Overwintering Plants in an Unheated Greenhouse

Illustration of staging and potted crops inside a greenhouse

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An unheated greenhouse shelters plants from wind and excessive rain, but it can still freeze. Choose plants that tolerate the minimum temperatures actually recorded inside, and have an alternative frost-free place ready for tender plants when cold weather is forecast.

Table of Contents

Understanding plant hardiness for unheated structures

To successfully overwinter plants without artificial heat, you must first categorise your plants by their hardiness. In a domestic unheated greenhouse, the environment is primarily used for half-hardy (H3) plants. These are species that can tolerate some cold but may suffer if exposed to direct winter frosts in the open garden.

If you choose to add minimal heating to keep the structure frost-free (typically around 4°C), you can expand your collection to include tender plants (H2). However, in a completely unheated setting, you should only expect to shelter plants that can withstand temperatures near or slightly below freezing, provided they are protected from damp and wind. For those seeking to grow warm temperate plants (H1C), such as certain bedding plants or salvias, an unheated greenhouse will generally be insufficient.

Preparing the greenhouse for winter

An unheated greenhouse is only as effective as its insulation and seal. Because there is no active heat source to combat draughts, preventing cold air infiltration is the priority.

Replace damaged glazing and use the seals specified for the greenhouse. Do not repair cracked glass with silicone or block designed ventilation and drainage gaps.
Check seals: Ensure that doors and ventilators fit snugly. If gaps persist, clear plastic sheeting can be taped over leaking areas, though you must ensure you can still access these vents for humidity control.
Insulate the walls: Adding a layer of re-used bubble wrap to the interior walls can provide a significant thermal buffer. While this may reduce light penetration by approximately 10%, this is usually a beneficial trade-off for the increased warmth. Note that plants requiring maximum light, such as winter lettuce or alpines, should be placed in areas without bubble wrap.

Tender plants and lifted tubers need a frost-free plan

An unheated greenhouse is unsuitable for frost-sensitive storage whenever its temperature falls below the plant’s safe minimum. Do not assume that bubble wrap or grouping pots makes it frost-free. Move tender plants to a suitable frost-free location before a cold spell if the greenhouse cannot maintain the conditions they need.

Lifting and storage methods differ between dahlias, cannas and begonias. Follow the guidance for the specific plant, including drying, storage medium and moisture; a single “keep all tubers moist” method is not appropriate. Check stored material periodically and remove rotting pieces.

Container-grown citrus and other tender shrubs also have different winter temperature requirements. Check the species and variety before moving a pot into an unheated structure, and monitor a maximum-minimum thermometer close to the plants.

Managing humidity and ventilation

The greatest risk in an unheated greenhouse during winter is not always the cold, but the damp. High humidity combined with low temperatures creates an ideal environment for moulds, rots, and botrytis.

To manage this, you must actively ventilate the structure. On mild or sunny days, open the doors and windows freely. This air exchange reduces the buildup of moisture on the leaves and soil surface. Be careful not to ventilate during a hard frost, as this will strip away the buffered warmth the greenhouse has accumulated.

Additional protection during cold snaps

Even in a greenhouse, a severe cold snap can drop temperatures below the tolerance level of half-hardy plants. In these instances, supplementary protection is necessary.

Fleece and Hessian: Drape horticultural fleece or hessian over individual plants or groups of pots. This provides an extra layer of insulation against the coldest air.
Grouping: Move the most sensitive plants to the centre of the greenhouse, away from the glass walls where temperatures are lowest.
Watering: Reduce watering significantly. Plants in a dormant or semi-dormant state have very low metabolic rates; excess water in cold soil is the primary cause of root rot.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

Horticultural fleece: Used to provide additional frost protection over plants during severe cold snaps within the greenhouse. Check the GSM (grams per square metre) for the level of thermal protection required; higher GSM is generally warmer. Search Amazon for Horticultural fleece.
Maximum/Minimum thermometer: Essential for accurately tracking the lowest and highest temperatures the plants experience in an unheated environment. Ensure it is a max/min model rather than a standard thermometer so you can see the overnight low while away from the greenhouse. Search Amazon for Maximum/Minimum thermometer.

Related garden guides

Sources and further guidance

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.