To winterize a greenhouse, clean it out in the fall, check the frame and foundation, seal every gap, add insulation, and make sure your heater, thermostat and vents work before the first freeze. Then keep plants healthy through winter with careful watering, good airflow, extra light and a close eye on pests. Start four to six weeks before your first frost date.
✅ Quick answer
- Clean and disinfect benches, pots and glazing to remove pests and let in every bit of winter light.
- Seal and insulate: fix gaps around doors, vents and the base, and add bubble insulation to single glass.
- Test your heater and thermostat now, and keep a small backup heater ready.
- Water less, ventilate a little: wet soil and still, damp air cause most winter losses.
- Brush heavy snow off the roof from the outside, and never climb on it.
We are a small, woman-owned greenhouse business in Georgia, and we help customers get ready for winter every fall, from mild Southern winters to deep snow in the mountains. This is the checklist we share with them.
When should I winterize my greenhouse?
Start four to six weeks before your average first frost. That gives you time to order parts, test the heater and fix anything you find. In the North that is often September. In much of the South it is October or early November.
Step 1: Clean out and disinfect
A clean greenhouse lets in more light and gives pests nowhere to spend the winter.
- Clear benches, empty old pots and remove dead or diseased plants.
- Scrub benches, pots and tools with a mild disinfectant and let them dry.
- Wash the glazing inside and out. Winter light is weak, so every bit counts. Use mild soap and a soft cloth on polycarbonate, never an abrasive pad.
- Clear gutters and leaves from the roof and base.
Step 2: Check the frame, foundation and anchors
Snow, ice and wind find weak spots fast. Walk around the greenhouse and look closely.
- Tighten loose bolts and check that every panel is seated in its seals.
- Check the anchors to the foundation. In windy areas, a tie-down kit adds holding power.
- Look for heaving or gaps along the base. Our practical guide to a greenhouse foundation explains how frost moves a base and how to stop it.
- Replace any cracked panel now. A small crack lets in cold air and can spread in a freeze.
Step 3: Seal every gap
Air leaks are heat leaks. Check around doors, roof vents, louvers, frame joints and the base.
- Seal gaps with greenhouse-safe caulk, foam tape or EPDM rubber strips.
- Replace worn door and vent seals. An old door is often the biggest draft.
- Lubricate hinges, latches and automatic vent openers with a silicone spray so they keep working in the cold.
- Recheck panel edges after the first hard freeze. Frames shrink slightly in the cold and can open new gaps.
Step 4: Add insulation where it pays
How much you need depends on your glazing. Twin-wall polycarbonate and insulated glass already hold heat well. Single-pane glass loses about twice as much heat, so it benefits most.
- Bubble insulation: clip horticultural bubble wrap inside single glass walls. It keeps light coming in while trapping a layer of air.
- North wall: it gets little winter sun. Foam board there cuts heat loss without costing much light.
- The base: insulating the perimeter stops cold seeping in at soil level.
- Thermal mass: dark water barrels under benches soak up sun by day and release it at night.
Not sure which glazing you have, or thinking of upgrading? See our glass vs polycarbonate buyer's guide.
Step 5: Get your heater ready
Test the heater before you need it. Run it for a few hours on a cool night and check the thermostat against a separate thermometer.
- Size it for your coldest night, not an average one. Our guide to the cost to heat a greenhouse in winter shows the simple formula and real numbers by size.
- Keep a backup. A small second heater can save a whole season if the main one fails on the coldest night.
- Add an alarm. A thermostat or sensor with a phone alert tells you right away if the temperature drops.
- Wire it safely. A 240V heater needs a dedicated circuit run by a licensed electrician, with GFCI protection in a damp greenhouse.
- Propane heaters: give them fresh air as the manual directs, and use a carbon monoxide detector.
Browse heaters for every size of greenhouse in our greenhouse heater collection.
🌡️ How warm should I keep it?
- Hardy greens, herbs and overwintering plants: at least 37 to 45°F
- Citrus and half-hardy plants: 45 to 50°F
- Tomatoes, peppers and tropical plants: 55 to 60°F at night
Each degree warmer adds to your bill, so heat only as much as your plants need.
Step 6: Keep air moving without losing heat
Closing everything up traps damp air, and damp, still air leads to mold, mildew and rot. The goal is gentle, controlled airflow.
- Run a small circulation fan to stop warm air pooling at the roof and to dry leaves.
- Crack a roof vent for an hour or two on sunny days to let out moist air.
- Aim for about 50 to 70% humidity. A cheap hygrometer tells you where you are.
- Heavy condensation dripping from the roof means too much moisture. Ventilate more and water less.
Step 7: Prepare soil, beds and containers
- Top up beds with compost before the cold sets in, then mulch to hold warmth.
- Raise pots off cold concrete or gravel. Pots lose heat much faster than ground beds.
- Use soil-warming cables or a heat mat for seedlings and tender plants.
- Plant crops that like the cold: kale, spinach, winter lettuce, chard, parsley and other hardy herbs.
How do I keep plants healthy all winter?
Winterizing gets the greenhouse ready. These habits keep your plants thriving until spring.
- Water less, and in the morning. Plants use far less water in winter. Wet soil freezes faster and invites root rot. Check moisture with a finger or a meter, and water early so leaves dry before night.
- Add light if you need it. Short, dull days slow growth. LED grow lights close to the plants help seedlings and fruiting crops.
- Watch for pests. Aphids, whiteflies and fungus gnats move indoors with the cold. Check under leaves weekly, remove dead material, and use sticky traps or neem oil early.
- Guard against frost. On the coldest nights, cover tender plants with frost cloth and group them together on one bench to heat a smaller zone.
- Use shelves wisely. Upper shelves are warmer and brighter, so put seedlings there and hardy pots below.
What should I do when it snows?
- Brush heavy or wet snow off the roof from the outside with a soft broom. Never climb on the roof, and don't chip at ice.
- Keep a little heat on during storms. A warm roof sheds snow faster.
- Close and latch all vents and doors before high winds.
- Clear snow away from doors and the base so meltwater drains away.
Choosing a new greenhouse for a snowy area? Our list of the most durable greenhouses for wind, snow and harsh winters compares snow and wind ratings.
📋 Winterizing checklist
- ☐ Clear out dead plants, old pots and debris
- ☐ Disinfect benches, pots and tools
- ☐ Wash glazing inside and out
- ☐ Tighten bolts, check panels and seals
- ☐ Check anchors and foundation
- ☐ Seal gaps around doors, vents and base
- ☐ Lubricate hinges, latches and vent openers
- ☐ Add bubble insulation to single glass and the north wall
- ☐ Test heater and thermostat; set up a backup and an alarm
- ☐ Set up a circulation fan and a hygrometer
- ☐ Top up beds, raise pots off cold floors
- ☐ Stock frost cloth, sticky traps and a soft snow broom
Frequently asked questions
How do I keep my greenhouse warm in winter without electricity?
Seal gaps, insulate the north wall, add thermal mass like dark water barrels, and use frost cloth on cold nights. Our guide to heating a greenhouse without electricity covers ten ways.
Should I close all the vents in winter?
No. Keep them closed on cold nights, but open a roof vent briefly on sunny days to let out moist air. Still, damp air causes mold and disease.
How often should I water greenhouse plants in winter?
Much less than in summer, only when the top inch of soil is dry. Water in the morning so plants and soil dry out before the night chill.
Does bubble wrap really work in a greenhouse?
Yes, on single glass. It adds an insulating air layer while letting most light through. Twin-wall polycarbonate and insulated glass rarely need it.
What is the minimum temperature for a greenhouse in winter?
Keep it above about 37 to 45°F for hardy crops and overwintering plants. Tender and tropical plants need 55 to 60°F at night.
Can I leave my greenhouse empty over winter?
Yes. Clean it, close it up, and still brush off heavy snow and check it after storms. A little ventilation on sunny days prevents mold.
Getting ready for winter?
Tell us your greenhouse and where you live. We will help you pick the right heater, fans and accessories, and let you know about any current sales.