There are at least two techniques for using a backpack sprayer when spraying an area or plot. You can hold the nozzle tip steady at a suitable height above the target surface, usually within a range of 14-18 inches. Or, you may choose to swing the wand back and forth in a pendulum motion as you walk. With either technique, make certain that you achieve uniform coverage of the treatment site. Use the same pressure, speed, and technique for the application as was done in calibration.
Watch this 8-minute video on How to Calibrate a Backpack Sprayer.
Calibration for treating small land areas
If you are applying pesticide to an area measured in square feet, calibrate the sprayer by staking out a 1,000-square-foot test plot (for example, 20 feet × 50 feet) on a surface similar to the treatment site.
Step 1. Fill the sprayer tank half full with water (no pesticide)
Step 2. Record the number of seconds it takes to spray the test plot evenly while walking at a comfortable, steady pace. (It is a good idea to spray the test plot two or three times and figure the average time.)
Step 3. Stand still and spray into a container for the average time found in Step 2.
- The number of ounces collected equals the amount of spray delivered to 1,000 square feet.
- With this number you can calculate the amount of pesticide and water needed to treat the target area.
Apply herbicide to a lawn 40 feet × 65 feet
Area to be treated: 40 ft. × 65 ft. = 2,600 sq. ft.
Test-plot time to cover 1,000 sq. ft. = 80 seconds
Amount of water collected in 80 seconds: 57 oz.
Sprayer output: 57 oz. per 1,000 sq. ft.
1. To determine the total spray mixture needed, set up the following ratio and cross multiply:
X = 148.2 oz. (round off to 148).
2. To determine the amount of herbicide needed, set up the following ratio and cross multiply:
X= 5.2 oz. of herbicide
3. To treat the target area, a little more than 5 oz. of herbicide should be added to 143 oz. of water (148 – 5). Because there are 128 ounces in 1 gallon, this will mean adding 5 ounces of herbicide to 1.1 gal. of water (143/128 = 1.1 gallons of water).
Calibration for treating small trees, shrubs and ornamentals
In this situation, you will find out how much water is needed to treat an average plant. If the label says “spray to wet”, spray as if you were painting the plant with spray paint. Try to avoid over-application and minimize dripping of the pesticide off the plant. Add water (no pesticide) to the tank and pressurize it. Then record the number of seconds it takes to spray a representative plant thoroughly. Now spray water into a container for that length of time. Use this number to calculate the amount of water and product needed.
Apply insecticide to 18 azaleas in a plant bed.
Labeled rate = 3 ounces of insecticide per gallon of water.
Number of plants to treat = 18
Seconds to spray one average plant = 12.
Amount of water collected in 12 seconds = 10 oz.
1. To determine the total spray mixture needed, multiply the total number of plants to treat by the amount of water collected to treat one plant.
18 × 10 oz. = 180 oz.
2. To determine the amount of insecticide needed, multiply the labeled rate by the total spray mixture.
3. Add a little more than 4 oz. of insecticide to 176 oz. of water (180 – 4) to treat 18 azaleas.
Calibration for treating rows of plants
Use the method described in the Ounces-to-Gallons unit of this Web site. Use row width to determine the distance to travel, and record the time it takes to spray (water only) both sides of the row if sprayed in two passes. In a container marked in ounces, collect the output from the nozzle for the time it took to cover the 1/128th of an acre calibration course. The number of ounces collected equals the application rate in gallons per acre.
Compiled by Wayne Buhler, PhD.