Weed Control in "Dormant" Bermudagrass

Weed Control in “Dormant” Bermudagrass Turf

Shawn Askew, Professor and Extension Turfgrass Weed Specialist

The following was an inquiry from a turfgrass manager in Williamsburg, Virginia.

Question (Turf Mgr.): "My question comes to you from Williamsburg where we have some new 'Tahoma 31' bermudagrass sod that was laid this fall, and it is very weedy. I think it is a lot of Deadnettle and or Henbit. I am considering trying to treat it on some of the upcoming warm days, but I am concerned that the new sod might be more susceptible to chemical damage since it has not had very long to root. I was looking at using Cool Power (a.i.’s: Isooctyl, Butoxyethanol Ester, and Dicamba) or a low rate of Glyphosate, but I am not sure how effective either one would be or how much of a risk it would be to the turf. So far, everything I have found listed to control deadnettle states "it is only for use on well-established grass". I would really appreciate it if you could help to point me in the right direction on this matter. I am wondering, is it even worth it to treat this, or if it would be better to wait until it just dies out this year? I have included a few pictures site so you may be able to determine what I am dealing with."

Response (Dr. Askew): We have done several studies on bermudagrass and zoysiagrass response to glyphosate and other nonselective herbicides during dormancy. In our work, studies were conducted on mature turf, but the primary difference between mature and immature turf is the resilience of the turf following herbicide damage and, especially, when using herbicides that impart residual soil activity.

Since you have young turf, we need to limit options to products that do not have any soil activity and, as with any nonselective herbicide, we need to limit the exposure of herbicide to green leaves or shoots of the bermudagrass. Our work shows that glyphosate does not start injuring bermudagrass until after the green shoots are visible above the dormant canopy. My rule of thumb is that dormant turf where green shoots or leaves of turf cannot be seen from a standing position is most likely safe to treat. When you do get down on your knees and dig into the turf canopy, we generally want to see less than 15 green leaves or portions of green leaves under an area the size of your hand if the mowing height is greater than 1 inch for bermudagrass. At athletic-field or golf-fairway heights of approximately 0.5 inch, the number of leaves can be up to 40 under the area of your hand and you will experience generally acceptable or mild injury from glyphosate. When the leaves exceed these numbers or start to punch through the upper canopy surface, we start to see exponentially greater injury from glyphosate, glufosinate, and diquat.

 Since your primary target is henbit and deadnettle along with, I’m sure, several other winter broadleaves and annual bluegrass, I would recommend the following approaches:

1. The safest: Apply Reward (a.i. Diquat Dibromide) at 2 pints per acre for henbit, deadnettle, and other broadleaf control, but this will not control annual bluegrass. It will be the safest approach and is unlikely to impart any noticeable herbicide injury. It will not address annual bluegrass which will become a much bigger problem this spring as the plants explode in size and start to produce white seedheads everywhere.
2. The second safest with broad-spectrum weed control: Apply glyphosate now at 24 fl oz product per acre. Wait 2 weeks and, assuming mother nature does not put on a heat wave and the turf foliage is punching through the dormant canopy, apply Reward at 2 pints per acre. This approach will address annual bluegrass and some broadleaf weeds now with glyphosate; then the follow up with Reward will seal the deal on broadleaves. We do not want to apply them together because the Reward and reduce glyphosate performance on annual bluegrass.
3. The third safest with broad-spectrum weed control (but easier one-shot approach): Apply Finale (a.i. Glufosinate-Ammonium) herbicide at 3 quarts per acre now. This will kill the annual bluegrass and broadleaf weeds in one shot. If this were mature turf, I would not blink at spraying. I doubt you will have any problems either way. But this approach is slightly less safe than glyphosate, but even that concern is mostly centered around zoysiagrass rather than your bermudagrass. I think this is still a very viable option.

Other things you should consider to maximize safety:

1. Get the product out ASAP to avoid more green shoots.
2. Pick a warm day with minimal wind and use spray tips that generate fine droplets, like TeeJet flat fan 11001, and spray a finer spray plume at lower application volume (5 GPA). This places a small quantity of fluid uniformly only over the top surface and minimal spray penetration down into the bermudagrass canopy where the small green leaves and green shoots reside.
3. It warrants mentioning that you can forego dormant sprays altogether, wait for summer conditions, push the bermudagrass with fertility, mow out the weeds, then clean everything up in late June with selective herbicide like Tribute Total (a.i.’s: Thiencarbazone-methyl, foramsulfuron, and halosulfuron-methyl).