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Pesticide Safety Education Program (PSEP)
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Shade Tree Spray Equipment


Donald R. Daum
Thomas F. Reed
The Pennsylvania State University
College of Agriculture, Extension Service
University Park, Pennsylvania

Shade trees are a vital part of our cities and parks. Preservation of shade trees is becoming more important as disease and insect problems continue to increase. Many areas already have taken steps to preserve the beauty of their shade trees. One important step is the implementation of a pest control program. A well-managed pest control program also reduces the high cost of tree removal.

A successful pest control program has four objectives; diagnosis of the problem, the right chemical and rate, proper timing, and thorough coverage. These are essential and cannot be overemphasized. Any one component is worthless without all the others. For example, often a pest is correctly identified and the right chemical chosen but the spray application does not provide thorough coverage.

Thorough coverage is probably the most difficult objective. Weather, size of trees, and available spray equipment all influence application. Weather conditions and tree size are beyond control, but you can select the proper equipment to meet your needs. Repeated applications are often uneconomical. Therefore, you must use equipment which gets the job done right, the first time.

Types of shade tree sprayers

A hydraulic (liquid) sprayer uses water as a diluent and carrier. The pressure developed from the pump is used to atomize the spray mix at the nozzle and deliver it to the target. An air-blast sprayer uses water as a diluent and both air and water as a carrier. Pressure from the pump provides atomization and a high-velocity air stream carries the droplets to the target.

Hydraulic tree sprayers

Hydraulic (liquid) sprayers are used for all sizes of trees. Tree sprayers are available as trailer models to be towed or skid-mounted models for use on pickup trucks or other vehicles. They vary in size from a 2.5-gpm pump and 2-HP engine for a homeowner to units with a 60-gpm pump and a 60-HP engine used by municipalities, institutions, or custom-spray operators. Tank sizes vary from 10 gallons up to 1,000 gallons. Some of the large trailer units can be powered from a tractor power-take-off shaft. Between these extremes there is a wide range of sizes and options.

The hydraulic sprayer uses large volumes of water and high pressure. The taller the tree, the higher the required pressure. However, since higher pressures tend to produce smaller droplets, larger orifices must be used to offset this effect. Droplets must be relatively large to have sufficient momentum to carry to the tree tops and to adhere to the surfaces. These factors are documented in the table of guidelines below.

        Tree         Pump          pressure           Hose           Nozzle
       height,       size       100-foot hose,      diameter,      pressure,
        feet         GPM             PSI             inches           PSI
      Up to 15        3              400               3/8         150 to 200
      Up to 25        5              400               3/8         250 to 300
      20 - 35        10              400               1/2         300 to 350
      30 - 45        15              400               1/2         300 to 350
      35 - 50        20              400               5/8         300 to 350
      45 - 65        20              800               5/8         400 to 500
      65 - 85        35              800               3/4         450 to 550
      85 - 115       60              800               3/4         450 to 550


As the tree height increases, the spray angle at the nozzle is decreased to concentrate the pressure in the spray stream. In spraying tall trees, a solid stream may be used with most of the liquid breakup being produced by the resistance of the air after the liquid leaves the gun.

Nearly all hydrualic tree sprayers use a hand-held gun. For short trees and shrubs a multiple-outlet gun may be used but the single-outlet gun with a pistol-grip valve is the most common. Many applicators use a variable discharge-angle gun; with a twist of the handle the spray angle can be controlled from a wide angle for short trees and shrubs to a solid stream for tall trees.

The working pressure at the gun is very important for satisfactory operation. Often the gun is a considerable distance from the pump. Runs of 200 feet or more are common where the sprayer must be parked away from the tree being treated, i.e. street parking when spraying a backyard tree for a homeowner. Whenever liquid flows through a hose, friction loss is unavoidable. However, this loss can be minimized by choosing the proper diameter hose for the desired flow rate. Of course the choice is a compromise among friction loss, cost, and handling ease (size and weight of hose with water).

Since friction (pressure) loss is directly proportional to length, you can extend the information to any length, i.e. a 100-foot hose has twice the loss of a 50-foot hose. Select hose size based on flow of the gun or other delivery means being used, not the rated pump capacity. It is interesting to note that for a given flow, a change of one commercial hose size has a big effect on friction.

Additional pressure losses occur at fittings, valves, and turns. To check pressure at a gun, simply 'tee' a guage into the line at the gun and read pressure while spraying at the desired rate. No friction loss occurs when the gun is off because there is no flow present. As you start spraying, you may observe a quick pressure drop at the gun; this drop is the friction loss in the hose as the flow initiates.

Generally, coverage by a hydraulic sprayer is relatively good with a high volume of water; if there are problem areas, they are usually in the tops of very tall trees. Height of effective coverage can be increased by using elevated truck-mounted platforms, gun extensions, or ladders.

The spray mix is relatively dilute and therefore relatively safe. Although the droplets are larger and more resistant to drift than those from an air-blast sprayer, drift can still be a serious problem. To insure adequate coverage, trees are usually sprayed to the point of runoff.

Air-blast sprayers

The air-blast (mist-blower) sprayer applies a concentrated pesticide mixture using a high-velocity, large-volume airstream. Since air is the carrier, the air-blast sprayer uses only a fraction of the water used by hydraulic units. With air providing the transport energy, the droplets must be relatively small. To accomplish this, air-blast sprayers are usually equipped with cone nozzles. If the air velocity is greater than droplet velocity at the point of injection, additional atomization takes place.

The small droplets give the necessary coverge for good disease and insect control. Although small droplets are desired for good transport and coverage, they must be large enough to deposit on the foliage. This balance between size and deposit establishes a minimum volume per area rate. Research has shown that more spray volume should be directed toward the top of the tree than the lower portions to obtain uniform coverage. The larger droplets, containing most of the spray volume, settle out of the air stream very rapidly.

The larger air-blast sprayers have capacities up to 20 gallons per minute. The combination of engine size, fan size, and type of air tunnel allow some sprayers to reach trees 80 feet tall.

Competent operation is essential when using air-blast sprayers since the spray pattern is almost invisible. It is also impossible to determine the extent of coverage since there is no runoff. Trees to be sprayed must be directly accessible to the sprayer unit because best coverage and distribution is obtained by spraying up through the canopy. If the distance from the tree to the sprayer is too great, the velocity will be insufficient to penetrate the canopy.

Most airstreams lose 75 percent of their velocity in the first 25 feet after leaving the sprayer. Therefore, the sprayer should be immediately adjacent to the tree.

Two factors affecting the coverage obtained with air-blast sprayers are airstream velocity and volume. In addition to canopy penetration, velocity is important in getting the spray to the top of tall trees.

Spray material must be forced into the foliage with a turbulent force. To achieve this, air velocity is nearly 100 mph when leaving the sprayer, and must be at least 15 mph at the tree surfaces.

Generally, increasing the volume of air applied improves the spray distribution. The blower must displace the volume of air in the tree with air from the sprayer containing spray droplets. When the available energy is fixed, the higher the ratio of volume to velocity, the better the distribution.

Air-blast sprayers are not trouble free. In addition to wind conditions, a potential problem during cold weather is freezing of the spray droplets both on the nozzles and while air-borne. Evaporative cooling may cause ice to accumulate on the nozzles. This can alter the droplet size as well as the distribution patterns. Also, sometimes after leaving the nozzle droplets will form ice crystals and coverage is negligible. To avoid freezing problems, air-blast spraying should be done only when the temperature is above 45 degrees Farenheit.

Although air sprayers can reach tall trees, their energy consumption far exceeds that of hydraulic sprayers. The energy need is greater because air-blast sprayers must move both air and liquid. Some air sprayers require as much as a 140-HP engine.

On the other hand, air sprayers cover trees faster and require less refilling, than hydraulic sprayers. When spraying large numbers of trees, timely operation can result in pest control equal to hydraulic sprayers, with lower overall costs.


The first key to good application is coverage. In other words, delivering the pesticide to all surfaces of the tree as uniformly as possible. This requires proper equipment and a competent operator.

Training and experience are very important. A limiting factor with shade tree spraying is the wind. Utmost attention must be given to wind speed and direction. Winds as low as 5 mph can disrupt the spray stream and should be avoided. Small droplets are very susceptible to drift and certain mixtures, such as dormant sprays, can be a problem on non-target areas.

Also, operators should avoid spraying during periods of low relative humidity; evaporation causes a significant reduction in droplet size from sprayer to tree. This can cause loss of momentum and impingement resulting in reduced coverage and control. Ideally, tree spraying is done during times of no wind and high relative humidity; often these conditions occur only at night or very early in the morning.

Often it is desirable to know the gallonage applied to a tree. If you know the flow rate of the sprayer, simply measure the time required to spray the tree and multiply by the flow rate. For example, if a gun delivers six gallons per minute and a tree requires three minutes to spray, 18 gallons of spray mix are applied. Delivery rate of a gun can be determined by catching the output for a timed period. For example, if a gun fills a 3-gallon pail in 30 seconds (one-half minute) the delivery rate is six gallons per minute.

Mixing is an important part of spray application. Always carefully and accurately measure the correct amount of pesticide for the mix. Stay within approved rates. If recommendations are in active ingredients, be very careful in determining the amount of formulated product. Technical materials may be formulated in more than one percentage of active ingredient, even by the same manufacturer. If you have trouble getting a uniform suspension with wettable powders despite agitation, try premixing as a thin slurry before adding to sprayer. In all cases, the agitator should be operating before any pesticide is added to the sprayer tank.

Sprayer maintenance

Proper maintenance of a sprayer provides good performance and long life as well as helping to control machine costs. Cleaning, checking and protecting your equipment prior to storage is especially important.

Sprayers need to be cleaned to prevent corrosion, cross contamination of pesticides, and possible tree injury. Trace amounts of one pesticide can react with another, or carry over to the next spray application, causing damage. Small amounts of some pesticides can damage sprayer components, including stainless steel tips, after long periods of contact.

Always try to end the day with an empty tank. Do not mix more pesticide than is needed. Always flush with clean water and drain, even if you plan to apply the same material the next day. Also rinse the outside of the sprayer. The use of surfactants with pesticides, when compatible with your needs, will provide some cleaning action in the sprayer. Some pesticide combinations (especially when oil is used) may produce a putty-type paste (buttering out) in the sprayer tank; flushing with water after each load may prevent an accumulation. If water alone does not dissolve and remove the buildup, add Stoddard solvent, kerosene, or other low flammable solvent; allow paste to dissolve, then agitate and flush. Next, flush with detergent and finally with clean water.

Whenever you change pesticides, or prior to storage, the sprayer should be given a thorough cleaning with a cleaning solution. Check the pesticide label for cleaning instructions; if none, use a strong detergent solution (1/4 pound in 25 gallons of water or 1 tablespoon per 2.5 gallons of water).

First, flush with water, then add the cleaning solution to the tank and thoroughly agitate before flushing. Always flush with clean water to remove the cleaning solution. Remove nozzle tip(s) and screen(s); clean them in a strong detergent solution or kerosene, using a soft brush such as an old toothbrush. Do not use a wire, knife, or other hard object that might scratch the orifice or puncture the screen.

Preparing spraying equipment for winter storage is similar to preparing other equipment; there are basically three requirements:

  • Cleaning a sprayer is extremely important because not only must dirt and grime be removed but, more importantly, as much of the chemical residue as possible must be cleaned from the system.
  • All parts of a sprayer, especially the pump, must be protected from deterioration during the storage period. Deterioration during storage can consume more of the useful life of some sprayer parts than actual use.
  • Sprayers and water-cooled engines must be drained or protected from freezing temperatures.

If your sprayer has no rubber parts (such as gaskets, diaphragms, hoses, pump rollers, etc.), put up to 5 gallons of new or used motor oil in the tank prior to the final flushing to help prevent corrosion. As the water is pumped from the sprayer, the oil will leave a protective coating on the inside of the tank, pump, and plumbing. Some operators leave dormant oil in the sprayer during winter storage for protection and then dilute and spray it out for the first spray in the early spring.

In all cases, the pump should be protected. If the pump contains rubber parts, disconnect the lines and put one tablespoon of radiator rust-inhibitor in each inlet and outlet ports. If the pump has no rubber parts, engine oil is satisfactory. Rotate the pump four or five revolutions by hand to completely coat interior surfaces.


Exposure to a large amount of almost any pesticide can cause illness. Some pesticides are so toxic even a small quantity can be dangerous to the operator. To apply any pesticide safely, your most important source of information is the label on the container. Read and follow the directions on it.

Avoid exposure to pesticides when mixing and spraying by using protective clothing and equipment. Applicators should wear long-sleeve shirts and trousers, gloves, waterproof boots, brimmed hats, goggles, and if necessary respirators. Consult the pesticide label for specific recommendations. Do not smoke or eat when handling pesticides. Apply chemicals at the recommended rate and time and only on the recommended crops. Avoid contamination of water sources due to runoff from trees or wash water from cleaning equipment.

When not in use, chemicals should be stored in their original, labeled containers. Place the containers in an approved locked storage building or room.

Empty pesticide containers are a serious problem. Before disposing containers, be sure they are rinsed and drained into the tank three times. Use a designated landfill site if available or bury the containers at least 18 inches deep and at least 500 feet from the nearest water. Empty cans should be punched with holes and glass containers broken to prevent use. Some large containers can be recycled. Check the label for specific disposal instructions.

Surplus pesticide mixes are a serious problem, also. Mix only what you need. Prevent contamination of food and water sources by disposing in an approved manner, preferably by applying in a labeled use.


The degree of success in any pest control program depends to a large extent on the equipment. Can you reach the tops of small trees? Does your sprayer give uniform, complete coverage.? Is a hydraulic sprayer or air-blast sprayer best suited for your needs?

Careful consideration must be given to these questions when under- taking a tree spraying program.

Remember the important elements to a successful pest control program are:

  • Diagnosis of the problem.
  • The right chemical and rate.
  • Proper timing.
  • Thorough coverage.

Select the sprayer which will get the job done right and insure continued success with good equipment maintenance. The proper equipment and its competent operation can be the key to your pest control program.

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