Warm Winter Weather Results in Many Questions for Weed Control on ‘Dormant/Transitioning’ Bermudagrass

Shawn Askew, Extension Turfgrass Weed Specialist, Virginia Tech

Question: We have quite a bit of broadleaf weeds and annual/perennial grasses in our bermudagrass athletic field and we thought it might be the right time to apply the Roundup and/or a preemergent herbicide.  However, I did some reading online and saw some information saying that, in some areas, the bermudagrass gets brown but isn’t dormant.  We certainly don’t want to kill it after all of this work to establish it.  How can we know if it’s dormant?  Our temperatures have been on a roller coaster this winter with lots of 70s here in February.  How long after the temps get warm does the grass usually start coming back for the season?   Did we miss the window for Roundup?

Answer from Dr. Shawn Askew: It all depends on how many green leaves are on the bermudagrass and what rate of Roundup you spray.  If the bermudagrass is still brown and all you can find is occasional green leaves down in the canopy, you can get away with spraying a low rate of Roundup.  For example if you put your hand down on a random place in the turf that is not full of weeds and all that you can find under your hand is 5 or less green bermudagrass leaves and these are down in an otherwise brown canopy, you can spray Roundup now at 20 fluid ounces product per acre and a low spray volume of about 0.5 gallons solution per 1000 sq ft.  But you must be accurate on these rates.  I just sprayed my zoysiagrass lawn today and to guarantee that my rate was accurate using a small garden pump sprayer, I marked off 1000 square feet of lawn, placed 0.5 gallons water and 13 milliliters of Roundup Pro Concentrate in the sprayer.  I sprayed the 1000 square foot area repeatedly (3 times) until the sprayer ran out.  Then I mixed a new tank and moved on to the next 1000 square feet.  If you have a more precision boom sprayer for larger areas,that is fine but the rate must be accurate.  When the bermudagrass is not completely dormant and the rate is 20 fl oz/A, Roundup will cause between none and a 2 week delay in greenup this summer.  At 40 fl oz/A the delay will be at least 6 weeks and maybe more if nondormant bermudagrass was sprayed.

                          

The target weeds for Roundup are annual bluegrass and other winter annual or perennial grasses like perennial ryegrass, tall fescue, roughstalk bluegrass, etc.  If you don't have a lot of these type weeds, you probably don't need to spray Roundup over the entire area.  For example, you can spot spray Roundup or dab the Roundup only on the winter grasses.  Be warned the spot treatment of Roundup on dormant or semi-dormant bermudagrass is usually not recommended because it is very hard to apply an accurate rate on small spots.  The result is round spots where the bermudagrass is about 2 weeks or more slower to greenup.  It all goes away once the bermudagrass completely wakes up but can look pretty bad for as much as a month.   But these are usually major "fails" where the applicator was grossly off on application rate.  It can be done, but just be aware that you risk having spots of slow greenup ("brown spots") if your rates are too high in the spots that are treated.  The alternative is to apply a perfectly uniform application of Roundup over the entire area, then you will not see anything unusual.  Even if you did cause a delay in greenup, you would not know it.  One last thing to mention about Roundup is that it is very sensitive to "hard water".  Buying and using a spray conditioner like ammonium sulfate is good insurance against hard water problems.  If you have your water tested and know you don't have excessive Ca and Mg ions, then the ammonium sulfate is not needed.  If you intend to use AMS, follow the instructions for rate and add the AMS to the water before you add the Roundup.

 

Roundup, especially at low rates, will not control winter broadleaf weeds.  It will only stunt most of them at best.  For broadleaf control, I would use Escalade II Herbicide or a similar broadleaf herbicide designed to be less injurious on bermudagrass (Trimec Southern, Speedzone Southern, Avenue South, Celsius, etc).  You can also use metsulfuron (MSM Turf, Manor, etc) at 0.5 oz product per acre with nonionic surfactant at 0.25% v/v.  Metsulfuron is slow but effective on a wide range of broadleaf weeds and far cheaper than the other options listed (about $6 to $15/A).

 

For preemergence control of crabgrass and goosegrass this summer, it is important to consider whether or not the use of a PRE fits your planned field use.  If it is aggressively trafficked and you want to do regular reseeding of bermudagrass, then stay away from the standard preemergent herbicides.  However, if it is infrequently trafficked and you are convinced that you will not have need to reseed, there are a variety of options available.  You can apply something more affordable like Barricade, Dimension, Pendulum or a generic version of these at a price of about $25 to $45/A.  You can apply Specticle (indaziflam) a new product by Bayer that generally improves spectrum of weed control but the cost will be closer to $100 per acre.  All of these are root-inhibiting herbicides and they are typically fine to be applied to lawn-height bermudagrass but they are not the safest option for trafficked bermudagrass sports fields.  If you were to suffer massive loss of bermudagrass due to a severe late frost or winter desiccation, these herbicides would prevent you from sprigging or plugging new bermudagrass to repair the areas this summer.   A safer option would be to use Ronstar (oxadiazon) or Sureguard (flumioxazin).  These are shoot-inhibiting herbicides that still control crabgrass and goosegrass preemergence but allow you to sprig bermudagrass at any time after spraying the herbicide.  The cost is about $150 to $200 per acre for Ronstar and about $100/A for Sureguard.  An added benefit of Sureguard is that it will control annual bluegrass postemergence by itself at 12 oz/A and does not need Roundup. 

 

If an preemergence herbicide is in your best interest, the cheapest option is to apply an affordable sprayable prodiamine product mixed with Roundup at 20 fl oz/A.  This will cost you about $40/A at most and 9 times out of 10 you will be very happy with the results.  A small risk in delayed bermudagrass greenup from Roundup and delayed winterkill recovery from prodiamine is the trade off if Mother Nature turns against you.  I would spray at 0.5 gallons solution per 1000 sq ft.  My target rates in the mixture would be 1 fl oz Barricade 4L (or equivalent) and 0.9 fl oz Roundup Pro Concentrate (or equivalent) per gallon of spray solution and one gallon of spray solution, in this case, would spray 2000 square feet.

 

If you have more green bermudagrass leaves than I indicated earlier and the area is loaded with annual bluegrass, I would use Sureguard at 12 oz product per acre with nonionic surfactant at 0.25% v/v or 1/3 fl oz per gallon of spray.  You will see a slight discoloration to the greening bermudagrass but it will shake this off quickly and start growing again after about 2 weeks of slow or sluggish growth.  This treatment will kill the annual bluegrass and give you a 2.5 month residual control of crabgrass and goosegrass for the summer season.  You can buy 1.25 acres worth of Sureguard online for about $175.