Cold Winter Could Impact Pre Herbicide Decision

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The cold weather this winter is reminiscent of the winters we had during the early 1990’s. From 1990 to 1995, there were varying amounts of winter injury on warm-season grasses throughout the mid-Atlantic. It is too early to determine if, and how much, winter injury has occurred this year. It is well-known that snow cover protects turfgrasses from subfreezing temperatures. Therefore, when temperatures dipped well into the single digits last week, those who had snow cover (particularly the eastern part of the state) got some protection. However, this recent cold snap was not the first time temperatures have been well below 20 degrees.

It is probably a good bet that widespread winter-kill has not occurred as of now. However, it is probably also a safe bet that warm-season grasses (particularly centipedegrass and shorter cut bermudagrasses) have been weakened by the cold weather. The bottom line is, the weather from now until the end of March will dictate the extent of winter injury. If there is some green-up in March followed by temperatures in the low 20s, there is a good chance we will have winter-kill. I think it is safe to conclude that the winter so far, has set us up for a potential problem if we have even mildly bad luck with the weather from now until the end of March.

Winter injury is seldom an “all or nothing” phenomenon. It can range from warm-season grasses being a little slower than normal to green-up to complete kill. It is also typically worse in shady or wet areas. North facing slopes are also more susceptible to winter injury.

Anytime there is a heightened concern of winter injury, it is wise to consider herbicide selection and use patterns on warm-season grasses. Because winter injury results in turf needing to be grown back, full rates of some herbicides can slow that process down. To further complicate the process, PRE herbicides need to be applied in February and early March. This is before we know the extent of winter injury, or in many cases, before we have winter injury because, as previously stated, significant winter injury can occur in late March.

Dinitroaniline herbicides (prodiamine, pendimethalin, oryzalin, etc.) and dithiopyr can inhibit root growth on stolons as turf recovers and grows into thin areas. Inhibiting stolon rooting may cause stolons to be cut off during mowing significantly reducing lateral spread and recovery.

Research conducted in NC in the late 1990s showed these herbicides can be safely used on thin warm season grasses if they are used at reduced rates (usually half rates). Therefore, anytime winter-injury is a concern, it is important to split the applications of the above-mentioned herbicides. For instance, instead of using 3 lbs active ingredient/acre (ai/a) of pendimethalin in late February, use 1.5 lbs ai/a in late February and follow-up with the remaining 1.5 lbs 8 to 10 weeks later. By late April or early May, it will be much easier to quantify if, or how much winter injury has occurred. If significant injury has occurred, you can then refrain from applying the remaining 1.5 lbs. If no injury has occurred, you can proceed with the application.

Recently, indaziflam (Specticle) has been introduced in the warm-season turfgrass market. We know Specticle can affect root growth much like the dinitroanilines and dithiopyr. However, to date, no research has been conducted on growing bermudagrass in with Specticle after winter injury. However, we might expect a similar response as with dinitroaniline herbicides and dithiopyr. Therefore, lower rates of Specticle (2.5 to 3 oz/a of the 20DF and equivalent rates of the Flowable) would represent a likely manageable situation in the case of mild winter injury.

Of course, if turf managers want to totally avoid any of this potential grow-in from winter-injury, oxadiazon can be used in commercial turfgrasses without any danger of affecting turf recovery from winter injury or any other stress that thins tolerant warm-season turfgrasses. In addition, in the event that bermudagrass, zoysiagrass, or St. Augustinegrass is severely injured such that sodding, sprigging, or plugging is needed, this can be done directly into oxadiazon treated areas with no problems with grow-in.