Summer Turfgrass Weed Control

Compacted soil at a dog park.

Compacted soil thins turf and gives an advantage to weeds like annual bluegrass pictured here. Photos by Eric DeBoer

Dry turfgrass.

Drought-stressed zoysia in Baton Rouge during the summer of 2023.

Best Management Practices

Regular mowing at the appropriate height for your grass type promotes dense, weed-resistant turf. St. Augustinegrass and bahiagrass perform best when mowed no lower than three inches. Aim for a 1.5-2.5-inch height of cut for bermudagrass, zoysia and centipedegrass. When mowing, never remove more than one-third of the total length of the grass blade at any one event. This rule is commonly referred to as the “one-third rule.” A simple method to help avoid breaking the one-third rule is to multiply your desired height of cut by 1.5, and then mow before it reaches that height. For example, if you’re mowing at three inches, you need to mow before the grass is 4.5 inches tall (3 inches × 1.5 = 4.5 inches). Frequent mowing is the No. 1 thing you can do to increase turf density while simultaneously discouraging competition from weeds. Many weeds can’t handle the repeated defoliation of mowing like our turfgrasses are able to. Just mowing your lawn at least once per week throughout the growing season can have a great impact on turf density and weed encroachment. Even though mowing is great to do, it will always be a stress for the grass. Sharp mower blades reduce the amount of stress placed on the grass from mowing. Dull blades fray the leaves, resulting in a brown tinge on the leaf tips and jagged wounds that don’t heal as quickly as clean cuts do.

I am going to sound like a broken record, but dense, healthy turfgrass is the best defense against weeds. Spring and summer are the main seasons to focus on nitrogen (N) fertilization of the warm-season grasses that grow throughout Louisiana. Nitrogen fertility ensures that your lawn has the nutrients necessary to outcompete weeds vying for a spot in the lawn. When fertilizing, N is typically measured as pounds of N per 1,000 square feet of grass. The amount of N contained in a bag of fertilizer can be determined by multiplying the weight of the bag by the percent N on the label (the first of the three numbers N-P-K on the bag). For example, a 50-pound bag of fertilizer with an analysis of 46-0-0 (N-P-K) contains 23 pounds of N (50 pounds × 46% = 23 pounds of N). You will need to determine your lawn size to know how many pounds of N are being applied per square foot. For bermudagrass and St. Augustinegrass, aim to apply up to a total of 3 pounds of N per 1,000 square feet from April through August, never applying more than 1 pound of N per 1,000 square feet in any one event. Zoysia, bahiagrass and centipedegrass can get by with up to 2 pounds of N per 1,000 square feet throughout the season, and carpetgrass will be able to get away with less than that.

Proper watering practices consist of deep and infrequent irrigation cycles applied in the early morning hours. Frequent or daily irrigation keeps the soil surface moist, which is exactly what weed seeds need to germinate and infest your lawn. Remember that deep and infrequent mean different things for different grasses and soils, but the idea is to maximize the window between irrigation events. Aim to water your lawn no more than three days per week. Irrigation windows can be tightened in the event of extreme heat and drought. Avoid overwatering to prevent waterlogging and disease. Limit irrigation to outside the hours of 9 a.m. to 6 p.m. as the evaporative potential is highest at this time of day. Watering in the early morning hours is ideal. Aim to deliver approximately 1 inch of water to your lawn weekly by supplementing rainfall with irrigation. Purchase a rain gauge to keep track of Mother Nature’s free irrigation and supplement with your irrigation system to reach an inch of water per week.

Additionally, aerating compacted soil can enhance water infiltration, gas exchange, root growth and overall lawn resilience, especially in Louisiana’s heavy soils. Many weeds can outcompete turf in compacted soils. Weeds like annual bluegrass (Poa annua), goosegrass (Eleusine indica) and dallisgrass (Paspalum dilatatum) can be indicators of soil compaction. April through July is a great time for a core-aerification to reduce compaction and increase root growth. Lawn care professionals can aerate your yard, or you can rent an aerator for a DIY approach.

Chemical Controls

When all of the best management practices for lawn maintenance still aren’t enough to keep weeds out of your lawn, chemicals may become necessary for difficult-to-control weeds. Summer is too late to control summer annual weeds with preemergence herbicides, and many postemergence options work best on seedling weeds, so the sooner you spray, the better off you’ll be. Always make sure to fully read the label of the herbicide you’re using and wear any suggested personal protective equipment. Reference the Louisiana Suggested Chemical Weed Management Guide, available as a free download from the LSU AgCenter website, for chemicals that control your specific weed in question. Remember that proper identification is the most important aspect of control. Consider investing in a phone app that can identify weeds or refer to the LSU AgCenter turfgrass website for weed profiles with photos and chemical controls.

5/22/2024 3:25:43 PM
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