How to Grow in an Unheated Greenhouse

Greenhouses for sale are the darlings of the season-extending cultivation. Such a goal becomes an achievable feat even for those budding green thumbs. Catering to novices and seasoned gardeners alike, these nurturing structures are a must!

However, the challenge lies in taming the costs of winter heating. Enter unheated greenhouse solution. While it demands planning and groundwork, the rewards are undoubtedly remarkable.

Join us as we explore the best practices for your growing success!

Growing in an Unheated Greenhouse Garden

BillyOh Harvester Walk-In Aluminium Polycarbonate Greenhouse
BillyOh Harvester Walk-In Aluminium Polycarbonate Greenhouse

Starting seeds outdoors during the colder months can be challenging. And in moments like these, greenhouse owners give thanks for their unheated investments. Not all winter vegetables would be happy underneath the snow anyway. Some might prefer flourishing in a greenhouse environment.

Unheated greenhouse buildings don’t rely on any external heating system. Unlike a polycarbonate greenhouse, which does to maintain a warm temperature inside. Instead, they use passive methods to regulate the temperature, such as:

  • Insulation
  • Proper ventilation
  • Thermal mass

The idea is to trap and retain heat from the sun during the day and release it slowly at night. This makes them ideal for growing cold-hardy vegetables and leafy greens. It’s also a more affordable and sustainable option than heated greenhouses. Other great unheated models include wooden greenhouses. With this material, timber acts as an effective insulator.

The primary challenge is to maintain a stable metal greenhouse temperature. To keep it at a reasonable level, insulation is key.

This can be achieved by using double-layered plastic sheets or bubble wrap. Using a thermal mass, such as water barrels or rocks, can also regulate the temperature inside. Also, avoid overwatering the plants, as this usually can lead to fungal diseases.

Starting seeds

Sprouting plants in a tray

(Image Credit: Flickr)

The first stage of seed germination tends to occur in colder temperatures. This makes the late winter the perfect time to start seeding.

Choose cold-hardy seeds that can withstand low temperatures and frost. Use a high-quality soil mix and water the seeds carefully. Cover them with horticultural fleece or frost cloths for extra protection. It’s also important to monitor the greenhouse temperature and humidity.

Top tip: Open the doors or vents of your greenhouse wooden during sunny days to allow for ventilation.

Hardening off

It’s essential to expose the plants to cooler temperatures gradually. To do so, open the vents during the day and close them at night. This will help the newly transferred batch adjust to the changing temperatures.

For seed trays, e.g. winter greenhouse plants, they can be acclimated a week earlier. For warm-season greens, begin two weeks before the last frost dates in your area. And for store-bought ones, you can place them straight inside for acclimation.

Invest in a thermometer

Thermometer inside a greenhouse

(Image Credit: Flickr)

Monitoring the temperature in your unheated greenhouse is crucial. Invest in a thermometer to keep track of it to ensure the plants are thriving.

Consider cold frames

DIY cold frame greenhouse

(Image Credit: Flickr)

Cold frames are a great addition to an unheated greenhouse. They are small, enclosed areas that can be used to start seedlings or protect plants in winter. Moreover, they can be made from a variety of materials, including old windows or even hay bales.

Basically, the concept is a simple box with a transparent roof for sun exposure. This keeps the greens warm and protected from harsh weather conditions. The materials and making process for this do-it-yourself approach are also simple. 

Follow our guide to DIY mini greenhouse (see #2) for more details!

Plants to Grow in a Greenhouse

Green vegetables, salad greens, and hardy herbs to grow

Cold-hardy veggies like kale, spinach, and lettuce can thrive in an unheated greenhouse. Additionally, herbs such as thyme, oregano, and rosemary can also withstand winter temperatures.

Root vegetables:

Freshly picked beets

(Image Credit: Pxfuel)

These crops can be harvested in the fall or early winter when the weather is cooler:

  • Beets
  • Carrots
  • Turnips

They can tolerate cooler temperatures and can be grown in containers or raised beds. Keep in mind though that they take longer to mature than leafy greens, but they are worth the wait.

Leafy greens:

Winter lettuce inside a greenhouse

(Image Credit: Flickr)

These crops can be harvested multiple times throughout the growing season:

  • Arugula
  • Kale
  • Spinach
  • Varieties of lettuce (e.g. winter lettuce)

This makes them a great choice for continuous harvests.


Herb greenhouse growing

(Image Credit: Wallpaper Flare)

Herbs can be grown in pots or raised beds, and they don’t require a lot of space. Some popular varieties to consider are:

  • Basil
  • Cilantro
  • Parsley
  • Thyme


A gardener inside their greenhouse

(Image Credit: Pexels)

Flowers can also be grown in an unheated greenhouse. Some popular options include:

  • Pansies
  • Snapdragons
  • Violas

Top tip: It’s important to research the specific plants you plan to grow. Also, ensure they can handle the colder temperatures of an unheated greenhouse.

Fruit trees:

Citrus tree

(Image Credit: Pixabay)

If you have a larger unheated greenhouse, you can consider growing fruit trees. Some popular options include:

  • Citrus trees
  • Fig trees
  • Olive trees

These trees can take several years to mature, so patience is a must. However, once they start producing fruit, you’ll have a bountiful harvest for years to come!

Tips for Successful Winter Gardening

Now that you know how and what to grow in an unheated green house, these extra tips can be a real game-changer:


Make sure you have enough greenhouse space for your plants. Overcrowding them can limit their growth potential and could cause them to die. Also, ensure the leaves don’t touch so they’ll thrive.

Raised garden beds

If you’re thinking of adding garden beds, thinning out the weak plants helps. This is to give the hardier greens room to grow.

Signs of disease

If you see any signs of disease on your winter vegetables, remove these plants straight away. This is to prevent this from damaging your other produce.

Dealing with pests

To keep pests at bay, consider planting marigolds. These have natural pest repellents in them! Or, invite the ladybugs in to keep those pesky critters under control.

What to Expect

During the winter months, plants won’t grow as quickly as they do during the summer. Some may not reach their full potential but that won’t be a problem when harvesting them – they’ll be just as good!

During late winter, like January and December, some plants may not grow for several weeks. But this also won’t be an issue. As long as these plants don’t die, they’ll be perfect to harvest and thrive in this environment.


Unheated greenhouses can still be a productive and rewarding space for gardeners. With the right crops and care, you can enjoy fresh produce all year round. Remember to monitor the temperature and ventilation to ensure your plants thrive!

If you’re still a little unsure about which of our greenhouses is best for you, take a look at our range below. If you lack space, consider investing in a polycarbonate lean to greenhouse. Alternatively, go for a wooden growhouse. Either way, both make a practical and aesthetically pleasing addition to any garden.

Moving on: How to Start a Garden Greenhouse

Shop Greenhouses


Yes! Your unheated greenhouse can hold onto warmer temperatures than the outside, meaning your plants can stay frost-free!

Yes! You’ll be able to wake up in the morning to plants that are alive and thriving.


Your unheated greenhouse will only be heated by the sun. This means that if the temperature is 0 degrees, for example, your greenhouse temperature will be 5 degrees celsius. It will always be 5 Celsius above the temperature outside.

Yes, you can. Take a look at our ‘What you can grow’ section above to see some of the perfect vegetables to grow in winter!

No. Your greenhouse will need a solid and level base to sit on. We recommend concrete, but decking or paving slabs would also be fine. Take a look at how to build a base!