My house sits in the corner of a fescue/bermuda pasture. It has provided a nice view for many years. And it has required three mowings each summer to maintain it. So last fall, I decided I wanted to convert about an acre of it back to Longleaf Savanna. But converting pasturelands back to woodlands is more difficult than one might imagine. Pasture grasses are tenacious and require a lot of encouragement to get them to relinquish their territory back to the trees.

A two year sequence of events (at a minimum) is required to remove the grasses so the pines can survive. The first part is the easiest. Spray glyphosate on the fescue in November, when it is green and growing. Being a cool season grass, it becomes thick and luscious in the late fall. Fescue is very susceptible to glyphosate, so when I sprayed it in November 2023, it was pretty much a 100% kill.

Next is the Bermuda, which is much harder to kill. Being a warm season grass, the bermuda grows most when it is hot. So June is the time when it is finally growing enough to spray it. While bermuda is knocked back with glyphosate, it really takes something stronger to kill it. Imazapyr. Even with imazapyr, follow up treatments throughout the summer will be needed to get it all.

So with imazapyr, dosage is very important. You want enough, but if you put too much out, it can kill nearby hardwoods and since it is soil active, remain in strengths high enough to harm your Longleaf transplants next January. So as a prelude to my spraying the bermuda, I needed to calibrate my ATV sprayer to get the required accuracy.

For Imazapyr on Bermuda, you need 2-3 pints of concentrated herbicide per acre of grass. I needed to calculate how much spray was being emitted by the sprayer nozzle at the speeds I would be driving to figure this out. A general explanation written by the excellent Craig Harper from University of Tennesee of what I would need to do can be found here. The only difference is that I would be using a single nozzle boomless sprayer instead of a boom sprayer with multiple nozzles.

So to do this, you need to know the spray width of the nozzle spread on the ground. In my case this was 10 feet. It is suggested that you take 85 % of that to account for overlap. So now 8.5 feet width. So you take the square footage of an acre(43560) and divide that by 128 so you answer will be in gallons of chemical per acre. So that comes out to 340.3 feet. Divide that by the previously mentioned 8.5 foot width. That gives me 40.o3 feet. Since that is such a short distance, we increase that by ten fold to get 400 feet which will be the distance we time with our ATV at the spraying speed (around 4 mph). It took me 109 seconds to go the 400 feet. So now I measure the output of my nozzle for that length of time and found that I get 145 ounces. Divide that by to take it back to the original scale and that gives me 14.5 gallons sprayed in the time it takes me to cover and acre. So I’m rounding that to 15 gallons of spray solution per acre at my speed and pressure. The recommended pressure is 20-40 psi, so I set mine at 30 (see photo). Is I put 15 gallons of water in my sprayer tank and then added 2 pints of imazapyr and then a surfactant to give better contact with the leaves.

So now any time I need to use my sprayer for an herbicide that gives the rates in amount per acre (all of them), I will know how much to add to create the correct mix!