Getting an answer to “how do greenhouses work?” will help you choose the most productive location, building and interior resources for your homestead’s greenhouse.
Greenhouses are useful for growing produce out of season or starting seeds and cuttings earlier than your climate allows. This jump start allows us to extend our growing season. When I first assembled my greenhouse, here in Tampa, FL, people thought that it was strange. It is Florida after all. I researched how to make a cheap greenhouse, like my neighbors did, but ended up purchasing a kit. Now that it is completed I use my greenhouse year-round.
Some of the best greenhouse plants include tomatoes, eggplants, peppers, and greens. Growing cucumbers in a greenhouse can also beneficial. My cucumbers and especially the tomatoes grew so well they reached the 16-foot peaked ceiling.
How do Greenhouses Work?
As a middle school science teacher, I can tell you that the sun provides us with a wide range of electromagnetic waves. Some of them we can easily detect with our eyes. These waves, which consist of the narrowest part of the spectrum, are called visible light. All the colors of the rainbow fall into this category. The sun, as you know, also provides us with heat. Heat also comes in the forms of waves.
Remarkably some animals can detect these heat waves. Some animals see wavelengths that are longer than visible light and some detect shorter wavelengths. Arctic caribou, pythons, rattlesnakes, vampire bats, birds, and butterflies to name a few, either perceive different colors than us or more!
Since greenhouses are transparent, or at the very least translucent, sunlight enters and visible light penetrates the glass. This is partly absorbed by the plants and soil and reflected on the inside. It’s reflected within the greenhouse and does not easily escape because the greenhouse materials only allow certain wavelengths to travel through. Shorter wavelengths like visible light and short infrared radiation (heat) can pass, but longer infrared radiation (heat) has a difficult time leaving.
Hot objects emit infrared radiation. The warmth of the sun, a radiator or fire all are infrared radiation. Some colors and materials are better at absorbing light and heat. Touching a smooth, black surface (in or outside a greenhouse) on a hot day will confirm this. By having dark colored objects in your greenhouse the more heat it will absorb. Light colored objects reflect heat.
When light enters a material but does not leave it, the light is absorbed. Absorption is the transfer of light energy to matter.
When the sun’s heat enters the greenhouse, where it is absorbed, it heats up the objects inside. For example, the sun heats up the soil and when the soil’s temperature increases the heat is then transferred to the objects next to it. This could be the air or a planted pot sitting on the ground. The air is then warmed and expands. The particles increase in speed and become less dense. The lighter, hotter, air rises, allowing cooler and denser air to take its place. This cooler air is then heated by the ground and the cycle repeats itself. Since the hot air cannot easily escape the greenhouse materials, the overall temperature goes up.
Understanding how greenhouses work highlights why ventilation is so important in keeping a greenhouse from overheating. Thinking about a closed car on a hot summer day will also bring you to the same conclusion.
How greenhouses work at night is similar to the principals during the day. The stored heat from the day continues to cycle. In the greenhouse, the cooling starts from the greenhouse roof. The stored heat from the greenhouse is available overnight, which allows the plants to stay close to the air temperature. You can maximize this nightly process and prevent daytime overheating, by storing black-painted barrels of water under the shelving.
Some gardeners, when space is permitted, fill bins of dark colored sand or rocks to absorb heat during the day to help maintain a steady temperature. These objects also slowly release their heat during the night.
One of the first things that will happen when the air starts to cool is that the moist air of the greenhouse condenses on the inside of the greenhouse roof. Water droplets further block the heat from escaping. Those, like myself, who live near a large body of water, understand the buffering effect of temperature water can have. The larger amounts of water you can store in your greenhouse the more heat they can give off during the night.
In addition to a temperature buffer, greenhouses provide shelter from wind, rain, and snow. Currently, in my greenhouse, I am sprouting a dozen “Flying Dragon” cultivars of Citrus trifoliata. This bitter citrus can be made into a marmalade but is mostly grown for its dwarf size and twisted contorted stems.
What are your favorite plants to grow in a greenhouse?