Preemergence Herbicide Use for Lawn Care
Timing and moisture are the two main keys to grassy weed control
Spring is a time to focus on control of summer annual weeds such as crabgrass (Digitaria spp.) and goosegrass (Eleusine indica). These species complete their life cycle in one year, germinating from seed in spring, growing throughout summer, and setting seed in fall. If left uncontrolled, both crabgrass and goosegrass can reduce the aesthetic and functional quality of warm- and cool-season turf lawns.
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LCOs should make their first preemergence herbicide application as soon as soil temperatures (at approximately 2 inches) reach at least 55 degrees Fahrenheit for a minimum of three days in spring.
PHOTOS COURTESY OF J.T. BROSNAN, UNIVERSITY OF TENNESSEE.
An effective means for controlling summer annual weeds is the use of preemergence herbicides. Numerous products can be used to provide preemergence control of summer annual weed species in residential and commercial lawns. A list of preemergence herbicides labeled for use on warm- and cool-season turfgrasses commonly found on residential and commercial lawns is presented in Table 1. Note that several of the products listed in Table 1 (page C4) require reduced application rates when applied to certain turfgrass species or can only be used on a particular species when turf is mowed above a certain cutting height. Be sure to consult herbicide labels for more detailed information on turfgrass tolerance to various preemergence herbicides.
Keys to success
There are two keys to effectively controlling summer annual weeds with preemergence herbicides. First, be sure to apply these materials before weeds have emerged from the soil. These herbicides do not prevent weed seed germination; rather, they prevent germinated seedlings from developing into mature plants.
Considering that the time frame between germination and emergence can be quite short, it is often recommended that these herbicides be applied once soil temperatures are favorable for crabgrass seed germination. Lawn care professionals should make their first preemergence herbicide application as soon as soil temperatures (at approximately 2 inches) reach at least 55 degrees Fahrenheit for a minimum of three days in spring.
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Goosegrass is a widespread species that tolerates a broad range of environmental conditions, but does not survive frost.
Forsythia plants can be a helpful indicator of when this benchmark soil temperature has been reached. The distinctive yellow flowers on Forsythia plants bloom at soil temperatures similar to those that facilitate crabgrass seed germination. Be sure to apply preemergence herbicides before forsythia plants have completed flowering.
The second key is to water preemergence herbicides into the soil after application. Most labels require that .25 to .5 inch 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 lawns without irrigation, try to time preemergence herbicide applications around a period of rainfall.
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Figure 1. Smooth crabgrass (Digitaria ischaemum) control five months after initial preemergence herbicide treatment at the University of Tennessee Center (Knoxville, Tenn.) in 2012. Means from the .6-inch (15-mm) and 2-inch (50-mm) heights of cut were pooled across six different herbicide chemistries.
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 eight to 10 weeks apart. A single 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 tend to provide a longer period of control, and will provide control of species that germinate later in the year than crabgrass (e.g., goosegrass, etc.).
Research conducted at the University of Tennessee 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 .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 .6 inches regardless of application regime.
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Foresythia plants in bloom indicate when soil temperatures are favorable for crabgrass germination and when you should apply a preemergence herbicide.
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 (.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; however, this height of cut may not be acceptable in all situations.
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.
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 athletic field traffic in spring; control measured 95 to 99 percent by five 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 belowground rhizomes rather than stolons.
Follow-up research was initiated in 2012, evaluating 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 does not affect hybrid bermudagrass traffic tolerance or recovery.
Numerous preemergence herbicides are available for controlling annual grassy weeds in warm- and cool-season turf. 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 www.tennesseeturfgrassweeds.org.
J.T. Brosnan is assistant professor University of Tennessee Plant Sciences Department and G.K. Breeden iss extension assistant Weed Control University of Tennessee.
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.