Backpack Sprayer Calibration: Single-seedling Plot Applications

(wood clacks)

- Backpack sprayer calibration for single seedling plot applications.

When planting trees, it's important to control weed and grass competition prior to planting, and then annually for up to five years following planting.

Treating the vegetation with herbicide in the spot where the tree is or will be planted greatly improves seedling survival and growth by reducing plant competition for sunlight, water, and nutrients.

It also reduces cover for rodents, particularly meadow vols, that may gnaw on and girdle trees.

Most hardwood tree plantings protect tree seedlings with a four to five-foot-tall plastic shelter, which is placed over the seedling and staked into the ground.

Tree shelters protect the seedlings from browsing animals and facilitate spraying by protecting the seedling from the herbicide.

A standard practice is to spray a six-by-six-foot plot with the seedling in the center, so those dimensions will be used for the example in this video.

Make a single seedling plot application using a backpack sprayer equipped with a spray wand and a conventional flat fan nozzle, which applies less liquid at the edges of the spray pattern.

This is different from an even flat fan nozzle, which does not taper at the edges.

Using a paved or gravel surface, place an object such as a roll of flagging or tape on the ground to mark the center of the plot.

Using plain water or water mixed with colored dye, practice spraying the targeted area on both sides of the tree spot.

Focus on covering the full six-foot length, maintaining consistent speed and nozzle height above the ground during each pass.

To treat a six-by-six-foot plot, apply two parallel spray bands, one on each side of the tree spot, both six feet long and three feet wide.

Each pass of the spray wand should be centered one and a half feet from the shelter.

Hold the nozzle approximately two feet above the ground so the pattern extends at least six inches past the shelter into the footprint of the other band.

This overlap of the two spray bands is necessary for uniform coverage across the pattern when using a conventional flat fan nozzle.

As much as practical, make sure the height above the ground, speed of the spray stroke, and sprayer pressure are uniform for each pass.

After spraying the practice plot several times, measure the dimensions for accuracy.

Once you feel comfortable spraying a six by six foot plot in a consistent manner, it's time to calibrate.

Calibration measures the amount of spray applied to a given area, and then allows you to calculate the application rate in gallons per acre.

The easiest method is to use a calibration area of 340 square feet, which is 1/128 of an acre.

Since there are 128 fluid ounces in one gallon, spraying 1/128 of an acre allows for direct conversion from ounces sprayed on the calibration area to a gallons per acre application rate.

For example, if you spray 15 fluid ounces onto the 340 square foot calibration area, that means your application rate is 15 gallons per acre.

For single seedling plot applications, you are treating a six foot by six foot plot.

This equals 36 square feet.

Now, to determine how many 36 square foot plots make up a 340 square foot calibration area, divide 340 by 36.

This equals 9.4.

We will round this up to 9.5 to make future calculations easier.

This means you will spray a six-by-six-foot plot nine and a half times to equal the 340-square-foot calibration area.

Keep in mind, a six-foot-by-six-foot plot is comprised of two three-foot-by-six-foot spray bands.

To spray the plot nine and a half times means you have to spray 19 individual bands.

9 and 1/2 times two is 19.

Before spraying the calibration area, make sure the sprayer is fully pressurized so the pump and hose are filled with water.

Then check that there is at least one gallon of water in the spray tank.

Mark the level of water in the tank.

You can fill it to a certain line already on the tank or draw a line with a pencil or marker.

Spray the 19 bands for your calibration in the same manner as your practice runs.

Next, fill a measuring cup to a known level with water, 30 to 40 fluid ounces should be sufficient.

Using the measuring cup, refill your sprayer to the original starting level.

Use subtraction to determine how many fluid ounces were applied to the calibration area.

For example, if the cup started with 32 ounces and had 14 ounces left after you refilled the sprayer to the starting point, then you sprayed 18 ounces.

32 minus 14 equals 18.

This means your application rate is 18 gallons per acre.

It's unlikely that you will get the same exact rate each time, so repeat this calibration process several times and calculate an average for your application rate.

Note, the amount of water sprayed onto this calibration spot will probably be small, and the level change in the backpack tank may be difficult to measure accurately.

To make measuring easier, you could double or even triple the calibration area and then divide the ounces used by two or three respectively to get the ounces applied to 340 square feet.

For example, you could double the calibration area by spraying 38 bands, and then divide the ounces sprayed by two.

If you apply a total of 30 ounces, that means you only applied 15 ounces to 340 square feet.

30 divided by two equals 15, and therefore applied at a rate of 15 gallons per acre.

Keep in mind that when you practice spraying and calibration, you will produce much heavier spray coverage than an actual application would.

This is because you are spraying one plot multiple times.

It may be helpful to use some colored water to practice spraying around a few tree shelters or other objects as you would in a real application, that is, spraying each band one time.

This can help you see how light the spray coverage will actually be.

Now that you have your application rate and gallons per acre, you can calculate how much herbicide to include in your spray mixture based on the herbicide rate per acre found in the product label.

Sprayer calibration is important.

Calibration takes some time upfront, but it is time well spent to ensure proper herbicide applications.