This article was co-written with Louis Damm (Dramm Corporation).
Greenhouse application equipment spans from the humble squirt bottle, to gas-powered foggers, to robots equipped with hydraulic vertical booms. The variety of spray equipment available reflects a variety of needs, just as a carpenter’s toolbox contains different tools designed to do different things.
Once the decision to spray has been made, it takes two things to achieve an efficacious application: An academic understanding of how spray droplets behave and a practical understanding of how to operate the equipment correctly.
The carrier in many greenhouse spray applications is water. Water has a high surface tension and tends to bead on targets (especially waxy leaves). The goal of spraying is to maximize the level of contact between droplet and target with as little waste as possible. Contact can be increased via mechanical spread or chemical spread. You can see chemical spread in action thanks to Dr. Heping Zhu’s (USDA ARS) video series on non-ionic spreaders. For more information on how droplets behave, download Purdue Extension’s new “Adjuvants and the Power of the Spray Droplet”.
There are many ways to categorize greenhouse spray equipment. Chapter 25 of the Ontario Grower Pesticide Safety Course Manual does so by sprayer design. Greenhouse spray equipment can also be grouped by the volumes they employ:
High Volume (HV):
These applications are performed at pressures ranging from 500 to 4,285 kPa (75 to 700 psi) employing flow rates of 3.9 to 5.7 L/min (1 to 1.5 US g/min). They use standard label rates to accomplish a dilute application by broadcasting droplets larger than 100 microns. The goal is to cover all surfaces without incurring run-off. Examples of HV application equipment include backpack sprayers, trailed sprayers and boom sprayers. Practice and self-calibration are necessary to achieve the desired results when using manual HV sprayers.
Targeted Low Volume (LV):
These applications are performed at high pressures around 20,685 kPa (3,000 psi) employing flow rates approaching 1 L/min (0.26 US g/min), covering 93 m2 (1,000 ft2). They apply reduced rates over a given area and create droplets between 25 and 100 µm. These are concentrated sprays that do not result in wet foliage. LV applications are particularly good in high-humidity environments, when it is desirable to minimize the moisture on leaves. Examples of LV application equipment include aerosol cans.
Ultra-Low Volume (ULV):
These applications employ flow rates approaching 2 L/min (0.52 US g/min), covering 930 m2 (10,000 ft2). They require concentrated solutions, but apply reduced rates per area using droplets less than 25 µm. ULV applications will not raise greenhouse humidity and are a good choice when days are short and nights are long. They are also an excellent way to apply disinfectants for complete space sanitation before starting a new crop. It is important to ensure vents are closed and fans are off during the application.
In this case, spray drift is desirable; Lateral air movement is very important to encourage plant canopy penetration and prevent droplets from merely settling on upward-facing plant surfaces. Examples of ULV application equipment include total release cans, auto foggers and thermal pulse foggers.
Thermal pulse foggers are unlike other ULV equipment and warrant special consideration. The design of the pulse fogger has remained virtually unchanged since the 1940’s. Smaller, 24 hp machines are used in smaller operations but range up to large 175 hp machines. Tank size ranges from 10 to 50 L, where 10 L should be enough to cover 4,645 m2 (50,000 ft2) in about 10 minutes, depending on crop density. Their range is about 35 m (115 ft) from the point of release.
Thermal Pulse foggers do not create aerosol using air shear – they use combustion (80 to 100 explosions per second) to shatter spray into a fog and propel it via positive pressure. Heat is a by-product of the engine, making it an unsuitable method for applying biological products. However, water-cooled foggers such as Dramm’s Bio Pulse Fogger reduce the exhaust temperature below 100 °C to make the application bio-rational. This has the added advantage of making droplet sizes more consistent and preventing spray from evaporating too quickly before it diffuses to the target.
It is recommended operators use approximately 1 L of carrier in 5 L of spray mix, but a higher proportion of carrier would be required for more viscous products. Start with a full tank of clean, high grade gasoline and once the fogger has been started, run it continuously until the application is complete. Leave it running even when moving between Quonset huts (Figure 5). Do not leave the manual fogger running unsupervised as an auto fogger: If they stay stationary, or aim directly at the canopy (as in hydraulic spraying), they could drench and potentially damage nearby plants.
When fogging, aim between the plants, such as the alleys and between hanging plants. This allows the fog to expand and permeate canopies for the best coverage. When spraying is done, be sure to release the pressure created in the spray tank to prevent accidental back flow into the gasoline tank.
HV, LV and ULV equipment requires model-specific cleaning and maintenance, according to manufacturer’s instructions. Even when sprayers are kept in prime condition, they are only as good as the operator’s understanding: When the wrong product is applied by the wrong machine using the wrong method, operators risk poor control, crop damage and increased potential for pesticide resistance.
For more information on greenhouse sprayers, try www.dramm.com.