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Calibrate weed sprayers

POSTED: May 22, 2013 4:00 a.m.

I have answered many calls at the extension office in the past few weeks dealing with weed control in home lawns.

Summer-annual and perennial weeds seem to be sprouting up everywhere. The recent warm temperatures, combined with large amounts of rainfall, have allowed these weeds to flourish.

For emerged lawn weeds, I will usually recommend certain herbicides depending on the type of turf and the types of weeds present. How well an herbicide will work in controlling weeds depends on the correct use rate. Most homeowners will use a hand-held sprayer or backpack sprayer to treat their lawns. These small, inexpensive sprayers are a great option, but they must be properly calibrated to be useful.

Most herbicides used for turf are designed to kill weeds without injuring the turf grass, but this depends on the rate. The rate is normally presented on the label as the amount applied to either 1,000 square feet or 1 acre.

For example, Trimec Classic, a popular 3-chemical mixture labeled for Bermuda and tall fescue, has a highest recommended rate of 1.5 fluid ounces per 1,000 square feet. At this rate, it will only cause slight injury to labeled turf grasses, according to UGA turf specialist Clint Waltz.

However, when applied at three times the recommended rate, some turf grasses will be brown or yellow for weeks. How do you know if your sprayer is properly calibrated to put out this amount of herbicide?

There are several ways to calibrate your sprayer, but I prefer the easiest method - the 1/128th acre method. Using some simple math, you can spray 1/128th of an acre, which equals 340.3 square feet. This is an area measuring 18.5 feet by 18.5 feet.

To calibrate, first mark off a plot 18.5 by 18.5 feet. Next, fill your sprayer to normal capacity with plain water.

Pump up your sprayer to the pressure at which you normally apply herbicides (for a new sprayer, follow product label directions for pressure suggestions). Spray the water over the entire plot area, maintaining constant speed and pressure. Record the time it takes to spray the plot area in seconds.

Next, using the same constant pressure as earlier, spray into a bucket for the same amount of seconds it took to spray the plot area. Then, measure the volume of water in fluid ounces in your bucket.

The number of fluid ounces you collect equals the number of gallons the sprayer would apply to 1 acre (if you use the same walking speed and pressure as during the test). This quick conversion method works because there are 128 fluid ounces in a gallon of water. Now all you have to do is find how much herbicide to add to your hand-held sprayer.

Let's do an example. You ran the test above and figured your sprayer applies 25 gallons per acre. The labeled use rate for your herbicide is 4.0 pints (0.5 gallons) per acre. Divide 0.5 gallons by 25 to get 0.02, or 2 percent.

Multiply this by the capacity of your sprayer. A 2-gallon sprayer will hold 256 fluid ounces, so 256 multiplied by 0.02 equals 5.1 ounces. This is amount of herbicide you need. Add 5.1 ounces of herbicide to your sprayer and fill it up with water to the 2-gallon mark.

If you follow these steps to calibrate your sprayer, you will greatly increase your chances of controlling weeds and reduce the chances of injuring your turf.

Clark MacAllister is the Dawson County extension agent. For more information, call (706) 265-2442.



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