Many growers have isolated patches of weeds that should be controlled to prevent their spread. In this case, spot spraying with a herbicide would be the most economically feasible approach. Some herbicide labels allow for spot-treatments. However, the recommended amount is often given in % volume of herbicide per volume of water, or a certain amount of herbicide per 1,000 square feet.
Before adding any herbicide to the spray tank, it is extremely important that the output of the sprayer is known. That is, it must be properly calibrated. This allows for reduced herbicide costs and optimum weed control. A simple calibration test for a single-nozzle backpack or ATV sprayer is shown in Table 1. No math is required for this calibration, and the end result is a known output volume for your sprayer in gallons per acre (GPA).
Hints for calibration:
Keep speed constant. This will ensure that you are walking the same speed at all times.
Keep the nozzle at the same height at all times—a height that is comfortable.
Modify the wand by adding a pressure gauge. Try to keep the pressure as constant as possible. Do not let the pressure fall below 10 psi.
Next the amount of herbicide to be added to the spray tank needs to be calculated. To do this multiply the amount of herbicide needed for 1 gallon by the number of gallons in the spray tank.
Example: A sprayer is calibrated with an output of 40 GPA and the tank holds 10 gallons. If the desired herbicide rate is 1 quart per acre, from Table 2, the amount of herbicide for 1 gallon of water is 4.75 tsp. Since the spray tank holds 10 gallons, 40.75 (41) tsp are needed, which is approximately equal to 7 fl oz.
1 teaspoon = 0.17 fl oz
1 tablespoon = 0.5 fl oz
3 teaspoons = 1 tablespoon
1 pint = 16 fl oz = 32 tablespoons = 2 cups
A no-math method for calibrating single-nozzle backpack or ATV wand sprayers for spot spraying herbicides.
Amount liquid herbicide to add to 1 gallon of water. Abbreviations: tsp=teaspoon, fl oz=fluid ounces.