How to Keep a Greenhouse Warm

On this page (9 sections)

1. How do you keep a greenhouse warm overnight?
2. Does insulation help a greenhouse stay warm?
3. Can water help hold greenhouse heat?
4. How can you keep greenhouse plants warmer?
5. When should you add a greenhouse heater?
6. How should you set up and care for greenhouse heating?
7. How do you prevent overheating and damp air?
8. Will a greenhouse stay warm in winter without heating?
9. Which plan should you choose to keep a greenhouse warm?

To keep a greenhouse warm, reduce heat loss first, measure overnight lows, and add thermostatic heat when plants need it. Sunshine can warm the space during the day, but that warmth may fade quickly after sunset.

Start with your plants' minimum temperatures, not a heater's coverage claim. Keeping hardy greens above their cold limit is a different job from maintaining warmth for tender tropical plants.

How do you keep a greenhouse warm overnight?

Close unwanted drafts and add a removable inner cover before sunset. Keep required ventilation available so you can release heat and moisture the next day.

Check door edges, loose panels, torn covers, and gaps where the frame meets the base. Repair the source of a draft rather than placing a heater beside it.

A fitted thermal curtain or inner cover reduces the space around plants that needs protection. Support it above foliage, keep it away from electrical equipment, and open it during the day when plants need light.

Don't make a permanent airtight enclosure. Closed vents may help retain warmth at night, but you still need a way to ventilate when sunlight raises the temperature.

How to keep a greenhouse warm using overnight readings

Measure the coldest hours at plant height, rather than judging warmth during an afternoon visit. A minimum and maximum thermometer records lows that you might otherwise miss.

Keep the sensor out of direct sun and away from heater airflow, doors, and glazing. Check another point near vulnerable plants to find cold corners.

Record the outside forecast, inside minimum, and covers used over several nights. These notes help you decide whether passive protection is enough and when supplemental heat becomes necessary.

Does insulation help a greenhouse stay warm?

Insulation slows heat loss and reduces the work a heater must do. It doesn't create warmth, so an insulated greenhouse can still become too cold during a long winter night.

Greenhouse bubble insulation can form a removable inner layer. Attach it securely to the frame without blocking vents, door access, or the clearances around a heater.

Avoid adding so many layers that plants lose useful daylight. Check behind insulation for trapped moisture and remove damaged or dirty sections that are hard to keep clean.

The structure matters too: compare polycarbonate and glass greenhouses for their light and insulation tradeoffs. If you're choosing a structure, our guide to backyard greenhouse types explains the main options before you plan heating.

Can water help hold greenhouse heat?

Water containers can buffer temperature swings by storing daytime warmth. They release that heat as the air cools, but they won't maintain a chosen temperature through every cold spell.

Use sealed, dark containers where sunlight reaches them. Keep them on a stable floor or a bench that can safely carry the weight, with walking routes and heater clearances open.

More water takes more space and needs enough sunshine to warm up. Several dull days reduce its usefulness, so don't count on water alone to protect plants from freezing.

Check containers for leaks and keep them away from plugs and cords. Open tubs add spill risk and expose more water to evaporation.

How can you keep greenhouse plants warmer?

Protect the plants that need warmth instead of heating unused space. Group pots by their temperature needs and move sensitive plants away from cold glazing and door drafts.

Raise pots off cold ground using a stable bench or insulating board. Keep drainage holes open, and avoid leaving containers standing in water.

A smaller covered enclosure inside the greenhouse can shelter a few tender pots. Check its temperature separately, because the air beneath the cover may differ from the main greenhouse.

See how to use a mini greenhouse for organizing a compact protected area. Don't place an electric heater inside a tight cover unless its instructions expressly permit that arrangement and all clearances can be met.

If most of your greenhouse is empty through winter, reconsider how much space needs heating. Our greenhouse size guide helps you match growing space to the plants and access room you need.

When should you add a greenhouse heater?

Add heat when measured overnight lows fall below your plants' needs despite insulation and covers. Choose around heat loss, moisture conditions, and power supply, rather than floor area alone.

A tall, drafty greenhouse loses heat differently from a compact, well-sealed one. The gap between the outside temperature and your target also affects how hard a heater must work.

All 3 heaters below have a maximum output of 1500 watts. Changing between them doesn't give you more peak heating power; the useful differences are controls, airflow adjustment, and suitability for the location.

ZIZIWIN for a separate temperature controller

The ZIZIWIN suits growers who want a heater and independent digital thermostat together. It has adjustable temperature and wind-speed settings, plus overheat protection.

Choose it when you want temperature-based switching rather than manually turning heat on and off. Position the controller's temperature sensor according to its instructions so it measures the growing area rather than the warm air leaving the heater.

Skip it if your greenhouse needs more heat than a compact electric unit can supply. ZIZIWIN describes coverage up to 120 square feet, but cold weather, glazing, and drafts determine whether it can hold your target.

SmterCon for directing warm airflow

The SmterCon suits a layout where you need to adjust the direction of warm air. Its tilt head, adjustable speed, and separate digital thermostat give you control over airflow and temperature switching.

Aim the airflow into an open route between benches, keeping it away from delicate foliage and loose covers. Crowding a heater behind pots restricts circulation and leaves other areas colder.

Choose it for its adjustable head when that solves a placement problem. Skip it if the available location exposes the unit or connections to water, or if you can't maintain the required clearances.

Roodike for sheltered spaces with lower heat needs

The Roodike suits a sheltered indoor growing space where a lower heat setting is useful. Its thermostat controls heating, while the blower moves warm air through the space.

Its indoor-use designation matters for greenhouse buyers. Skip it for a damp greenhouse unless its instructions allow the actual conditions where you plan to put it.

Roodike recommends coverage up to 120 square feet and warns that winter heat is limited in larger or colder spaces. Use the lower setting for a smaller heat demand, then check the farthest plants rather than assuming warm airflow means the whole area is warm.

For more sizing help, read how to heat a greenhouse in winter and our greenhouse heater guide.

How should you set up and care for greenhouse heating?

Give the heater a stable, clear location and keep water away from electrical parts. Follow the unit's instructions for mounting, connections, moisture exposure, and clearance distances.

Use an appropriate protected electrical supply, and don't assume a household extension cord or power strip can carry the load. Have a qualified electrician resolve any uncertainty about the greenhouse supply.

Keep watering routes away from plugs, controllers, and heaters. Check overhead surfaces too, since condensation can drip from glazing even when the floor stays dry.

Unplug the heater before cleaning and let it cool. Remove dust and dry leaves from around air openings as directed, and inspect cords and connections for damage.

Check temperature control while you're present before relying on it overnight. Keep a separate thermometer near plants, since the controller's reading won't reveal every cold corner.

How do you prevent overheating and damp air?

Open covers and ventilate when daytime temperatures rise. A thermostat stops heating at its set point, but it can't remove heat from sunshine or release excess moisture.

Water early enough for wet foliage and surfaces to dry before night. Avoid leaving plants crowded against cold glazing, where moisture can collect.

Look for condensation, wilting, and large temperature differences between benches. Adjust airflow and spacing before raising the heat setting to solve a problem caused by poor circulation.

Our guide to greenhouse ventilation with vents, fans, and doors explains how to plan airflow. A fan moves air; ventilation exchanges that air with the outdoors.

Will a greenhouse stay warm in winter without heating?

An unheated greenhouse can suit cold-tolerant plants if its measured lows stay within their limits. Tender plants may need relocation even when daytime temperatures feel mild.

Plan for cloudy stretches and sudden cold spells, not just an average winter night. Keep a warmer backup location ready for plants that cannot tolerate a heater failure or power outage.

Read whether greenhouses work in winter for realistic expectations. With a lightweight structure, also check whether you can use a pop-up greenhouse in winter before building your plan around it.

Which plan should you choose to keep a greenhouse warm?

Choose passive protection for plants that tolerate your recorded lows; add controlled heat for plants that need a higher minimum. For a few tender pots, a smaller protected area or an indoor move may be more practical.

Choose ZIZIWIN for a separate digital controller, SmterCon when its adjustable head helps your layout, or Roodike for a suitable indoor space where lower heat output is useful. None adds heating capacity beyond the shared maximum.

The final step in deciding how to keep a greenhouse warm is checking whether your plan holds the required temperature through the coldest expected nights. Use our winter greenhouse heating guide to plan supplemental heat before those nights arrive.

Frequently asked questions

How can I keep a greenhouse warm at night?

Seal drafts, add a removable inner cover, and measure overnight lows at plant height; keep required ventilation clear and covers away from heating equipment. If temperatures fall below your plants' minimum needs, add suitable thermostatic heat or move tender plants to a warmer place before the next cold night.

Does a greenhouse stay warm without a heater?

A greenhouse can retain some daytime warmth after sunset, but it may still freeze overnight; insulation and sun-warmed water containers slow cooling without creating heat. An unheated setup suits plants that tolerate its measured lows; tender plants need extra protection or another location when those lows become too cold.

What temperature should a greenhouse be at night?

Set the night temperature around the needs of the plants you're growing, rather than choosing one number for every crop; check each plant's minimum and allow some margin for colder corners and changing weather. Group plants with similar needs so you aren't warming the whole greenhouse for a few sensitive pots.

Is it safe to use an electric heater in a greenhouse?

Use an electric heater only when its instructions allow the greenhouse's moisture and temperature conditions; keep it stable, clear of leaves and covers, and protected from watering and drips. Follow its electrical and clearance instructions, including any restrictions on extension cords; overheat protection doesn't replace correct placement or regular checks.