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How Does a Greenhouse Work?

Wendy Connick
Glass Greenhouse

A greenhouse can help increase plant growth and fruit production and even allow you to grow plants that wouldn't normally survive in your climate. Understanding how the process works can help you to get the most out of your greenhouse.

Greenhouses Trap Heat and Light

Plants need light, warm temperatures, air, water, and nutrients to survive and grow. Different plants have different requirements for each of these necessities. A greenhouse works by providing the first two requirements for your plants, but the last three are up to you.

Step 1: Light Comes In

In order to provide light, greenhouses need to have some way for the light to come in. This is why greenhouses are made of mostly translucent materials, like glass or clear plastic. This gives the plants inside maximum access to sunlight.

Step 2: Heat Is Absorbed

When the light comes in the glass walls of the greenhouse, it is absorbed by the plants, ground and anything else in the greenhouse, converting it to infrared energy (aka heat) in the process. The darker the surface, the more energy it can absorb and turn into heat. This is why black pavement gets really hot in the summer. It's absorbing a lot of heat.

Step 3: Heat Gets Trapped

Once the light energy gets converted into infrared energy (heat), it has a different "shape" than light energy - what scientists refer to as wavelength. The change in the wavelength makes it so that the heat can't easily escape out of the greenhouse's glass walls. So while getting in was easy, getting out is harder.

Step 4: Warming the Greenhouse

The trapped heat warms the air inside the greenhouse and because a greenhouse is relatively air-tight, the warmer air stays inside, raising the entire building's temperature. This is the same effect that you've no doubt experienced when getting into a car after it's been sitting in a sunny parking space for a few hours. It is nice and toasty.

greenhouse diagram

Step 5: Staying Warm

With sufficient sunlight, the temperature inside a greenhouse may become much higher than the outdoor temperature; in fact, on a hot sunny day you may need to ventilate the greenhouse all day to keep from literally cooking the plants inside. On overcast days, less sunlight means that the greenhouse will heat up more slowly, if at all. For that reason, greenhouses are most useful in areas that have plenty of sun.

Step 6: Promoting Photosynthesis

All this light and warm temperatures give plants ample access to the sunlight and temperatures need to grow. This is because they have the right conditions for photosynthesis to occur. Photosynthesis is the combining of carbon dioxide from the air and energy from sunlight to make simple sugars, which the plant then uses as food. You might use a cheeseburger to get big and strong, well a plant uses the sun. On average, plants need about six hours of sunlight per day, although this varies depending on the type of plant; placing your greenhouse where it will get full sun all day will ensure that the plants inside get enough light.

When There's No Sun

The plastic or glass that makes up most of a greenhouse's exterior is great for letting in the maximum amount of light, but it's a poor insulator (it doesn't hold heat well). This means the heat energy travels though it eventually to escape to the outside world. As long as the sun is shining this doesn't matter because light energy comes in faster than the heat can get out. But at night, all that heat energy will quickly leave, which will leave your plants at the mercy of lower nighttime temperatures. In order to protect your tender plants, you need to either store excess heat during the day or use an artificial heat source at night.

Storing Heat During the Day

Different materials take different amounts of energy to heat up (bricks take longer to get warm than dirt or gravel), a characteristic known as thermal mass. The higher a material's density, or how packed together it is, the more energy it takes to raise the temperature of that material. So, high density materials can store a lot of heat. Examples of high-density materials include:

  • Stone
  • Brick
  • Water

Flooring Matters

Adding a brick floor to your greenhouse means that it will take longer for the building to heat up during the day, but during the night, all that extra heat energy will slowly be released into the air inside the greenhouse. This will keep your plants warm and toasty even after the sun goes down.

Dual Purpose Features

Some enterprising greenhouse owners multi-task by keeping large fish tanks inside their greenhouses. The water in the tank provides lots of thermal mass to store heat, the fish grow faster and are more productive thanks to the extra warmth, and the waste retrieved when cleaning out the fish tank makes excellent fertilizer for the greenhouse plants.

Adding Artificial Heat

If adding more thermal mass to the greenhouse isn't an option, you can always resort to putting in an artificial heat source such as a space heater. Ideally, you'd want a heat source that you can link to a thermostat so that it automatically keeps the temperature inside within the desired range. Greenhouse supply stores carry many types of artificial heaters designed specifically for use in greenhouses.

Meeting Your Plants' Needs

While greenhouses are highly efficient at their two strong points - providing light and heat - your plants will still need your help to meet their other needs. In fact, the high levels of light and heat often cause plants to use up nutrients and carbon dioxide at an accelerated rate, meaning you have to fertilize regularly and vent the greenhouse periodically to provide fresh air. The high temperatures also make water evaporate faster, so diligent watering is critical, especially for container plants. Taking care of these simple tasks will help your greenhouse plants to thrive.

The Effective Greenhouse

Greenhouses are a great alternative for growing plants in the winter months or even the summer. By trapping the light and turning it into heat, these ingenious creations keep the plants fed and warm. Adding elements like bricks and stone, along with water can help to trap the heat for those cold nights. You can also consider adding an external heating source.

How Does a Greenhouse Work?