Will Plants Freeze In A Unheated Greenhouse?

Picture this: a serene greenhouse surrounded by a winter wonderland – its glass walls protecting the vibrant plants within from the harsh elements outside. But wait, there’s a twist – this greenhouse is unheated. Your mind might immediately conjure up images of wilted and frost-covered plants, but fear not! In this article, we will explore the fascinating question of whether or not plants will freeze in an unheated greenhouse. Buckle up for a journey into the world of horticulture, where we’ll discover the secrets of survival for these resilient and resourceful green inhabitants.

Factors That Influence Freezing in a Unheated Greenhouse

When it comes to maintaining a healthy and thriving greenhouse throughout the winter months, understanding the factors that influence freezing is crucial. With an unheated greenhouse, you may be concerned about whether your plants will freeze or not. However, by considering various factors such as outside temperature, inside temperature, type of plants, plant hardiness, and humidity levels, you can effectively manage the freezing risk and provide the best possible conditions for your plants.

Outside Temperature

The outside temperature plays a significant role in determining the temperature inside the greenhouse. During cold winter nights, the temperature outside can drop dramatically, and without any heating system, the greenhouse will naturally cool down as well. This cooling effect can lead to freezing conditions inside the greenhouse, potentially damaging your plants.

Effect on Greenhouse Temperature

The outside temperature directly affects the greenhouse temperature, influencing the overall climate inside. The colder it is outside, the harder it is to maintain a warm environment within the greenhouse. It is essential to monitor the outside temperature regularly to anticipate any significant drops that may impact the temperature inside.

Cold Hardiness of Plants

To prevent your plants from freezing in an unheated greenhouse, it’s crucial to choose plants with suitable cold hardiness. Cold hardiness refers to a plant’s ability to tolerate freezing temperatures without sustaining damage. Some plants have a higher tolerance for cold temperatures and can withstand freezing conditions better than others. By selecting plants that are naturally more cold-tolerant, you can reduce the risk of freezing in your unheated greenhouse.

Inside Temperature

While the outside temperature is an important factor, managing the inside temperature of the greenhouse is equally essential. Several factors contribute to maintaining a stable and favorable temperature inside the greenhouse during colder periods.

Thermal Mass

Thermal mass refers to the ability of an object or material to absorb and store heat. Utilizing thermal mass in your unheated greenhouse can help regulate temperature fluctuations. Materials such as water-filled containers, bricks, or concrete floors can absorb heat during the day and release it at night, providing a more stable environment for your plants.

Solar Gain

Taking advantage of solar gain is another way to maintain a higher temperature inside the greenhouse. By strategically positioning the greenhouse to maximize exposure to sunlight, you can harness the sun’s natural heat and keep the greenhouse warmer during the day. This trapped heat can help counterbalance the cooler outside temperature, minimizing the risk of freezing.


Proper insulation is essential for reducing heat loss and maintaining a consistent temperature inside the greenhouse. Insulating the walls, windows, and doors can help prevent the cold air from permeating the greenhouse, ensuring a warmer environment for your plants. Additionally, using double-glazed or thermal-insulated materials can provide extra insulation and help retain heat more effectively.

Type of Plants

The type of plants you choose to grow in your unheated greenhouse can greatly influence their susceptibility to freezing. Some plants are more sensitive to cold temperatures, while others are naturally cold-tolerant.

Sensitive Plants

Certain plant species are more sensitive to freezing temperatures and require additional protection or a heated greenhouse. Tropical plants, such as orchids or citrus trees, are examples of plants that cannot tolerate extreme cold. If you have sensitive plants, it’s important to consider alternative options, such as using cold frames or bringing them indoors during severe cold spells.

Cold-Tolerant Plants

On the other hand, some plants are naturally well-suited to colder conditions and can withstand freezing temperatures without significant damage. These cold-tolerant plants include varieties of kale, cabbage, broccoli, and certain types of lettuce. By choosing cold-tolerant plants for your unheated greenhouse, you can enjoy fresh produce year-round without worrying about freezing.

Plant Hardiness

Understanding the concept of plant hardiness is crucial for selecting suitable plants for your unheated greenhouse. Plant hardiness refers to a plant’s ability to survive in specific climatic conditions, particularly cold temperatures. The United States Department of Agriculture (USDA) has developed a plant hardiness zone map, which divides the country into different zones based on average annual minimum temperatures.

Understanding Hardiness Zones

The USDA hardiness zone map provides valuable information about the lowest temperatures a particular region experiences. By determining your location’s hardiness zone, you can select plants that are adapted to the temperatures typically found in your area. For example, if you live in USDA zone 4, you should choose plants that can tolerate temperatures as low as -30°F (-34°C) to ensure their survival in an unheated greenhouse during the winter.

Choosing Suitable Plants

Once you have determined your hardiness zone, it’s essential to choose plants that are suitable for your specific region’s climate. Local nurseries or online resources can provide valuable information on which plant varieties are best suited for your area. By selecting plants that are well-adapted to your zone, you can greatly reduce the risk of freezing and ensure the success of your greenhouse gardening endeavors.

Humidity Levels

Humidity levels play a vital role in preventing freezing inside the greenhouse. Proper management of humidity can help create a more stable environment for your plants, reducing the risk of freezing and providing optimal growing conditions.

Effects on Freezing

Humidity levels affect the freezing point of water, which directly influences the likelihood of freezing inside the greenhouse. Higher humidity levels can result in a lower freezing point, making it more challenging to prevent freezing even at slightly cooler temperatures. It’s important to strike a balance in humidity levels to prevent excessive condensation and potential freezing.

Maintaining Optimal Humidity

To maintain optimal humidity levels inside your unheated greenhouse, consider incorporating humidity controls such as fans or vents. These mechanisms can help regulate moisture levels and prevent excessive condensation, which may lead to freezing. Monitoring the humidity with a hygrometer can also provide valuable information for adjusting ventilation and watering practices.

Protective Measures for Unheated Greenhouses

While an unheated greenhouse presents challenges during the winter, there are several protective measures you can implement to safeguard your plants from freezing.

Using Cold Frames

Cold frames are essentially mini greenhouses designed to provide additional insulation and protection for your plants. By placing cold frames inside your unheated greenhouse, you create an extra layer of warmth, shielding your plants from freezing temperatures.

Adding Insulation

Insulation is key to preventing heat loss in an unheated greenhouse. Adding insulation materials such as bubble wrap or horticultural fleece to the walls, windows, and doors can help retain heat and maintain a more stable temperature inside. Additionally, insulating the ground with a layer of straw or mulch can help protect the roots of your plants from freezing.

Using Row Covers

Row covers are lightweight fabric covers that can be placed over individual rows or beds of plants. These covers act as a barrier against cold temperatures, trapping warm air and protecting your plants from freezing. Row covers are an effective and cost-efficient way to shield your plants during colder periods.

Installing Thermal Mass

As mentioned earlier, thermal mass can help regulate temperature fluctuations inside the greenhouse. Installing heat-absorbing materials such as water-filled barrels or thermal mass walls can provide additional thermal stability. These materials absorb heat during the day and release it at night, preserving a more favorable environment for your plants.

Monitoring and Managing Temperature

To ensure the well-being of your plants in an unheated greenhouse, it’s crucial to monitor and manage temperature fluctuations effectively.


Installing thermometers throughout your greenhouse allows you to monitor and track temperature variations accurately. Place thermometers at different heights and locations to capture a comprehensive view of the greenhouse’s climate. Regularly checking the temperature readings will help you make informed decisions regarding protective measures for freezing conditions.

Space Heaters or Heat Lamps

In extreme situations, or for particularly sensitive plants, using small space heaters or heat lamps can provide supplementary warmth. However, it’s important to exercise caution when using these heating sources, as improper placement or unmonitored usage can be a fire hazard. Always follow the manufacturer’s instructions and use heaters or lamps specifically designed for greenhouse environments.


Proper ventilation is essential for maintaining a healthy greenhouse environment, even in the winter. While it may seem counterintuitive to open vents during colder temperatures, ventilation helps control humidity, prevent excessive condensation, and regulate temperature. By adjusting vents and utilizing natural airflow, you can prevent overheating during the day and ensure adequate air circulation to prevent freezing.

Considerations for Winter Greenhouse Gardening

Winter greenhouse gardening requires careful planning and consideration to maximize success and protect your plants from freezing.

Planning for Winter Conditions

Before winter arrives, it’s important to evaluate your greenhouse structure and make any necessary improvements or repairs. Ensure that all windows, doors, and seals are properly insulated to prevent heat loss. Additionally, consider strategically positioning the greenhouse to maximize solar gain and protection from harsh winds.

Choosing Cold-Tolerant Varieties

Selecting cold-tolerant plant varieties is crucial for successful winter greenhouse gardening. Research suitable plant species for your climate and choose varieties that have been specifically bred to withstand freezing temperatures. By focusing on cold-tolerant varieties, you can enjoy a productive and thriving greenhouse throughout the winter months.

Utilizing Season Extension Techniques

Season extension techniques, such as using row covers or cloches, can help prolong the growing season and protect your plants from freezing. These techniques provide an additional layer of insulation and act as a barrier against cold temperatures. By utilizing season extension techniques, you can extend the period during which your plants can thrive in an unheated greenhouse.


While an unheated greenhouse presents challenges during the winter months, by considering factors such as outside temperature, inside temperature, type of plants, plant hardiness, and humidity levels, you can successfully navigate freezing conditions. Implementing protective measures, utilizing thermal mass, and carefully monitoring temperature fluctuations are essential for maintaining a healthy and thriving greenhouse. With proper planning, suitable plant selection, and the use of season extension techniques, you can continue to enjoy the benefits of a greenhouse garden even in the coldest months of the year.

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About the Author: Jake Scott