How Do Greenhouses Work?
Your DIY Greenhouse has a long history. Humans invented greenhouses in their attempt to control nature and adapt it to human needs. Fruits and vegetables are not naturally available at all times during the year, in all climates and in all geographical locations. Humans started growing plants in greenhouses to get around this fact and create a controlled environment in which plants can grow when and where humans need them.
In order to grow plants need sunlight, water and nutrients. Greenhouses are built to provide all those needs at all times. At its most basic, greenhouses collect the sunlight and convert it to heat; store that heat by preventing it from escaping back into the outside, colder air; keep the temperature and humidity more or less stable by keeping the cold air outside; preventing too much rainwater from inundating plants, and protecting plants from pests and diseases.
Short and Long Wavelengths
Glass (as well as other transparent materials such as plastic) allows most of
solar (sun) wavelengths to pass through, except the long (thermal infrared) wavelengths. So, solar radiation can come into the greenhouse, where the plants convert it into thermal, long wavelengths (heat). Since the glass is not transparent to the thermal wavelengths, they get trapped in the greenhouse, heating the air, plants, soil and the greenhouse structure itself.
When heated by the sun, soil warms up, whether it is in the greenhouse or outside of it. The air around the soil warms up as well and, as it is less dense than the cool air, starts expanding and raising. Outside of the greenhouse, the warm air goes into the atmosphere, where it eventually cools off. In the greenhouse, hot air is trapped, so the temperature in the greenhouse keeps rising throughout the day. The heat causes water to evaporate, creating high humidity making the greenhouse atmosphere better for plants’ growth.
Heating and Cooling
Of course, we cannot stop the sun from shining and heating the greenhouse, so the temperature can become too high for plants. It is up to us to design a system to control the amount of heat that remains in the greenhouse. Well-designed greenhouses have vents that automatically open when the air temperature reaches certain point, and close when the temperature drops below the one desired. Fans help to keep the temperature within the entire greenhouse even by circulating the air – moving the hot air higher up and mixing it with the cooler air at the lower parts of the greenhouse.
Automatic misting system can also help with keeping the temperature and humidity stable.
Of course, the air is not the only thing that gets heated by the sun. Everything else in the greenhouse gets heated to a different extent. Wood, water, soil and bricks get heated slowly and release heat slowly. Iron and aluminum warm up fast, and release heat fast. This is particularly important at night, when the stored heat or ‘thermal mass’, slowly releases the heat, keeping the temperature in the greenhouse warm even when the sun is not there to heat it up. That is why it is so important to design the greenhouses carefully, using materials that have the ability to store and release large quantity of heat slowly. Wooden frame, brick greenhouse floor, open plant trays full of soil, all store and release heat slowly, and are more useful to keeping the greenhouse temperature optimal at night than iron or aluminum, which heat fast, but lose the heat fast as well.
Plants in the greenhouse get a fair amount of needed water through water vapor, but it is not enough for a fast growing process. Automatic watering system provides water to plants when it is needed, and the greenhouse structure prevents too much water, such as excess rain, from inundating plants.
A well-designed greenhouse will have a system or gutters and drainage for managing the excess water inside and out.
Greenhouse also acts as a ‘house’ for plants – it keeps them protected from insects, pests, domestic pets, hail, strong wind, falling debris, flying baseballs and anything else that can damage your precious buds and seedlings. Keep in mind that pollinators, like bees, are also kept outside, so choose self-pollinating plants or introduce a few bees inside.
It is important to learn how things work and why greenhouses do such a great job of providing perfect environment for our plants, so that we can improve our DIY Greenhouse design and learn even better ways to control our growing environment.