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Dec 04, 2023

Mist and fog propagation systems

Mist and fog propagation is finding greater use in the vegetable industry. With

Mist and fog propagation is finding greater use in the vegetable industry. With more growers producing grafted plants, there is a greater need for this technology. Grafted plants, such as tomatoes, pepper, and eggplant are gaining in popularity and are now available to homeowners through seed catalogs.

Mist and fog systems probably go back to the indigenous people that propagated under mist and fog adjacent to waterfalls. Modern-day mist and fog is first recorded in published research papers starting about 1940.

Fog particles are generally considered to be less than 50 microns (0.002 inch) in diameter. Mist on the other hand, is particles from 50 to 100 microns. As a comparison, human hair is about 0.004 inch in diameter and equals 100 microns.

The humidity in the air affects the evapotranspiration rate from the leaf surfaces. To get good propagation, a balance between humidity and transpiration is needed to allow water and nutrient uptake without excess dehydration. In a crop with a dense foliage canopy and without much air movement, a boundary layer of moisture approaching saturation develops around the plants.

On the other hand, when the air temperature is high and leaf temperature increases, water loss can exceed the ability of the plant to take up moisture and stress can build up within the plant. The use of fog and mist at this time can reduce the air temperature and increase the humidity within the plant canopy without saturating the plant medium. With more oxygen in the root zone, faster rooting occurs. Once the root system is established, the relative humidity can be reduced.

A mist system contains piping, nozzles, filter, pressure regulator, solenoid valve and timer or controller. Several types of nozzles are available that develop mist size droplets. These include the deflector or impingement type which operates at 30 – 60 psi water pressure. A leak prevention device (LPD) is frequently added to eliminate dripping. A 100 – 150 mesh strainer in the line will prevent the fine holes from clogging. Oil burner type nozzles have also been used by some growers.

Installation should follow the system manufacturer's recommendations. Nozzles can be supported above the bench on risers or suspended from a cable overhead. Most misting nozzles should be placed 3 feet to 5 feet above the crop. Spacing is usually on a grid of 3 feet to 5 feet. Overlap of 100 percent or more is necessary to get uniform coverage of the crop.

A solenoid valve is needed to turn the water on and off. The valve should be the type that normally closes with a snap action operation and it should have the same voltage as the time clock or controller. A 24 volt system is safer than a 115 volt one.

Fog has several advantages over mist. Due to the smaller droplet size there is less saturation of the soil and more oxygen available in the medium. This also can lead to a reduction in problems such as, fungus, moss, grey mold and fungus gnats. Fog also reduces water use by 75 percent or more. Research has shown that many cutting root in a shorter time under fog. Due to the small droplet size it is harder to control the area that receives the fog.

Several methods are used to produce fog. A typical system uses a high pressure pump, distribution piping and nozzles that break the water stream into very fine droplets. These are frequently assembled into a self-contained unit. Piston pumps are needed to develop the 800 to 1,200 psi pressure to get the 10 to 20 micron size droplets. These systems are frequently referred to as dry fog.

Copper, stainless steel, and re-enforced flexible hose are used for piping. Diameter is frequently ¼ or 3/8 inch as the water supply required is only 1 to 2 gallons/hour/nozzle. For propagation, lines of pipe are evenly spaced above the crop area.

Plastic, ceramic, and stainless steel are used for nozzles. Nozzles should have anti-drip check valves to prevent dripping after the system shuts off. An integral strainer will keep the nozzle from clogging.

The greatest problem associated with fogging systems is nozzle clogging from chemical and particulate matter. Calcium deposits can coat the inside of the pipe and nozzles, reducing flow. Water treatment or the use of rain water or bottled water can solve this problem. Several levels of filtration of particulate matter should be installed.

Fog can also be produced by a system using a high-speed fan with water channeled to the tip of the blades. The shearing action as the water exits the blades produces a fine fog. The fan distributes the fog above the crop canopy. This system has the advantage of less clogging as no nozzles are used but some growers have had to remove the system because of the high noise level.

Water at household pressure, injected through a nozzle into a stream of compressed air will also produce a fine fog. Each nozzle requires both a water and air supply. Different flow rates and droplet sizes can be achieved by adjusting the water and air pressure. Distribution can be through ducts, HAF fans, or nozzles evenly spaced over the crop.

Ultrasonic fog generators have also given good results for some growers. They produce a micro fog.

Mist and fog systems can be controlled with a time clock and timer, light-operated interval switch (LOIS), humidistat or controller. The time clock governs the time of day the system operates. The timer turns the mist on for several seconds every few minutes (Example: 3 seconds every 3 minutes). The humidistat is a switch that senses the humidity level of the air and activates the solenoid when the level falls to a preset point.

Controller systems frequently operate with a sensor that measures vapor pressure deficit (VPD). The difference between saturation water vapor pressure and ambient water vapor pressure is the VPD and represents the evapotranspirational demand of the surrounding atmosphere as well as the proximity to the dew point.

Due to the fact that relative humidity varies with temperature, it is better to manage propagation with VPD. By maintaining the VPD below one, water stress within the plant can be kept at an acceptable level.

John Bartok is a regular contributor to Greenhouse Management and an agricultural engineer and emeritus extension professor at the University of Connecticut. He is an author, consultant, and a certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.

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