Preemergence crabgrass control on athletic fields

Crabgrass species are annual grassy weeds that are problematic on most every athletic field. If left uncontrolled, crabgrass infestations will decrease the aesthetic and functional quality of any athletic field.

The forthcoming loss of an effective, economical postemergence herbicide like MSMA renders the use of preemergence herbicides for crabgrass control increasingly more important. While registrants of MSMA for use on sports turf can no longer sell the product, distributors will be able to sell products purchased before December 31, 2009 until December 31, 2010. After December 31, 2010 existing stocks of MSMA can legally be used for weed control on sports field turf until they are exhausted provided that these uses comply with the EPA-approved label and labeling of the affected product1. Once supplies have been exhausted, preemergence herbicide applications will be one of the main tactics used to control crabgrass infestations on athletic fields.

Crabgrass characteristics

There are two primary crabgrass species that infest athletic fields: large crabgrass and smooth crabgrass. Large and smooth crabgrass can be differentiated by examining the leaves and stems of each species. Large crabgrass has pubescence (hairs) on its leaves and stems, while the leaves and stems of smooth crabgrass have very little pubescence except on the collar region (the intersection of the leaf and stem of the plant). Both large and smooth crabgrass are summer annuals that germinate primarily in the spring, grow through the summer, produce seeds in the fall, and die following the first killing frost.

Germination and preemergence herbicides

Correct application timing is an integral part of controlling crabgrass with preemergence herbicides. A common misconception is that preemergence herbicides act by preventing weed seeds from germinating. These herbicides actually prevent germinating seedlings from developing into mature plants. For preemergence herbicides to work properly, they must be applied before seed germination and need approximately 0.5 inch of rainfall or overhead irrigation within 24-48 hours after application in order to be activated.

Large and smooth crabgrass seed germinates in the spring when soil temperatures exceed 55 F for 4 consecutive days and nights. Initial applications of preemergence herbicides for crabgrass control should be made before these temperatures occur in your region. In the transition zone applications are normally made in late February through early April.

The blooming of the forsythia plant, also known as golden bells (Figure 3), is a visual indicator that soil temperatures are increasing to a level conducive for crabgrass seed germination. Using this visual indicator, the first preemergence herbicide application of the season should be made before the last forsythia bloom falls from the tree.

Preemergence herbicides will generally provide crabgrass control for 12-16 weeks after application depending on application rate. However, the level of control provided will dissipate over time. Using a split application strategy where an herbicide is applied twice at a lower rate can extend the length of residual control provided by that application. Additionally, a split application strategy will offer improved preemergence goosegrass control.

Research

Each year the University of Tennessee evaluates numerous preemergence crabgrass herbicides in different locations and environmental conditions. In most years, labeled rates of prodiamine (Barricade), dithiopyr (Dimension), pendimethalin (Pendulum Aquacap), oxadiazon (Ronstar), and prodiamine + sulfentrazone (Echelon) all provide effective crabgrass control (> 90%) throughout the season when applied properly.

Some preemergence herbicides do provide activity against crabgrass plants that have emerged from the soil seedbank. Early postemergence applications of Dimension (1 tiller or less) and Echelon (1-3 leaf) have been found to provide a level of smooth crabgrass control similar to applications of the same herbicides at recommended preemergence timings. While these are the only two preemergence crabgrass herbicides that exhibit postemergence activity, control from these applications has been reported to be inconsistent in other locations. In general, preemergence herbicides should be applied before crabgrass seed germination. Any postemergence activity should be considered an added bonus when these herbicides are made at later than optimal timings

Developing a preemergence program