Fog is commonly used as an alternative to mist to enable germination as well as propagation. It comes with various benefits such as more uniform wetting across the area where the plant is, ability to penetrate better in the foliage, lower maintenance cost and a higher efficiency. For the sake of propagation, it is possible to maintain a humidity level that is relatively high without having to saturate the medium used for growing. Fog can also be used for the purpose of evaporative cooling.
Choosing between fog and mist
The fog particles have a diameter of less than 50 microns. When it comes to high pressure greenhouse fog systems, the particle size that is used is usually around 10 microns. On the other hand, mist has a particle size of 50 to 100 microns. When one gallon of water is broken into droplets of 50 microns, it can produce around 58 billion droplets of fog.
When these tiny droplets are injected into air, they will stay suspended until when they evaporate. On the other hand, mist is usually heavier will can take longer to evaporate. This means that mist is more likely to fall out of the air and thus wet the plant surface. The mist particles that are the smallest are usually carried by the air currents. These continue becoming smaller to the point where they get vaporized.
Propagation using mist
The vapor-transpiration rate is affected by the humidity from the surface of the leaf. If you would like to get good propagation, you will need to ensure that there is a balance been transpiration and humidity necessary to enable nutrients and water intake without causing a lot of dehydration. If a crop has a foliage canopy that is dense and there is no enough movement of air, there will be a moisture boundary that will develop around the plants. In case the growing medium also gets saturated, problems may arise as a result of moss, Botrytis, fungi and fungus gnats.
In case there is high temperature in the air and there is an increase in the leaf temperature, the loss of water can exceed the plant’s ability to absorb moisture and this may cause the plant to be stressed up. When fog is used at such a time, the air temperature and the humidity among the plant canopy may reduce without causing the saturation of the growing medium. When the amount of oxygen in the root zone increases, this may help in faster tooting. The moment the root system gets established, this will cause a reduction in the relative humidity.
There is need for experience in determining the right level of humidity. Some of the guidelines that can be used include:
- Establishment phase. A relative humidity of 60 to 80 percent
- The phase of rapid growth. Humidity of 55 to 70 percent
- Hardening phase. Humidity of 45 to 50 percent
The fog system has an advantage in that it is possible to apply fungicides, foliar feeding and insecticides through this kind of system. This can enable you to save time even as it ensures a uniform application.
How to control fog systems
It is possible to use time clocks, humidistat and sensors to control fog systems. The manner in which the time clock operates is such that the fog will be turned on for some few seconds and for several times in a day. It is also possible to control fog systems using a controller that is able to measure the vapor pressure deficit. Since relative humidity will vary depending on the temperature, propagation can be better managed using VPD. When the VPD is maintained to below one, it is possible to keep water stress to acceptable levels within the plant.