W E E D WAT C H Keys to Successful Preemergence
CRABGRASS CONTROL By J.T. Brosnan, Ph.D.; G.K. Breeden; T.J. Samples, Ph.D.; and J.C. Sorochan, Ph.D.; The University of Tennessee nfestations of crabgrass (Digitaria spp.) occur annually on athletic fields, golf courses and residential and commercial lawns throughout Tennessee. Both smooth crabgrass (Digitaria ischaemum) (see Photo 1) and large crabgrass (Digitaria sanguinalis) (Photo 2) are summer annuals that germinate primarily in the spring and grow throughout the summer. If left uncontrolled, crabgrass infestations will decrease the playability, usability and overall aesthetic quality of any turfgrass stand during the growing season. Uncontrolled crabgrass plants will also produce seeds in the fall that overwinter in the soil and germinate the following spring, ensuring that infestations will be a problem that becomes more prolific every year. Most turfgrass managers opt to control crabgrass in spring with preemergence herbicides. Numerous preemergence herbicides are available, and research conducted at The University of Tennessee has shown that these products perform similarly when applied correctly under the same environmental conditions. For information on specific products and rates, refer to UT Extension Bulletin W146, “Crabgrass Control in Turfgrass.” This publication can be downloaded for free from http://tennesseeturfgrassweeds.org. While research has regularly reported that many preemergence herbicides provide an equal level of crabgrass control, failures are reported every year; most of these failures can be attributed to improper application. Steps can be taken to ensure that these herbicides are applied correctly and effectively, resulting in successful crabgrass control this coming season.
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Application timing Improper timing is arguably the most common reason that preemergence herbicide applications fail to control crabgrass. For preemergence herbicides to work properly, they must be applied before seedling emergence; once seedlings are visible, a preemergence herbicide application is too late. Crabgrass seed germinates in the spring when soil temperatures exceed 55°F for four consecutive days and nights. This usually occurs sometime between late February and early March in West Tennessee and in mid-March to early April in East Tennessee. The blooming of the forsythia plant, also known as golden bells, is a visible indicator that soil temperatures are increasing. Therefore, be sure to make the 20
TENNESSEE TURFGRASS
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first preemergence application of the season before the last forsythia bloom falls from the tree. What happens if preemergence herbicides are not applied during these ideal timings? All hope is not lost. Certain herbicides (dithiopyr, marketed as Dimension; and sulfentrazone + prodiamine, marketed as Echelon) can be applied to control newly emerged (1-3 leaf) crabgrass seedlings and provide preemergence control of those that have not yet emerged.
Irrigation or rainfall Preemergence herbicides need to be activated with water. Most labels indicate that treated turfgrass areas should receive approximately 1/2 inch of irrigation or rainfall within 24 to 48 hours. The bottom line is that if the site does not receive irrigation or rainfall, the herbicide will not work. When applying materials to turfgrass sites that are not irrigated, try to coordinate the preemergence herbicide application with a rainfall event.
Application strategy Generally, most preemergence herbicides will provide crabgrass control for 12 to 16 weeks after application, depending on application rate. However, the level of control provided will dissipate over time. Research at The University of Tennessee has found that using a split application strategy — where each herbicide is applied twice at the lower labeled rate — can extend the length of residual control provided by a preemergence herbicide. Additionally, this strategy can provide increased control of other summer annual grassy weeds — like goosegrass (Eleusine indica) — that germinate later in the spring and are harder to time than crabgrass. For split applications, make the first application sometime during late February or early March in West Tennessee or midMarch to early April in East Tennessee. The second application should then be made six to eight weeks later.
Conclusion Always refer to the product label for specific information on proper product use, tank-mix compatibility, and turfgrass tolerance. For more information on turfgrass weed control, visit The University of Tennessee’s turfgrass weed science website, http://tennesseeturfgrassweeds.org.
Email TTA at: tnturfgrassassn@aol.com