Calibrate your sprayers before herbicide application

Moisture, moisture, moisture! Larimer County has been hit by a lot of snow in the month of April. This has delayed the weed control efforts about a month especially at higher elevations. And to top it off we had big snow storm in the beginning of May. This time last year, the Weed District was already getting calls from land owners with “weed emergencies” and were reserving our sprayers and buying herbicide.

By Clara Pilcher, Larimer County Weed District

We often get questions of “How much herbicide am I suppose to put in my sprayer?”. Our next response will be “What is your sprayer calibrated at?” Usually the answers from callers are “I don’t know.” Because herbicide is expensive, you don’t want to put too much herbicide in your sprayer. You also should not over apply herbicides and need to use amounts according to the label (Remember, the label is the law).

For these reasons, it is very important to calibrate your sprayers. Every time we explain to the landowners that they have to calibrate their sprayers, the response is often “That is too much work” or “I need to do what?”. It might seem unnecessary or there is too many steps involved but it is actually quite easy to calibrate your sprayer. These are the instructions how to calibrate a backpack sprayer:

1. Measure a calibration plot that is exactly 18.5 ft X 18.5 ft (128th of an acre).
2. Spray the plot uniformly with water, keeping the sprayer pressure constant. Note the number of seconds required.
3. Spray into a bucket for the same number of seconds, again keeping the sprayer pressure constant.
4. Measure the number of ounces of water in the bucket.
5. Number of ounces of water measured from the bucket is equal to the number of gallons per acre (GPA) the sprayer is delivering.

According to your calibration, then you can calculate how many ounces, pints or quarts of herbicide you would need to add to each gallon of water. Make sure you read the chemical label to determine the how much herbicide to apply per acre.

Do you not have a sprayer and don’t want to buy one? Well, we can solve the problem. We have a sprayer-loan program available at the Weed District. Call us to reserve a sprayer. The district carries a variety sizes of sprayers starting from a 25-gallon sprayer with a handgun attached to approximately 30 feet of hose. It is designed to lay in a pickup bed or on an ATV. 65-gallon and 110-gallon sprayers are also available which is on a trailer that can be easily pulled by a truck, lawn tractor or ATV. These sprayers are also equipped with a handgun with 30 feet of hose and a fold-out boom that sprays an 8-foot wide swath. A 65-gallon boomless sprayer is also available.

For those who don’t have a lot of acreages to spray, you can purchase a backpack sprayer with a 4-gallon capacity at a local hardware store to be more economical. All the sprayers at the Weed District are calibrated and cleaned and maintained for your convenience. Sprayers can be picked up and dropped off at the shop yard at 614 E. Vine Drive.

The Larimer County Weed District also provides services such as free site visits, plant identification, advice on controlling noxious weeds, herbicide sales, and land and pasture management guidelines. Contact the Larimer County Weed District at 970-498-5768 or

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