Several choices of hand sprayers can provide adequate coverage on trees. If you plan to apply herbicides underneath or around your fruit plants, you will need to have two sprayer—one for the application of herbicides and one for fungicides, insecticides, or foliar nutrients. The popular types are described below.
A knapsack sprayer is suitable for small plantings up to an acre in size. This sprayer is entirely manual and is carried on an operator's back. Also called a backpack sprayer, the knapsack sprayer is designed to be as light as practical.
A typical empty weight is about 12 pounds. With 4 gallons of water, the weight is approximately 45 pounds. The parts of the knapsack sprayer are the same as those found on most sprayers: a tank to hold the spray mix; a pump to produce pressure and flow; a regulator to control the flow; and at least one nozzle to atomize the spray mix (Figure 2.2). Tanks on the knapsack sprayer typically are made of plastic or steel and hold three to five gallons. Some knapsack sprayers have a mechanical agitator that moves when the pump is used. Others have jet agitation. Before spraying, shake the entire sprayer to ensure a uniform mix.
The sprayers have a built-in piston or diaphragm pump that is operated by hand. Some models can be adapted to either right- or left-hand pumping; the other hand is needed to operate the flow-control valve and the wand. The pumps are capable of relatively high pressures (80 to 180 psi). Some backpack sprayers are equipped with pressure gauges as well as pressure regulators. The chamber that pressurizes the chemical liquid is very small, so the operator must pump while walking and spraying. At least one company manufactures a knapsack sprayer with an engine-powered pump. This eliminates hand pumping but increases the empty weight of the sprayer by about 50 percent.
The distribution system includes an on-off valve, usually with a pistol-grip handle, and one or more nozzles on a wand. The nozzle is often mounted at an angle on a 16- to -20-inch-long wand to aid spray placement on the trees. Some designs allow for interchangeable nozzle tips so that the nozzle can be better matched to the job. Others use an adjustable nozzle that can be varied from a wide discharge angle to a solid stream. Although the solid stream setting will throw the liquid farther, it should not be used on fruit trees because there is little or no atomization. Poor coverage and pest control results.
The time and energy needed to use a knapsack sprayer on fruit trees limits the device to small plantings. When labor is of minimal consideration, such as with homeowners and hobbyists, the knapsack sprayer can be effective. Its size, however, is not practical for applying high rates of water per acre. Considerable practice is required to obtain thorough coverage of trees without overspraying, which creates wasteful runoff and may increase the risk of phytotoxicity.
A great deal of skill is needed to obtain a uniform application. The rate of spray will be less reliable by hand than if a tractor sprayer is used. Application rates, walking speeds, and coverages also will vary with operator fatigue caused by temperature conditions, the time of day, the slope of the terrain, and the walking surface. In addition, the risk of overspraying and underspraying is increased because the knapsack sprayer uses a small volume of water. Extra care should be given to coverage and uniformity.
Knapsack Mistblower Sprayers
The powered knapsack sprayer, also called a mistblower, has a small engine and fan. It is actually a small, back-carried air-blast sprayer.
Powered knapsack sprayers are equipped with two-stroke cycle, three- to five-horsepower engines. These relatively lightweight engines require that oil be mixed with the fuel. They operate at 5,800 to 8,000 revolutions per minute and are rather noisy; operators should wear ear protection. The sprayers weigh between 17 and 25 pounds when empty and are much heavier than the manual models (Figure 2.3).
Powered Knapsack Sprayer
The engine propels a centrifugal fan that delivers 200 to 450 cubic feet of air per minute (cfm) at a discharge velocity higher than 200 mph. With this high velocity, air shear nozzles are sometimes used. Hydraulic orifice and rotary nozzles also are popular because they can easily form and inject droplets into the airstream. The air from the fan is fed through a flexible tube with an air nozzle on the end that the operator directs to deliver the spray. Because of the high discharge velocity, the air nozzle should be kept at least 6 feet from the trees. The airstream should be aimed downwind so that air currents can assist in carrying the droplets away from the operator. Mistblowers should not be used to apply herbicides.
Powered knapsack sprayers can spray trees much faster than the manual sprayers. The airstream will assist in delivery and coverage even at lower application rates. The number of trees that can be sprayed, however, is still limited because the sprayer tanks are small. Because a tankful will cover only a relatively small area or a few trees, refilling and measuring the chemicals is time consuming.
Handguns are basically hand sprayers with an engine-powered pump. Hydraulic handheld gun sprayers might be equipped with more than one nozzle.
The multiple-nozzle guns are, in effect, very small handheld boom sprayers. A handgun is connected by a hose to a powered pump. Two or more handguns can be used on one pump if its capacity is sufficient. Piston or diaphragm pumps are usually needed to provide the required high pressure. Because the pump and tank are carried or towed by tractors or other vehicles, their tanks can be much larger than is feasible with the knapsack sprayers. This permits using more water and fewer refills.
As with any hydraulic nozzle, the handgun sprayers use pressure to atomize the spray liquid into droplets. The droplets' velocity when discharged from the nozzle must carry the spray to the target. The farther the tree is from the handgun, the higher the pressure must be to adequately deliver the droplets. Handguns with pistol grip-handled valves are preferred for spraying fruit trees because the operator can turn off the spray easily when going from one tree to the next.
Because the operator controls the travel speed, or time at each tree, variation in application rates similar to those of the knapsack sprayer can be expected. Long and heavy hoses contribute to operator fatigue and result in less uniformity. Some people build platforms on the sprayer or tractor on which the handgun operator can ride. This provides comfort but limits maneuverability. If you do build a platform, be sure that guards and railings are adequate to protect the operator.
Two or more people often are used in handgun spraying--one (or more) operates the handgun(s) and one drives the tractor through the plantation. This increases the number of people involved in comparison to knapsack spraying but allows treatment of many more trees in a given time period.
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