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Calibrating a Backpack Sprayer for Chemical Mowing

Prepared by Doug Hundley and Jeff Owen

Many North Carolina Fraser fir Christmas tree growers are using Roundup® Original or a similar glyphosate formulation at reduced rates to suppress weed growth between rows of trees (middles). This fact sheet describes a method for calibrating backpack sprayers for this “chemical mowing” process.


  • Single nozzle backpack sprayer
  • 150 degree (wide-angle) nozzle, Tee Jet brand: TK or TQ series.
  • 50 mesh nozzle screen
  • Yellow pressure regulator (14 psi.) (Available from Solo Company)
  • Stop watch or watch with second hand
  • 2-quart measuring cup
  • Graduated cylinder or any device capable of fine measurement (milileters)


  1. Follow the recommended rates. This will suppress weeds and allow beneficial groundcover to flourish.
  2. The person treating the field should always do the calibration.
  3. When spraying the field, maintain the same speed, pressure, and nozzle height used in calibration.
  4. It may be necessary to re-calibrate for different areas of the field if some areas are much steeper, or for changes in the applicator’s energy level (time of day).
  5. Allow the groundcover to green-up before treatments.
  6. To allow most weed seeds to emerge before treating, let the tallest weeds reach at least 12 inches.
  7. The calibration instructions provided in this publication are based on the active ingredient and adjuvant content of Roundup Original®


  1. Clean the nozzle and screen in soapy water with a soft brush. (Never use a knife or wire to clean nozzles. It will ruin them.)
  2. In a place away from any wells or water supplies, rinse the spray tank thoroughly and partially fill it with clean water.
  3. Reinstall the nozzle and screen.
  4. Pressurize the sprayer and check output for even pattern on a dry surface. If the pattern is uneven, replace or clean nozzle. Inspect the sprayer for leaks. Repair as needed


  1. Always calibrate in the field you are planning to treat.
  2. Add 1-2 gallons of clean water to the backpack. Walking at a steady pace, practice spraying down the center of the rows while keeping the 150 degree nozzle about knee high. The goal is to cover the middles, 5-6 feet across, without waving the nozzle back and forth. Foliage at the base of treees will be hit without damage as long as the recommended rates and date windows (see table on the next page) are followed accurately. Some overlap should occur along the tree row.
  3. Measure the width of the spray band. Divide 340 by that number to determine the length of the calibration course. The result will give you the distance you need to walk to spray 128th of an acre for that width of a spray band (340 square feet = 1/128th of an acre). By using 128th of an acre for your calibration area, calculations are simplified (1 ounce caught = 1 gallon sprayed per acre)
  4. Tag a tree down the row that comes closest to the specified distance.
  5. Using a consistent, comfortable pace, record the number of seconds it takes to spray the calibration course. Take an average of at least two trips, once in each direction. Always use the pressure regulator.
  6. While standing still and using the same pressure applied in the calibration course, spray into the cup for the same number of seconds it took to walk the course. The number of ounces collected is the exact number of gallons per acre (GPA) being applied (1 oz. = 1/128th of a gallon, therefore, # oz. per 1/128th of an acre = # gal. per acre).

[Note: When spraying a 6-foot wide swath and using a TK-2 or TQ-15004 nozzle, yellow (14 psi) pressure regulator, and a comfortable walking speed, you should obtain an output in the range of 7-12 gallons of water per acre. Under windy conditions, fine droplet size can contribute to drift and uneven treatment. Use a nozzle with a larger opening to increase droplet size and reduce drift (a TK4 instead of a TK3, for example). You should use the same rates per acre and date recommendations with these larger nozzles. Re-calibrate if you change nozzles.

  1. Now that you have determined your output in GPA, you can mix glyphosate with water at the rate you intend to apply (see table below). Divide the rate of herbicide you want to apply per acre by the GPA measured in calibrating the sprayer. The result is the amount of glyphosate needed per gallon of water (Acre rate / GPA = rate per gallon).

If you prefer to premix glyphosate in a nurse tank, you may stop at this point and mix. Be aware that you must maintain the pace established during calibration to maintain your accuracy.

  1. Decide on the volume of mixture you will be carrying per backpack. Then multiply this volume by the ounces of glyphosate per gallon. This will give you the amount of herbicide to add to each backpack (rate per gallon X gallons per sprayer = rate per backpack). For better accuracy, use the three gallon mark on the backpack. Make sure the backpack is level when filling.
  2. Use a graduated cylinder marked in millimeters to measure glyphosate. One ounce equals roughly 30 milliliters. Simply multiply your ounces of glyphosate per backpack by 30 to get milliliters per backpack tankful.

Worksheet of Calibration Procedure Steps 1-9 Above

Fraser Fir Growth Stages and Roundup® Original Application Rates for Chemical Mowing


Warning!! When spraying in fields where trees are getting larger and closer together, it is easy for the spray nozzle to get within 18 inches of the tree foliage. This causes the rate of glyphosate hitting the tree foliage to rise greatly, and unexpected damage can occur, even at the 4oz. per acre rate. Solution!! When you pass between larger trees close together, lower the spray nozzle to avoid hitting tree foliage at close range. When glyphosate is applied at exactly the recommended rate, this “close range” damage has only occurred between May 10th and July 10th.

Working With a Crew

It is easy to keep track of your own sprayer and to recognize when something has changed or gone wrong. Keeping a crew of sprayers calibrated is more complex. Whether they work from a premixed nurse tank or by mixing each backpack individually, crews of workers must adjust their speed to a leader who sets a reasonable pace. Do not pre-mix herbicide in your nurse tank based on a rule of thumb. Calibrate your sprayers first. The crew must walk together in a line to apply herbicide at the same rate. The leader determines the average time it takes him to cover 340 square feet (see Calibration steps 1-5 above). All the sprayers are then calibrated to his time interval and output. This may require that some sprayers be outfitted with different sized nozzles (see step 3 below).

Adjust calibration to terrain.

1. Because nurse-tank mixtures are based on calibrated walking speed, use different nurse tank loads (or nurse-tank mixtures) for gentle and difficult terrain. For example, a farm might have 20 acres of gently rolling land and 15 acres of very steep land that would be slow-going. One nurse tank full can be used to treat all easy-walking areas. A separate nurse tank load can be used for the difficult terrain or slow-walking areas. The crew leader must direct the spray crew to the right areas. Once a nurse tank is mixed to a calibrated walking speed, it should all be applied at that pace.

2. Only put as much water into the nurse tank as will be needed on that site for a selected walking speed. For example, applying glyphosate to 20 acres in 8 gallons of water per acre will take 160 gallons of water.

Matching the leader’s calibration time.

3. Equip each backpack sprayer with the appropriate TK or TQ nozzle and a pressure flow regulator. No matter what the sprayer capacity, all sprayers should be filled with the same total amount of water (not 4, 3.5, or 3 gallons together). Check all the sprayers for the crew leader’s timed interval to make sure that they are all spraying similar amounts of water in that time interval (see page 2, step 6). If the amount applied by any sprayer varies from the leader’s sprayer by more than 1 ounce (which is equal to 1 gallon per acre), you may need to clean the filter and nozzle, or you may need to change the nozzle. Adjust and recalibrate until all sprayers are within 1 ounce of the leader’s sprayer.

For questions regarding human health and pesticides, call the Carolinas Poison Center 1-800-848-6946 (1-800-84TOXIN).

The use of brand names and any mention of commercial products or services in this publication do not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact an agent of the North Carolina Cooperative Extension Service in your county.

This fact sheet was prepared with information adapted from Doug Hundley, Avery County IPM Program, Avery County Extension Center. The procedures have been developed and tested by the Avery County IPM Research Group and the North Carolina Cooperative Extension Service involved in the On-Farm Grower Participant Research Project.

Technical Reviewer: Wayne Buhler, Ph.D., College of Agriculture and Life Sciences, NC State University

Print version sponsored by the North Carolina Environmental Stewardship Project of the CropLife Foundation, 1156 15th St NW, Suite 400, Washington, DC 20005. 202.296.1585, and the North Carolina Christmas Tree Association

PDF Version

Written By

Jeff OwenArea Extension Specialist (Christmas Trees) (828) 684-3562 jeff_owen@ncsu.eduForestry & Environmental Resources - NC State University
Page Last Updated: 3 years ago
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