Gardening often requires the use of a cold frame or a greenhouse, even in warmer climate. A good way to think of the difference between a greenhouse and a cold frame goes beyond heating.
- A greenhouse is a tool that allows the gardener to fully control the environment within that closed space. It goes beyond temperature to include aspects of growing plants such as humidity, lighting, and watering.
- A cold frame on the other hand generally protects plants from the worst of cold weather including frost and damaging wind. Some cold frames are heated, especially in really cold areas.
When should you use a greenhouse and when will a cold frame do the job? In this article, we explore the differences between the two and offer some advice on when to use one and not the other.
One of the biggest problems for gardeners when it comes to greenhouses is that the name means a few things. Most of us have a definition of a greenhouse that actually fits a cold frame. They key of what makes one a greenhouse and the other a cold frame is all about the control one has over the inside environment.
The Temperature Factor
Greenhouses are either hot or cool and the definition of what this means is all about nighttime temperature.
- A cool greenhouse keeps the night time temperature within the greenhouse above 45°F.
- A hot greenhouse keeps the nighttime temperature above 55° at night.
The reason for these different terms is that plant root temperature or soil temperature is critically important for plants to thrive and for seed germination. This of course is species dependent. Most orchids need a hot greenhouse and Japanese maple trees might do well in a cool greenhouse.
At the other end of the spectrum is daytime temperatures. A quality greenhouse allows the gardener to control both the nighttime temperatures and the daytime high temperatures and those buildings that are really sophisticated can do so to very specific degrees.
Humidity and Moisture Factors
Some plants need a very specific range of humidity – either moist air or dry air or something in between. These types of plants are generally grown in an environment that is very different from their native environments. An example would be plants that thrive in desert conditions but are being cultivated in a wet and cold area. A greenhouse might have a humidity control system that adds or removes moisture from the air.
Lighting Factors for Greenhouses
Here in the inland area of California where I live we have a Mediterranean climate. The summers are hot – 85°F – 100°F and with highs to the 110°F range. In the winter are weather is cool and wet – usually. The daytime high might hit 60°F on a warm day and at night the temperature drops down into the 20’s and 30’s but on a really cool night, it can hit 10°F.
This type of climate is fairly rare in the world and we are very lucky to have the largest swath of it in the world. It means that we can grow commercial crops here that cannot be grown elsewhere. Still, we have a growing season -March through October. We have a winter growing season for some plants but not many plants tolerate the drop in daytime light.
They may grow just fine during the summer, but in the winter months their growth becomes very slow or stops altogether. That change in light which happens from October – March reduces the types of plants that we can grow here during the warmer part of the year.
In a greenhouse, there is often some form of light control that allows plants to thrive in climates that have less light then is required by the plants that are grown.
A really interesting industry that requires indoor lighting is the cannabis industry. Those greenhouses have very sophisticated lighting systems so that those plants receive the exact amount of light they need not only to thrive, but to thrive in such a way as to produce the most potency for whichever species of cannabis that is being grown.
Lighting, Humidity, and Temperature are just three of the factors that a greenhouse is likely to control. The more elaborate systems have smart-systems that monitor and control all of these elements in very specific ranges.
This technology has spilled over into things like lawn systems which homeowners now can add in features like soil moisture levels so that their lawn is only watered when the soil reaches a specific dryness or that the sprinkler system shuts down if the system senses rain.
For most of us, this level of greenhouse is not how we define what a greenhouse is. A closed structure where we grow plants. My neighbor has a small nursery and when I talk with him it is always about the weather. His greenhouses are not “uber” sophisticated but they tough on all of the above factors.
He has a little weather station that monitors the humidity during each day and temperatures, etc.
- If the daytime temperature is too hot, then he turns on the misters or opens the big doors to vent the houses.
- If the temperature is too cold, he has propane heaters that he turns on to warm the space.
If the summer is too hot or too bright, he has shade cloth that covers the greenhouse. In the winter, he takes the shade cloth off the buildings so that they take in the most light possible. For a few species of plants that he grows, there is a florescent light bar with grow bulbs in them. He grows trees for bonsai and he is forever fussing about the Maples.
His setup is what I imagine is the most beneficial for the prepping community. Take out the bonsai trees and replace them with food crops and there is no reason that he could not be growing watermelons in December.
The Cold Frame and Its Uses
A cold frame is simply a greenhouse without all of the control features. They range in size by a few square feet to an acre long. The purpose of a cold frame is only to protect plants from the worst of the winter weather. That might be protection from wind, rain, or freezing cold spells. A cold frame works with plants that are adapted to a region.
If I were to try to grow a tropical plant in my neighbor’s cold frame during the winter it would most likely die. The cold frame is not going to raise the temperature more than about 10°F. When I discussed earlier the winter lows usually in the 30’s to low 20’s and sometimes as low as 10°F those are the temperatures that a cold frame is designed to improve.
Most of my plants would die in 10°F weather. As it is, we use sprinklers and misters to protect the fruit trees here on really cold nights. If the temperature here should drop to 10° then the plants in the cold frames would be in temperatures in the high teens or low 20’s. In a really dire situations, we could add a portable heater to a cold frame and at that moment we are blending the boundaries between what a cold frame is and what a greenhouse is.
There are little tricks to make a cold frame perform better in colder weather and one of those is second layer of plastic that provides a 3-6-inch space between the inner layer. That air pocket helps raise the temperature even more by creating a buffer that allows the growing space to remain warmer.
I have used gallon jugs of water that are warmed by the sunlight in the day time and then placed in box with a glass lid on it as heating sources during winter. That confined space allows the water to cool very slowly keeping the cold frame a bit warmer during the night. Again, that is pushing the upper limits of what a cold frame is supposed to do.
What Do you Need to Grow Your Own Food?
How you answer this question really depends on where you live and what type of weather you face on a daily basis. Most of us need something a little more than a cold frame and not as extravagant as a greenhouse.
The basic greenhouse kits that are available are generally not greenhouses at all but fancy versions of cold frames. As long as you know what you need to meet your growing demands, then you can adapt a basic cold frame to fit into your growing environment. It is really easy to add a PVC sprinkler system to a cold frame and turn it into a greenhouse.
It is just as easy to rudimentary control temperatures inside by venting or adding a heat source as simple as solar warmed water. Even a small electric heater works in a confined space. Increasing the ambient lighting also does not have to be overly scientific. A simple florescent light strip with a grow bulb works just fine. All of these things you can find at your local big box store.
The first place to start is by defining what you need to grow and the requirements needed to produce a thriving crop.
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