How to Protect Greenhouse Plants from Frost: Questions Answered
Seven questions decide whether your plants survive a frost. The wrong answer to any one of them can be costly. The critical factor is knowing which plants are hardy enough for your specific greenhouse conditions.
Can I just leave my greenhouse door open on frosty nights?
Leaving the door open on a frosty night defeats the purpose of having a greenhouse because it lets the heat escape and lets the cold air in. You need to maintain a stable microclimate inside, which means keeping the structure sealed to trap the warmth generated by the sun during the day. Full details in our guide to How Warm Does an Unheated Greenhouse Stay at Night.
Will bubble wrap really stop my plants from freezing?
Bubble wrap acts as an insulating layer that traps a pocket of still air between the fabric and the glass, which slows down heat loss significantly. It works best on the inside of the glazing, where it reflects heat back into the greenhouse rather than letting it escape through the glass. Full details in our guide to How to Protect Greenhouse Plants from Frost.
Is frost fleece the same thing as plastic sheeting?
Frost fleece is a non-woven fabric that allows water, light, and air to pass through it, whereas plastic sheeting traps moisture and can cook plants on sunny days. The weight of the fleece determines its effectiveness, with heavier grades providing better insulation for colder nights. Full details in our guide to Using Fleece and Frost Covers Effectively.
What is the minimum temperature my unheated greenhouse can reach?
An unheated greenhouse typically stays around 5–8°C warmer than the outside air on a cold winter night, though this can vary based on insulation and weather conditions. This buffer is enough to protect some plants but not others, so you need to know exactly where your plants stand on the hardiness scale. Full details in our guide to How Warm Does an Unheated Greenhouse Stay at Night.
Which plants can I safely leave in the greenhouse over winter?
Plants that are hardy or half-hardy can usually survive in an unheated greenhouse, but tender plants like citrus or bougainvillea will need active heating to survive the cold. You should check the RHS hardiness ratings for your specific plants to determine if they can handle the 2–5°C temperature drop at night. Full details in our guide to What to Overwinter in a Greenhouse UK.
Why do my plants keep dying even though I thought it wasn’t freezing?
Overwatering is often the culprit when plants die in winter because stagnant, damp compost in a closed greenhouse is a faster killer than a light frost. You need to drastically reduce watering frequency during the colder months to prevent the roots from rotting. Full details in our guide to What to Overwinter in a Greenhouse UK.
How do I know if my greenhouse is actually cold enough to kill my plants?
A standard thermometer only tells you the temperature right now, but you need to know the lowest and highest temperatures reached overnight to protect your plants properly. A min/max thermometer records these extremes over a 24-hour period, giving you the data needed to make informed decisions about heating and insulation. Full details in our guide to Using Min-Max Thermometers for Greenhouse Monitoring.
Do I need to buy a heater for my greenhouse?
Most UK gardeners can protect their plants with insulation alone, but tender plants like citrus or bougainvillea require active heating that insulation cannot substitute for. You should assess the hardiness of your plants and the temperature your greenhouse actually reaches before deciding whether to invest in heating. Full details in our guide to How to Protect Greenhouse Plants from Frost.
Can I use old windows or doors to insulate my greenhouse?
Using secondary glazing like old windows or doors is one of the most effective ways to insulate a greenhouse and stop heat escaping through the glass. This method creates a barrier of still air between two layers of glass, significantly reducing the temperature drop on frosty nights. Full details in our guide to How to Protect Greenhouse Plants from Frost.
What is the best way to insulate a greenhouse floor?
Insulating the floor is just as important as insulating the glass because cold air rises and can seep up from the ground, chilling the plants and compost. You can use bubble wrap on the floor or cover it with a thick layer of organic mulch to keep the roots warm. Full details in our guide to How to Protect Greenhouse Plants from Frost.
Why does my greenhouse feel warm during the day but freezing at night?
A greenhouse traps heat during the day, but it does not store it the way a house does, so the temperature can plummet rapidly once the sun goes down. This rapid cooling is why it is crucial to insulate the structure and use thermal covers to maintain a stable temperature overnight. Full details in our guide to How Warm Does an Unheated Greenhouse Stay at Night.
Will putting a blanket over my plants help them survive the frost?
Covering plants with a blanket or hessian can provide temporary protection against frost, but it is not as effective as purpose-built horticultural fleece. Fleece allows light and air to pass through while trapping heat, whereas a blanket might suffocate the plant or trap excess moisture. Full details in our guide to Using Fleece and Frost Covers Effectively.
The next step is to get a number. Is your greenhouse actually warm enough for the tender plants you want to protect? Find the answer in How Warm Does an Unheated Greenhouse Stay at Night . For the full picture, start with our How to Protect Greenhouse Plants from Frost .
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