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inches) measure ≥ 55 degrees Fahrenheit for a minimum of three days in spring. Ornamental forsythia plants can be a helpful indicator of when this benchmark soil temperature has been reached. Forsythia plants produce distinctive yellow blooms at soil temperatures similar to those that facilitate crabgrass seed germination. Thus, the presence of yellow petals on forsythia plants serves as an indicator of when preemergence herbicides for summer annual weed control should be applied in spring. Turf managers should be sure to apply preemergence herbicides before forsythia plants have completed flowering. A second key to effectively controlling weeds with preemergence herbicides is to water them into the soil after application. Most labels require that 0.25 to 0.50 inches of irrigation or rainfall be applied within 24 to 48 hours after application. These herbicides are absorbed by germinating seedlings in the soil profile so moving them into the rootzone is critical. Failure to irrigate after application can also lead to material being lost due to volatilization. On fields without irrigation, try to time preemergence herbicide applications around a period of rainfall. Safety in 2012 evaluated the effects of mowing height on the efficacy of single and split applications of preemergence herbicides for crabgrass control. A total of six different herbicides were evaluated. At a 0.6-inch mowing height, split application regimes provided greater crabgrass control than single applications regardless of product. When mowing height was increased to 2 inches, no significant differences were detected between single and split application regimes regardless of product (Figure 1). Five of the six herbicides tested provided greater crabgrass control when applied to turf maintained at 2 inches compared to 0.6 inches regardless of application regime. While this experiment will be repeated again in 2013, these preliminary results indicate that split application regimes provide better control than single applications at low (0.6 inch) heights of cut. Additionally, increasing mowing height can improve the efficacy of preemergence herbicides for crabgrass control. Increases to 2 inches may reduce the need for split application programs altogether. Conclusions Split applications Split (also referred to as "sequential") application programs of preemergence herbicides tend to provide more consistent control of summer annual weeds throughout a growing season. These programs typically apply the total amount of active ingredient for the season in two applications spaced 8 to 10 weeks apart. A single herbicide application in spring for preemergence control of crabgrass will slowly be broken down by soil microbial activity over the course of a summer, often leading to crabgrass breakthrough by fall. Split application programs delivering active ingredient two times throughout a season tend to provide a longer period of control. Additionally, split application programs will control species germinating later in the year than crabgrass (e.g., goosegrass, etc.). Mowing height Research conducted at the University of Tennessee Center for Athletic Field www.landscapeirrigation.com Research was conducted at the University of Tennessee Center for Athletic Field Safety during 2009 and 2010 evaluating the effects of four preemergence herbicides on Tifway hybrid bermudagrass traffic tolerance and recovery (1). Over the course of the two-year study, no differences in smooth crabgrass (Digitaria ischaemum) control were detected among herbicide treatments after being subjected to traffic in spring; control measured 95 to 99 percent by 5 months after treatment. Moreover, no differences in Tifway traffic tolerance or recovery were reported in either year. We hypothesized that this response was due to Tifway recovering predominately from below-ground rhizomes rather than stolons. Follow-up research was initiated in 2012 to evaluate the effects of preemergence herbicide applications in spring on Tifway traffic tolerance in fall. After the first year of the study, no differences in fall traffic tolerance were detected due to herbicide treatment in spring. To date, these findings illustrate that use of preemergence herbicides to control weeds on bermudagrass athletic fields does not affect traffic tolerance or recovery. Figure 1. Smooth crabgrass (Digitaria ischaemum) control five months after initial preemergence herbicide treatment at the University of Tennessee Center for Athletic Field Safety (Knoxville, TN) in 2012. Means from the 0.6 inch (15 mm) and 2 inch (50 mm) heights of cut were pooled across six different herbicide chemistries. Concerns over traffic tolerance It is well documented that many of the preemergence herbicides used to control annual grassy weeds can inhibit bermudagrass root growth. Reductions in root growth in the uppermost portion of the soil profile could potentially compromise bermudagrass traffic tolerance and recovery; thus, rendering the benefits of effective weed control moot. Numerous preemergence herbicides are available for controlling annual grassy weeds. Always refer to the product label for specific information on proper use, tank-mixing compatibility and turfgrass tolerance. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the University of Tennessee Institute of Agriculture. For more information on turfgrass weed control, visit the University of Tennessee's turfgrass weed science website at LI www.tennesseeturfgrassweeds.org. J.T. Brosnan is assistant professor, turfgrass weed science; G.K. Breeden is an assistant specialist; J.C. Sorochan is associate professor, turfgrass science & management; and A.W Thoms . is research leader, all at the University of Tennessee in Knoxville. Literature Cited 1. Brosnan, J.T., Breeden, G. K.,Thoms, A.W., and Sorochan, J. C. 2011. Preemergence herbicide efficacy under athletic field traffic. Online. Applied Turfgrass Science. doi:10.1094/ATS-2011-1128-01-BR. Landscape and Irrigation 17