Calibrating Hand-held Sprayers

Source(s): Clint Waltz, Extension Turfgrass Specialist, The University of Georgia

Hand-held and backpack sprayers are extremely useful for treating small turf areas infested with weeds. But they have to be calibrated to apply the recommended rate of a herbicide.

Most herbicides used in turf grasses control weeds without injuring the turf. But that depends on the rate applied. The rate is usually on the product label as the amount to be applied to 1 acre or 1,000 square feet.

For example, the highest recommended rate of Trimec Classic for Bermuda grass and tall fescue is 1.5 fluid ounces per 1,000 square feet.

Applied at this rate and by the label directions, it will cause only slight injury to labeled turf grasses. But if you apply three times that amount, some turf grasses would be yellow or brown for weeks.

If a sprayer isn’t calibrated, it can’t apply herbicides at recommended rates.

It’s simple

You can use a number of methods to calibrate sprayers. One that’s simple, easy to do and easy to remember is called the 1/128th-acre method.

In this method, you spray 1/128th of an acre. That’s 340.3 square feet. This figures out to 18.5 feet by 18.5 feet. Here’s how to do it.

  1. Mark off a plot 18.5 feet by 18.5 feet.
  2. Fill the sprayer to normal capacity with water.
  3. Pump the sprayer to the pressure normally used to apply herbicides.
  4. Spray water over the plot area while maintaining normal and constant operating pressure.
  5. Record the time in seconds it takes to spray the plot area.
  6. Using the same constant pressure used in step 4, spray into a bucket for the same time (number of seconds) it took to spray the plot area.
  7. Measure the volume of water in fluid ounces.

The number of fluid ounces you collect is equal to number of gallons the sprayer would apply to 1 acre if you use it at the same pressure and walking speed you used in the plot area.

This quick conversion works since there are 128 fluid ounces in 1 gallon of water. It doesn’t get much easier than this. Now all you have to do is find out how much herbicide to add to the pump-up sprayer.

How it applies

Let’s say your sprayer applies 25 gallons per acre. The right rate for Trimec Classic is 4.0 pints, or 0.5 gallons, per acre. So divide 0.5 by 25 to get 0.02, or 2 percent.

Multiply that by the sprayer capacity. A 2-gallon sprayer would hold 256 fluid ounces, and 256 times 2 percent would be 5.1 ounces. So add 5.1 ounces of Trimec Classic to the sprayer. Then fill it to the 2-gallon mark with water.

Remember, you have to walk at the same speed, maintain the same pressure and hold the spray nozzle tip at the same height you used in the plot area.

If you do these things, you’ll apply the recommended rate of the herbicide, control the target weed and, most important, drastically reduce the chance of injuring your turf grass.

Reviewer(s): Clint Waltz, Extension Turfgrass Specialist, The University of Georgia, November 2005.

Center Publication Number: 153

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