Once you have put up your greenhouse, it is time to make it do what it was intended to do – foster the rapid and healthy growth of the plants inside it. The strength of a greenhouse lies in the ability to control the climactic conditions inside.
One such significant climactic condition is temperature. You need to have the right temperature if you are to realize optimal plant growth. Light is another important consideration. Note that one climactic factor may have an impact on another. For instance, too much sunlight inside the greenhouse can raise the indoor temperature beyond the appropriate levels.
There are a number of actions you can take to manage the impact of external weather changes to the greenhouse’s internal atmospheric conditions.
a. Ventilation – Every greenhouse should be built with two forms of ventilation in mind – natural and artificial ventilation. For natural ventilation, you need only appropriately position a number of outlets and inlets to allow the natural flow of air in and out of the greenhouse. Electricity may or may not be used but this would only be for controlling opening and closing the inlets or outlets. While natural ventilation is a great cost saver, it is ineffective in locations with high outdoor temperatures.
And this is where automatic ventilation comes in. Auto-ventilation regulates indoor temperatures using electricity powered louvers and exhaust fans. The difference between automatic ventilation and electricity-powered natural ventilation is that the former is usually part of a larger air conditioning system that detects temperature and powers the louvers and fans accordingly.
b. Cooling – When ventilation alone is inadequate to manage the temperature within the greenhouse, a different approach is required. Two common strategies used to control temperature and that can augment ventilation are: fog systems and pad-and-fan systems.
Fog systems use nozzles installed every 50 to 100 feet in the greenhouse to evenly distribute a cooling ‘fog’. It is expensive because clean water must be used to prevent the tiny mouth of each nozzle from getting blocked.
Pad-and-fan technology goes hand in hand with automatic ventilation. Evaporative pads are installed at air inlets and cool the air entering the greenhouse. The cool air circulates within the greenhouse picking heat in the process before it is pulled out via the exhaust fans.
c. Humidity – Humidity must be kept in check to prevent plants from succumbing to disease. High moisture content in greenhouse air increases the amount of condensation on plants thus hampering respiration. High humidity is also a breeding ground for pests and fungal plant diseases. To better manage humidity, the vapour pressure deficit (VPD) must be measured regularly and maintained at the optimum level of 0 to 1 psi. VPD is a calculation of atmospheric humidity versus the humidity at which water condensation will start.
d. Shading – Shading is a means of temperature and light control that uses automatically operated shades or blinders. The shades close when there is too much sunlight during the day or if warm temperature needs to be retained in the greenhouse at night. A sensor detects internal temperature and triggers the shades’ opening or closing.