As a field applicator of aquatic herbicides, it is your responsibility to:

  • maintain your equipment,
  • calibrate carefully and regularly, and
  • use the correct herbicide at the correct label rates.

Remember that using too little wastes herbicide, time and money; using too much herbicide violates the law.

Laying Out a Test Square Acre

One acre = 209 feet per side.

To lay out a test acre, use a 209' rope with a floating buoy attached. Mark point one with a fixed marker or anchored buoy. Drag your rope behind the boat until the floating buoy meets the anchored buoy at point one, and mark point two with a fixed marker or anchored buoy.

Take a compass reading, add 90 degrees to make a right angle or square corner, and follow this reading dragging the rope to mark point three. Repeat to finish the square acre.


Calibrating for Spray Boom Use (particularly for treating submersed weeds)

To calibrate this system, you need to:

  • check for nozzle uniformity,
  • measure the output of the nozzle in gallons per minute,
  • measure the swath width to determine how many passes will be required,
  • and calculate time, required to cover one acre. 
Nozzle uniformity test:

A uniform spray pattern reduces "hot spots". To check for nozzle uniformity, fill your herbicide mix tank with water, get the air out of the system, and put each nozzle into a separate bucket. Turn the pump system on and, using a stop-watch, let the system run for one minute. Measure the output in each bucket. The output should be the same, or uniform, in each bucket. If the output between nozzles is off by >10%, check your hoses or change orifices. 

Measuring output in gallons per minute (GPM):

To determine the GPM, combine the contents of the buckets from the nozzle uniformity test into one bucket and measure; this is the total output in gallons per minute. 


GPM = 4

Measuring swath width:

Because backwash from the boat and natural diffusion spread water soluble herbicides, experience shows that you can skip 1-1/2 times the width of the airboat, or about 13', to obtain uniform coverage.


airboat = 6' + 3.5' on each side (or 7') = 13' swath width

To calculate the number of trips required for uniform coverage, divide 209 (side of a square acre) by swath width.


209' (acre) ÷ by 13' (swath width) = 16 trips, or 8 round trips

Calculating time:

To calculate the time it will take to treat one acre, measure the time it takes to make one round trip through the measured acre.


one round trip = 82 seconds.
82 seconds x 8 round trips = 656 seconds.
656 secs ÷ by 60 secs (one minute) = 11 mins (rounded figure),
which is the time it will take using your system to treat one acre.

Total mix:

Using the figures from the above calculations, you can determine your total mix:

GPM (system output) x time (to treat one acre) = GPA (gallons per acre) of total mix


4 GPM x 11 minutes = 44 GPA
Herbicide use "label" rate = 2 GPA
42 gals water + 2 gals herbicide = 44 gals total mix

Discussion: keep your pump at a constant speed and pressure. Check your time at each round trip to be sure it matches your time calculation. Halfway through the acre, you should have half a tank of herbicide mixture left. If you run out of mixture before you reach the end of the acre, or if you have a lot of mixture left after you complete the acre, recalibrate. If you are within 2-3 gallons either way, you are close enough. Distribute any remaining mix or rinsate in the plot, not at the boat ramp!

Note: boat speed changes with wind and weight of tank. Check time and mixture levels often. This may seem complicated, but this comes as second nature with experience.


Calibrating for Hand Gun Application

The handgun is basically a one-nozzle boom. The principles are the same. To calibrate this system, once again you need to know:

  • output (GPM),
  • swath width, and
  • time

Determine your gallons per minute (GPM) by spraying the hand gun for one minute into a bucket or into a plastic bag in the bucket to prevent excess splashing, and measuring the total output.


GPM = 5

Swath width:

Determine your swath width by spraying water at the normal pump speed and pressure over the ground and measuring the distance. If you change your pump speed or pressure, you must recalibrate.


swath width = 35'


Measure the time it takes to make one round trip through the measured acre.


time = 2 mins 23 secs or 143 secs

Total mix:

Using the figures from the above calculations, you can determine your total mix.

209' ÷ by 35' (swath width) = 6 round trips to cover one acre;
one round trip = 2 min 23 secs or 143 secs x 6 trips = 858 secs or 14.3 min to treat one acre

GPM x time = GPA


GPM = 5, Time = 14.3 minutes
5 GPM x 14.3 minutes = 71.5 GPA total mix
Herbicide use rate = 1.5 GPA
70 gals water + 1.5 gals herbicide = 71.5 gals Total mix

Discussion: Do not spray directly in front of the boat, because the boat wash rinses the herbicides off floating and emergent vegetation. If your tank is too small for the total mix, divide water and herbicide amounts in half and treat half an acre per tank.


Calibrating for Granular Spreader Use

Again, find the output (pounds per minute), swath width and time.

PPM (output):

Measure output in pounds per minute (PPM) by running the granular spreader for one minute into a container and measuring the total output. To facilitate collecting the granules, tape a large garbage bag around the spreader disk to collect the granules. Duct tape and plastic bags are essential tools for the aquatic applicator.


PPM = 20

Swath width:

Determine swath width by running your spreader at the proper setting and measuring the distance that the pellets are thrown.


swath width = 30'


Determine time by doing a timed run through the measured acre.


time = 1 minute

Pounds per acre:

Using the figures from above, you can determine how many pounds of granular herbicide you will use.


time = 1 minute, swath width = 30'
209' ÷ by 30' = 7 trips per acre
Output = 20 pounds per minute (PPM)
20 PPM x 7 minutes = 140 pounds per acre (PPA)

Discussion: To reduce or increase granular output, it is easiest to change the speed of the airboat (time). For more output, slow down the boat; for less output, speed up the boat.


10 mins. x 20 PPM = 200 PPA

Doubling the opening of the spreader orifice does not double the output. Recalibrate! If you use a lot of granular herbicide, get blanks from the manufacture and use them to determine your output in pounds per minute at each of the various settings on your spreader. Know your output!

Note: In this page ' denotes foot (a unit of measurement for distance).

For Convenient Conversion Factors click HERE.


  • Determine your output, swath width and time.
  • Follow label instructions and wear safety gear.
  • Calibrate regularly and carefully and know your equipment.


Last updated: 28 November 2011