertilizer can encourage weeds to take hold in your landscape.
For more information on caring for lawn, please visit Clemson Home and Garden Information Center at hgic.clemson.edu.