How Warm Does an Unheated Greenhouse Stay?

Though a lot of people opt to install fancy heating systems into their greenhouses, there are many others who choose to grow their plants the old-fashioned way and leave the structure unheated. So how warm can you expect your unheated greenhouse to be? 

An unheated greenhouse will stay 5-10° warmer than the outside temperature so long as the structure has been properly insulated. These numbers can vary during extreme hot or cold temperatures. Sunlight also plays an important role in a greenhouse’s temperature. 

In the rest of this article, I’ll discuss everything you need to know about using an unheated greenhouse in greater detail. I’ll also answer a few frequently asked questions about growing plants in this kind of structure. So if you would like to learn more about the practice, read on!

Do Unheated Greenhouses Actually Work?

First, it’s essential to understand if you can actually achieve success growing in an unheated greenhouse. Is using an unheated plant nursery worth it? Can you grow healthy plants in an unheated greenhouse? 

Unheated greenhouses work so long as you live in a warm climate or only grow plants during the warmer months. The better insulated your greenhouse is, the longer you will be able to grow as the seasons become colder. How much sunlight your greenhouse gets will affect its interior temperature. 

Another approach that can help your plants survive is to grow cold-resistant varieties as the months start to cool off. This will ensure that your plants live a bit longer before succumbing to the cold. 

Cold hardy crops would include:

  • Spinach
  • Garlic
  • Broccoli
  • Cabbage
  • Kale
  • Arugula
  • Turnips
  • Brussel sprouts 

These are just a few of the more popular cold-resistant crops people like to grow. They will do exceptionally well in your unheated greenhouse. 

Do You Have To Heat Your Greenhouse?

All gardeners strive for their plants to grow healthy and strong, and accomplishing this with as little upkeep as possible is always a plus. However, do you have to go through the hassle of installing heaters for plants to thrive in a greenhouse? 

You do not have to heat your greenhouse unless you live in an area that experiences frequent cold weather. However, there are ways you can naturally bring up the temperature in your greenhouses, such as insulation, solar warming, and compost heating. 

You’ll have to plan your growing seasons when using an unheated greenhouse in colder climates, as the colder months will kill your plants. However, many only garden during the warmer months anyway. Therefore, it ultimately comes down to what your gardening preferences are. 

Ways To Keep Your Unheated Greenhouse Warm

Luckily, if you wish to leave your greenhouse unheated, there are several alternative ways to naturally increase its temperature. Creating more warmth isn’t necessarily a priority during the summer months, but it becomes crucial as the seasons get colder.

So let’s dive in and go over a few methods you can try to keep your unheated greenhouse warm for longer. 

Things you can do to keep your unheated greenhouse warmer include:

  • Adding insulation. You can use several different types of insulation, ranging from bubble wrap to greenhouse insulation panels. Insulating is a great way to trap in heat so that your greenhouse can stay warmer for longer. 
  • Ensuring your greenhouse is in full sun. How much sun your greenhouse gets during the winter really does matter. So if your greenhouse happens to be in the shade, it’s probably a good idea to move it into full sun. 
  • Adding a compost pile inside of your greenhouse. You can use compost to heat your greenhouse as the organisms that break down the waste grow hot as they work, thus warming the space.
  • Using large rocks. Large stones can be placed along the floor of your greenhouse to help trap heat during the day so that your space will stay warmer longer. 
  • Sealing up all holes. Warm air will escape through holes in your greenhouse. So it’s always a good idea to do an inspection and seal up any spots you notice. You can easily do this with some caulking. 
  • Placing cut-off plastic bottles over all of your plants. Plastic bottles are an excellent cheap way to lock more heat into your greenhouse and keep your plants warm as temperatures plummet, especially during the night. You simply have to place the cut bottle with the lid on over your plants, and then you have your very own little terrarium. 

So there are certainly a few options if you wish to keep your greenhouse unheated. The most effective being properly insulating your structure. Oddly enough, most people swear by the bubble wrap insulation method, and it’s relatively inexpensive.

It’s also important to note that the type of greenhouse you have will affect how well your structure retains heat, i.e., plastic vs. glass greenhouses. Both types of greenhouses will work fine, but one might need a little more insulating than the other. 

Will an Unheated Greenhouse Keep Plants From Freezing?

So now that you know a few ways you can keep your unheated greenhouse warmer for longer, it’s time to ask some critical questions. Will an unheated greenhouse keep plants safe from frost? 

An unheated greenhouse will not keep plants from freezing during cold weather. However, unheated greenhouses are still excellent options during the warmer months, and a heater can always be added in the winter if you wish to continue gardening. 

Ultimately, you will have to decide what type of gardening you’re looking to get into and for how long you want to be able to grow throughout the year. For example, if you are simply looking to grow some crops during the warmer months, then an unheated greenhouse might be perfect for you.

However, if you want to extend your growing season into the cold winter months, you may need to look into a heating source of some kind. Otherwise, your plants will likely freeze and die. Even frost-resistant plants will not do well in temperatures below 55°F (13°C). 

When Should You Plant in an Unheated Greenhouse?

Deciding when to start planting in your unheated greenhouse can feel a bit tricky; however, you should treat planting just as you would in an outdoor plot. Greenhouses are typically only 5-10° warmer than the outside temperature after all.

You should plant during the months of March and April in an unheated greenhouse. If you live somewhere with warmer weather, you can get away with planting mid-February. However, you should do your research and monitor if cold fronts are expected as these will kill your new plants.

The gardening zone where you live plays a huge role in when you can start planting. People who live in warmer climates are less restricted than those who live in biomes with regular cold fronts and snow. Therefore, it’s essential that you understand when the last cold front is meant to hit your area.

Moreover, should you find that you have planted too early, you can always take precautions, such as covering your plants or adding a heater to combat the approaching cold front. Therefore, there’s always a little leeway for experimentation. 

If you’re fully against adding external heaters, however, I recommend erring on the side of safety and planting only once the weather becomes warm enough.


An unheated greenhouse will generally stay several degrees warmer than the temperature outside. Provided the structure has been insulated, and there are no significant tears/openings for heat to escape from. 

Unheated greenhouses are an excellent option for those who live in tropical or desert climates since those areas tend to stay warm longer into the year. However, if you live in a cold climate, an unheated greenhouse is still a good option for the summer months. Plus, if you want to garden longer into the year, adding a heater is always an option.

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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