As you are aware, a greenhouse is used to grow plants and vegetables throughout the year with the help of temperature regulation. It is mostly a closed structure, and hence greenhouses do need ventilation, irrespective of the time of year, to maintain optimal conditions for the plants growing in them.
Why do greenhouses need ventilation?
Ventilation in a greenhouse performs some essential tasks like temperature regulation, humidity regulation, and resets the carbon dioxide levels in the greenhouse. In addition, it helps with pollination and prevents the buildup of unwanted things like pests and fungus.
Greenhouses are built to retain heat in areas where the ambient temperature might not be high enough to cultivate a particular type of plant. However, it is quite possible that the amount of heat in the greenhouse becomes too much for the plants, so ventilation will help with temperature control.
Getting a thermometer to monitor the temperature is always a good idea. Electronic thermometers that log temperatures and that can show you a record of the preceding 24 hours are also available – a little research can help you find some great temperature – related tools for your greenhouse.
Plants give off carbon dioxide and water vapour during the course of the day and night, and without adequate ventilation, the levels might become too high. Too much carbon dioxide is bad, since plants require a balance of CO2 and oxygen for optimal growth. It is a similar story with the water vapour: too much water vapour will help retain heat, and the moisture-rich environment will encourage unwanted growth, as that of fungus, mold, or mildew. Getting rid of these unwanted growths is a lot of trouble, so it is best to avoid their formation in the first place with adequate ventilation.
A couple of tips to maintain the right humidity are to get yourself a device that can measure humidity, called a hygrometer – devices that measure temperature as well as humidity are available on e-commerce sites. They are a combined thermometer and hygrometer. This is a useful device to have, since temperature and humidity are related.
The buildup of pests is also reduced with ventilation, since a warm, humid environment is ideal for the breeding and survival of crop-destroying pests like aphids, gnats, shore flies, slugs, and snails.
Ventilation also helps agents of pollination like bees and insects enter and exit the greenhouse and do their job, thus helping the plants reproduce better. If you’re cultivating flowers, this is a very important factor.
Types of greenhouse ventilation
Greenhouse ventilation can be categorised into two broad categories: natural ventilation, and mechanical ventilation.
Natural ventilation does not need any power, which makes it extremely efficient if done right. It does not require any running cost, since it requires no power. The simplest form of natural ventilation is to build vents into the roof and the sides of the greenhouse. Hot air rises, so it will escape from any open roof vents, and the open vents on the sides will let in cool, fresh air.
A semi-mechanised way of helping this along is to have solar-powered vent openers that open the vents when it is sunny. The installation cost will be higher than a simple natural ventilation system, but it requires no extra investment beyond the installation.
Another natural ventilation system can be designed relatively easily if there are prevailing winds or breezes in the location of the greenhouse – the side of the greenhouse from which the wind predominantly blows should have vents built low down, and the side of the greenhouse downwind from it needs to have vents built high up on the walls. This will allow the prevailing wind to push cool air into the greenhouse, and since the vents on the opposite end are high up, hot air will be blown out of the greenhouse.
However, natural ventilation needs to be thought of from the design stage onward to be fully effective. Mechanised ventilation can be effective even when installed at a later date, or when installed in a suboptimal manner.
A simple mechanized ventilation system is one that can open or shut vents at will, either on the sides of the greenhouse or its roof. If the greenhouse has walls made of flexible material like plastic sheets, a system to raise the walls by rolling them up can be designed relatively easily, to facilitate ventilation and encourage easy access to pollination agents.
Another simple ventilation system using the idea from the ‘natural ventilation’ playbook, that of putting vents low down on one side and high up on another, can be made with fans playing the role of the prevailing wind. Of course, one must remember to install fans that send air in to the greenhouse low down, and out of the greenhouse when mounted high up. Either intake or exhaust fans will work, depending on the requirements.
Greenhouse ventilation in summer
Ventilation of a greenhouse in summer is very closely related to lowering the temperature of the greenhouse as well, as a lot of the systems to regulate temperature are geared towards lowering the temperature by removing the excess hot air from the greenhouse.
As mentioned before, a natural way of doing this is to install vents on the roof and sides to facilitate circulation of air through the escape of the rising hot air via the roof vents, thus creating a vacuum in the greenhouse that sucks in cool air through the vents in its sides. A general rule is that if your roof vents are a fifth of the area of the floor, they will replenish all the air in the greenhouse every five minutes. If you’re using roll-up walls, you can add a skirt to the ground to 1-2 feet high to prevent extremely cold air from entering the greenhouse.
An oft-overlooked aspect of ventilation is to clean insect screens. If they are not cleaned regularly, they can hinder airflow significantly.
If using fans, placing them as intake fans low down will be more beneficial for practical reasons – maintenance will not involve climbing anywhere, and installation will not mean building a structure that can support a fan with a heavy electric motor.
Greenhouse ventilation in winter
Winter ventilation is not as straightforward as ventilation in summer. Simply pulling in air from outside on a sunny winter day when the temperature in the greenhouse goes too high can pull in air that is too cold. Therefore, there are a few systems devised to prevent air that is too cold from entering the greenhouse.
One of the methods is the ‘fan and tube’ method. The intake air is not piped directly into the greenhouse; instead, it is piped through a pipe at or near the top of the greenhouse, which heats the incoming air from the greenhouse heat itself. Only then is it allowed to circulate into the greenhouse itself. Other heating options include fans with inbuilt heaters.
Humidity cannot be allowed to build up in the greenhouse even in winter, so a simple method to prevent buildup and ventilating the greenhouse is to open the vents on sunny days and letting the stale air out. If sunny days are in short supply, then you have no option but to resort to mechanised means of greenhouse ventilation in winter, either with the heated fan option, or by turning heaters on in the greenhouse before opening the vents, to make sure that the temperature doesn’t drop drastically.
A good rate of ventilation to aim for in winter is two or three complete changes of the greenhouse air hourly.
Remember that a thermometer and hygrometer become even more important in winter while ventilating a greenhouse, since the temperature and humidity will change quite drastically.
Whatever the season, ventilation of a greenhouse is integral to the health and optimal growth of the crops in it, as it helps regulate temperature, humidity, and thus help control pests as well.