Preemergence herbicides and bermudagrass sports turf
It seems like a topic too often revisited, but preemergence herbicides really are a linchpin of an integrated weed management strategy. Next to proper cultural practices that promote a healthy and competitive turf, preemergence herbicides are one of the most important factors influencing weeds present throughout the growing season. Annual weeds, including crabgrass, goosegrass, and annual bluegrass, can be controlled with properly timed preemergence herbicide applications in the spring and fall.
Some preemergence herbicides inhibit health of desired turfgrass, which complicates recovery from wear and tear, as well as from seed, sod, or sprigs. Some are safer than others, but safety is rate and timing dependent. Our research at Mississippi State University evaluated effects of almost a dozen common preemergence herbicides on grow-in from sprigs. Over a 2-year period, Erick Begitschke, myself, and colleagues, evaluated establishment of field and greenhouse grown bermudagrass to determine how commonly used herbicides impact grow-in as well as root architecture, considered to be an important factor in resource allocation from the soil, as well as a key factor in shear strength.
In many ways, sprigging establishment isn’t that much different from regrowth and recovery of sports fields. The wear and tear of routine use and maintenance damages foliage and roots; bermudagrass stolons and rhizomes are torn or crushed, and fine root hairs are dislodged from the soil that they are mining for nutrients and water.
Many herbicides, if not all, can affect grow-in and stand resilience if not applied at the appropriate rates and grass maturity. Although labeled for sports fields, many preemergence herbicides should only be applied once turf is well established to avoid having negative effects. Sports turf managers must also take into account the spectrum of weeds controlled by each herbicide. What follows is a brief discussion about pros and cons of several herbicides commonly used in sports field management as they relate to general weed control and grow-in/regrowth effects.
Mitotic inhibiting herbicides specifically inhibit cell division in the roots, causing a signature “stubby” or “bottle-brush” root symptomology. These herbicides have been known to initially injure bermudagrass roots and delay grow-in of sprigs. However, in many cases, bermudagrass is able to recover from these initial injury symptoms, especially when lower rates are used. It is important to read and follow all label instructions, as some of these herbicides are prohibited from being applied prior to perennial ryegrass overseeding.
All three of these herbicides can provide adequate fall and spring preemergence activity on annual grasses and most broadleaf weeds. Pronamide on the other hand, doesn’t provide much crabgrass or goosegrass control relative to others mentioned above. It does, however, uniquely control annual bluegrass both pre- and post-emergently. One can debate the effectiveness, but when it works it works very well. Other times it just fails due to environment or perhaps plant size.
Regardless of mitotic inhibitor, all lack the longevity for adequate season long control of goosegrass. For this reason, multiple applications (roughly twice in the spring/early-summer on an 8-week reapplication interval) are required for season-long control. On healthy turf, that amount of preemergence might not be a problem, but on thinned or compromised stands, it might cause some frustration.
Another concept to be familiar with is the combination of preemergence with postemergence herbicides. As an example, the product Echelon (prodiamine + sulfentrazone) has been marketed for its combination pre/post-emergence activity on goosegrass. Sulfentrazone has no preemergence activity to speak of, so Echelon or other sulfentrazone containing products must be applied after goosegrass has germinated in order to take advantage of the pricey sulfentrazone treatment. Even then, no product works well on goosegrass postemergence unless applied at very early growth stages, so a close monitoring of emergence is necessary to leverage the post activity.
Furthermore, other effective goosegrass post-emergence herbicides, such as Revolver (foramsulfuron) or Speedzone (2,4-D, mecoprop, dicamba, carfentrazone) may also be combined with mitotic inhibitors for pre/post activity on early-emerging goosegrass.
Photosystem II (PSII) inhibitors
PSII inhibitors disrupt the flow of light energy within the plant. There are many reported instances of herbicide resistance to these herbicides, particularly in populations of annual bluegrass, which limits their effectiveness on sports turf. However, atrazine and simazine are especially notable for having both pre- and postemergence activity on many winter broadleaf weed species. Applications of PSII inhibitors have been known to initially injure bermudagrass, particularly during the middle of the growing season in high temperatures. These PSII inhibitors are not effective for prevention and control of crabgrass and goosegrass. However, they are commonly applied in the fall and winter with preemergence herbicides, such as mitotic inhibitors, for increased viability.
In our research, PSII inhibitors rarely reduce time to grow-in, but they do have the potential for reducing root length and total root biomass. These effects are poorly understood, but still warrant caution on weakened turf stands.
There are several advantages that may outweigh potential detriments. For one, both herbicides are cheap. So the saying goes, “it’s too cheap not to spray.” Our research, as well as that of others, indicates that simazine in combination with prodiamine as a fall preemergence is extremely effective against annual bluegrass and winter annuals. Likewise, combinations of simazine plus trifloxysulfuron, foramsulfuron, or flazasulfuron are an effective early post-emergence treatment for annual bluegrass.
PROTOX inhibiting herbicides target a crucial step in the chlorophyll biosynthesis pathway. When applied as a liquid to non-dormant turf, both oxadiazon and flumioxazin can burn the leaf tissue. Both can be applied on a fertilizer or inert granular carrier, which minimizes the risk of injury. However, just like other herbicides discussed, both of these herbicides have the potential to reduce root growth temporarily after use. In terms of weed control, the two herbicides are somewhat different.
Of all preemergence herbicides, granular applied oxadiazon is probably the safest for bermudagrass grow-in and recovery. For this reason, it’s frequently used in sod production, where unhindered bermuda growth is paramount. When applied on a granular carrier, oxadiazon provides good to excellent preemergence control of annual bluegrass, but it somewhat lacks preemergence broadleaf control. As a liquid application on dormant turf, or as a granular application during spring green up, oxadiazon provides good control of crabgrass and excellent control of goosegrass. For best results, apply at 3 lbs. oxadiazon per acre rate. A common spring rotation might involve an initial liquid oxadiazon application while dormant, then a follow-up treatment with a mitotic inhibitor and some effective postemergence goosegrass/nutsedge material. Or, another spring rotation might rely upon a mitotic inhibitor for the first preemergence crabgrass application followed by a granular oxadiazon application.
Flumioxazin is rarely used on a granular carrier, but when applied as a liquid to dormant turf, it provides good postemergence control of annual bluegrass and other winter annual weeds. It has limited preemergence activity on crabgrass and goosegrass.
Very Long Chain Fatty Acid synthesis inhibitor
S-metolachlor (Pennant Magnum)
Similarly to the mitotic inhibiting herbicides, S-metolachlor and dimethenamid-P have been known to delay grow-in and initially injure bermudagrass roots. Both are labeled for control of annual grasses and sedges in warm-season turfgrass species. Research indicates that annual grass control is somewhat limited relative to other options discussed herein. However, tank mixing with either simazine or atrazine is a common practice to enhance S-metolachlor’s preemergence activity on annual bluegrass and broadleaf weed species. Both of these products provide some suppression of nutsedge. For this reason, it seems they’re most often tank-mixed with late spring preemergence goosegrass applications. Dimethenamid does have some postemergence activity on goosegrass, but it’s not clear whether it should be relied upon solely. Both of these herbicides can cause foliar injury and turf thinning of bermudagrass, but it is transient.
Cellulose Biosynthesis inhibitor
Indaziflam is particularly harmful to establishing or recovering sports fields. However, its unique mode of action and excellent weed control, even at low use rates, makes this herbicide a viable option where others may no longer work or where it is the only option available for herbicide resistant populations of annual bluegrass. Its usefulness is somewhat limited in scenarios requiring overseeding, as it has at least a one-year restriction on application prior to seeding.
Editor’s note: The information given here is for educational purposes only. References to commercial products, trade names, or suppliers are made with the understanding that no endorsement is implied and that no discrimination against other products or suppliers is intended.
Jay McCurdy, PhD, is an Assistant Professor for Mississippi State University’s Department of Plant and Soil Sciences.