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From seasoned farmers to amateur houseplant enthusiasts, many have asked me this question over the years. Most people are surprised how much physical science is behind the answer. So today I will break down how it works to make understanding greenhouses easy.
Greenhouses make plants grow quicker by creating an environment that meets their needs to the extreme. They use glass or polycarbonate walls to trap the gases plants need to breathe, as well as the heat and moisture seeds need to grow. They can even provide irrigation and temperature management.
Understanding a greenhouse means understanding a plant’s needs. Luckily, a plant’s needs are not that different from our own: They need sunlight, food, and water just like us. But how does a greenhouse provide these things?
Greenhouses are Mother Nature’s Gasmask
The most advanced greenhouses are designed with a highly scientific understanding of the plants that live within them. They will have sensors that detect everything from heat, to humidity, to the micro-bacteria in the air.
In order to stimulate optimal plant growth, these sensors will communicate with heaters, air vents, sprinklers, and specialized lights that produce exactly the wavelength of light the plants need to grow the fastest.
But that is for the most cutting-edge of greenhouse technology. The fundamentals of greenhouses are much simpler.
To begin with, greenhouses are sealed shut to keep all the gases plants produce and consume contained.
Plants breathe carbon dioxide during the day and produce oxygen as a by-product. Then at night, they do the opposite, breathing in oxygen and producing carbon dioxide.
Keeping a greenhouse sealed allows plants to have constant access to their own breathing cycle. It’s as if the greenhouse was a giant gas mask for the plants inside.
Besides trapping air, the most basic function of a greenhouse is to trap heat.
Sunlight passes through the glass panels of the greenhouse, but once it reaches the ground it bounces off as a different kind of light. This light is infrared light, and the glass panels of a greenhouse are designed to hold it in.
This in turn holds in its heat, which is why greenhouses are so warm. Plants love this warmth, though it’s more accurate to say the plant’s seeds love this warmth.
Germination: The Silent Life-Giver
The reason heat benefits plants so much is because of a process called “germination”. Germination is the process of a seed growing into a plant. Seeds are inanimate balls of enzymes encased by a protective shell, all surrounding an embryonic plant.
They remain dormant in this state until they are given dirt in which to root, water to drink, and heat to stimulate their growth. They are so dormant that seeds as ancient as 2,000 years old have been able to grow into plants.
Most seeds will remain dormant until exposed to sufficient heat, which greenhouses specialize in providing. In addition to heat, the seeds need water to stimulate their metabolisms. The heat does not just trigger growth though, it also stimulates it.
Heat increases the rate of chemical reactions. That is where the humidity of greenhouses comes in: The lingering heat of the greenhouse warms the water inside the soil, making it react with the seed’s enzymes more potently. The more potent the reaction, the greater the growth.
A Structure for all Seasons
However, it would be an oversimplification to simply say that more heat means more growth. If that were true, the Sahara Desert would be a lush and vibrant place. There’s nuance to the environment of a greenhouse.
Heat stimulates growth, but it can also dry out the soil and leave seeds with nothing to feed their plant embryos. Therefore many greenhouses will have vents to manage the heat of their ecosystems.
The vents are electrical and automated, turning on to release heat once the temperature of the greenhouse gets above a certain threshold.
All that technology just to grow some carrots makes the process seem more complicated than it is. Back in ancient Roman times, some of the earliest recorded artificial environments were achieved by growing potted vegetables in carts.
The carts would be wheeled from the sun to the shade depending on their growth needs. Sounds much easier than installing sensors, vents, and LED sunlamps.
Part of the purpose of a greenhouse is for it to be low maintenance. Just remember that technology can increase maintenance as much as it can decrease it.
Many greenhouses will be built with trees situated outside their western-most wall. This is because the trees will provide shade during the summer when the days are long and the heat is most likely to negatively affect plant life.
However, the trees will shed their leaves during the winter, meaning that the shade will disappear, and the much-needed sunlight will be allowed in for the whole day.
Winter is Always Coming
That brings us to one of the great questions of growing any plant life: What about the cold, can you grow in a greenhouse during winter? How can anything, even in a greenhouse, survive that?
There is less light in the winter, and even when the sun does show itself heat will be scarce. As stated before, greenhouses stimulate the germination process. Most seeds simply do not germinate in the winter due the obstacles posed by the temperature.
But people still need their plants to grow during the winter. This problem has plagued humanity for our entire history, and as such there have been many innovations addressing it.
Perhaps the simplest factor in getting heat into a greenhouse is where the greenhouse is built. A greenhouse will usually have two long walls on its sides and two short walls for its front and back. When the front and back of the greenhouse face east and west respectively, it allows the greenhouse to receive the most heat throughout the day.
This is because the sun will slowly travel over the length of the greenhouse. This is especially true during the winter, when the angle of the sun will be less favorable to a greenhouse facing north and south.
Another solution to the winter chill is insulation, but not the kind of insulation one would use for a house. That would block the sunlight, and if plants need anything more than heat it is the sun.
No, much better than normal insulation is bubble wrap (Amazon link). Bubble wrap is transparent, meaning light will still pass through it, as well as being thicker and absorbing more heat than most types of glass a greenhouse will be made of. The pockets of air in the bubbles themselves will also act as “traps” for the cold, creating a web of protection for your greenhouse.
If one cannot get their hands on several dozen feet of bubble wrap, then there is another solution to the problem of heat in the winter: A thermal mass. This can be anything large and dense enough to hold a lot of heat.
The easiest thermal mass to make is a black-painted jug of water; the black paint will absorb heat from sunlight, which will then be stored in the water.
When night rolls around the area around the thermal mass will cool. This will essentially “pull” the heat out of the thermal mass, turning it into a makeshift radiator.
This method of warming a greenhouse is particularly interesting, as it relies on one of the core rules of thermodynamics: Heat does not just disappear in the cold. Thermal energy is constantly trying to balance itself out.
When something warm is placed up against something cold, the heat will transfer into the cold object until both contain the same amount of heat. The greenhouse must act as a buffer against the cold. Insulation can make the transfer of heat slower, while a thermal mass will use the rules of heat to store warmth for later.
These techniques of warming a greenhouse will usually only produce a few degrees of warmth. But plants are resilient. A few degrees can mean the difference between a seed growing with sustainable germination, or the water in its system freezing.
Greenhouses Make Plants Die Slower
During the spring and summer, a greenhouse’s job is to act as a breathing mask for plants. During the fall and winter, it is a blanket. And as along as the plants inside can last till the next sunrise, it is doing its job.
It can be mind-boggling to think of how much science goes into something as seemingly simple as a greenhouse. Plant biology, thermodynamics, and meteorology all combine to determine the sustainability of a greenhouse’s ecosystem.
But with the right know-how and little intuition, all of these disciplines can easily be manipulated to turn a lifeless glass box into a life-giving haven for even the most sensitive of plant specimens.