Miller, James H.; Manning, Steven T.; Enloe, Stephen F. 2010. A management guide for invasive plants in southern forests. Gen. Tech. Rep. SRS–131. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 120 p.
Backpack sprayers vary greatly in design and capabilities, and many versions are available commercially. The most desirable components of a backpack sprayer are a streamlined shape (to make it easy for the applicator to move through dense vegetation), an impact-resistant tank equipped with a large opening (for filling), and a solid screen (to trap debris). All backpack sprayers and spray guns should have chemical-resistant seals. Viton® seals are made of the most durable materials and allow handling of both oil- and water-based mixtures. Backpacks with internal pressure regulators can help the applicator ensure uniform rates of application and minimize drift that otherwise might result from using too much pressure. Some models feature kink-resistant hoses for improved safety and reliability. Comfort and support in the design of a backpack sprayer cannot be overstated. Padded shoulder straps with harnesses that attach securely to the frame can reduce operator fatigue and accidents, especially in dense vegetation or on uneven terrain. Backpack sprayers come with a piston or diaphragm pump. Piston pumps must have a regulator because they generate higher pressures. An added pressure gauge on the wand handle can help the applicator maintain a spray pressure appropriate for the given treatment. Another useful addition to a backpack sprayer is a more durable metal spray wand with a more precise shutoff valve than the valve on a flexible plastic wand. Quality backpack sprayers, which are preferred by commercial applicators, usually come with a 1- to 3-year warranty.
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Spray Nozzles
Choosing the best nozzle will usually require that you purchase another nozzle and not use the one that comes with your sprayer. Correct nozzles can greatly improve effectiveness and efficiency. Nozzles consist of a nozzle body, a cap, and a tip with the orifice. The spray tip is the most important part of your sprayer because it breaks the liquid into droplets of the correct size and forms the spray pattern.
Tips are made of a variety of materials. Most readily available tips are made of plastic, brass, and stainless steel. Tips of harder materials initially may cost more but will pay for themselves in the long run with durability and consistent performance. Plastic tips tend to have very irregular spray patterns. Brass tips can be easily damaged during cleaning, and such damage will cause irregular spray patterns. It is a good practice to discard plastic or brass nozzles that come standard with a sprayer and replace them with tips of stainless steel designed for long-term commercial use. Stainless steel tips provide a uniform and consistent spray pattern over longer periods and are known for having fewer out-of-the-box defects.
Air induction tips are newer modifications. These tips produce large, air-filled drops that help reduce the potential for drift in winds. They work by drawing air through two holes in the nozzle sides, which blends the air with the herbicide mix. They emit a spray of large droplets filled with air bubbles and virtually no drift-prone droplets. Other nozzle alternatives include “low-drift” and “extended-range” tips that produce fewer small stray droplets and aerosols that contribute to drift compared to conventional nozzles. When choosing air induction or stainless steel nozzles, consider models with a plastic body and a steel tip, because these models are as durable as more expensive all-steel nozzles.
Low-volume directed sprays are often applied with a backpack sprayer and a spray wand equipped with a full cone, flat-fan, or adjustable cone spray tip. The tip has the orifice, which is usually identified by a four-digit number representing its spray pattern angle and flow-volume specifications; for example, an 8002 tip has a spray pattern angle of 80 degrees and a flow volume of 0.2 gallons per minute (all flow volumes are measured at 40 pounds per square inch of pressure). When your application is for invasive plants, models SS4004E (SS = stainless steel, E = even pattern); SS8004E; SS2504; and a wide-angle flooding nozzle like the Floodjet TK VS3 yield good coverage, useful flow rates, and are good additions to your toolkit. A range of tips should be carried to the field so that the best nozzle for plant heights, terrain, and weather can be selected. To purchase nozzles and spray components and parts, consult an agricultural products outlet, farm tractor dealer, or farm store. Major sources are Spraying Systems Company (630–665–5000, www.spray.com, info@spray.com); Dultmeier Sales (888–677–5054, www.dultmeier.com/div_agricultural. asp, dultmeier@dultmeier.com); and Forestry Suppliers Inc. (800–752–8460, www.forestry-suppliers.com, www.forestry-suppliers.com/c01_pages/contact.asp). These few are provided to assist the reader, while there are other manufacturers and distributors.
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Sprayer Preparation
Prior to calibration of equipment and application of herbicides, applicators must properly prepare their equipment. Thoroughly clean all nozzles, screens, and filters in detergent solution, using a soft bristle brush, and then rinse. Make certain that all nozzles are the same size, prescribed for the application, and made of the same material for uniformity by all applicators and especially when several nozzles are along a short boom. Replace nozzles that do not have uniform spray patterns (often determined by spraying along a concrete surface). Check flow rates periodically by catching spray in a container over a timed period. Stay alert for dripping nozzles and use check values to prevent drips. Select an operating pressure consistent with the desired gallons per acre output and spray pattern relative to wind conditions. Check all PPE, replace faulty items, and make certain that all applicators are trained in use and maintenance of all items. Always use the appropriate PPE for each operation, and perform daily maintenance on this equipment.
Backpack Sprayer Calibration
Prescriptions for invasive plant applications are often given as percent-solutions of specific herbicides. But on herbicide labels, most prescriptions are given in herbicide volume or weight (dry herbicides) per acre. This requires calibration of a spray system using the same nozzle(s) and pressure that will be used during application. Calibration is simply the determination of how much spray volume is applied to a specified area of land. Calibration requires timing an application to a known area, which is then expressed as gallons per acre. This should be done in the treatment area because factoring in the speed of the applicator who works in those conditions is crucial for accurate calibration. Water alone should be used for calibration procedure.
- For calibrating broadcast sprays, measure the width of the spray swath in feet and multiply this measurement by the distance traveled in feet for 1 minute. Convert this result to an acre basis by dividing the square feet by 43,560 square feet per acre.
- Catch the spray from the nozzle(s) for 1 minute and measure the volume in ounces. Convert this measurement to gallons by dividing by 128 ounces per gallon.
- Gallons per acre then can be calculated by dividing the measured gallons by the part of an acre treated.
The calculated “gallons per acre” output during application will be a mixture of herbicide, water, and any additional adjuvants. The amount of herbicide prescribed per acre is specified on the label or in this book; this is the amount you should mix with the “gallons per acre” determined during calibration. For example, if a prescription calls for 2 gallons of herbicide per acre, and if your sprayer is calibrated to apply 20 gallons per acre, then, to treat 1 acre, you must mix 2 gallons of herbicide in 18 gallons of water and adjuvants. For mixing an individual batch for a 4-gallon backpack sprayer, you would divide these quantities by 5. If you need to apply more or less gallons per acre as specified on the label or dictated by vegetation conditions, the nozzle size(s) and/or pressure should be changed. Higher pressures or larger tips will increase output, while increasing pressure may increase drift potential.
You can use the same calculation methods for foliar-directed sprays and for sprayers mounted on tractors and other equipment. For foliar sprays, you will treat individual plants in the measured area (some part of an acre) and then you can refill your sprayer according to a marked pre-application level to determine the gallons applied. For calculations of mounted sprayers, you will need to consider a large calibration area and capture spray from all nozzles, both over a timed period as before.
