Start before the frost hits. Once nighttime temperatures are consistently dropping into the single digits Celsius (upper 40s Fahrenheit), it is time to clean, check equipment, and add insulation, not wait for a frost warning to show up on the forecast.
Clear out and clean first. Remove finished summer crops, wash down any plants staying in for winter, and pencil in a deep clean for mid-November before things get too cold to tackle it comfortably.
Check your equipment, then seal in the heat. Inspect fasteners, vents, and fans while the weather still allows for it, then add insulation, thermal mass, row covers, or soil warming to hold onto heat as temperatures fall.
Ease off watering and switch crops. Cut back as growth slows down, and swap warm-season plants for cold-hardy crops like kale, spinach, and Swiss chard to keep the greenhouse productive right through winter.
There is a moment every fall when you walk out to the greenhouse and notice it. The air feels different. The glass has a light fog on it in the morning, and the tomatoes that were thriving in August are starting to slow down. That is your sign. Fall is here, and your greenhouse needs a little attention before the cold sets in.
The good news is that fall prep does not have to mean shutting things down. With the right cleaning, a few structural checks, and some smart insulation, your greenhouse can keep producing food and providing a warm, green escape well past the first frost. This guide walks you through everything to do before the temperature drops, from clearing out summer crops to choosing what to plant next.
It is tempting to let the greenhouse coast once the peak of summer growing is behind you. But the tasks you skip in September tend to show up as problems in December. Leftover summer growth invites pests and disease to overwinter alongside your plants. A vent that sticks in October will only get harder to fix once it is covered in frost. And a greenhouse that loses heat overnight can undo a whole season of care in a single cold snap.
A little bit of fall prep protects the investment you have already made, and it sets you up to keep growing right through the colder months instead of putting everything on pause.
Frost dates vary depending on where you are, but the safest approach is to start preparing well before the forecast calls for it. Watch for the early signs:
Once nighttime temperatures are consistently dropping into the single digits Celsius or the 40s Fahrenheit, it is time to move from "keeping an eye on it" to actively prepping.
Waiting until a frost warning shows up on the forecast puts you in reactive mode. Starting a few weeks earlier gives you time to clean, check your equipment, and get insulation in place without scrambling.
A clean greenhouse going into fall is a healthier greenhouse coming out of winter. This is the point in the season to take stock of what stays and what goes.
If you are extending your season, pull out any summer crops that have finished producing. Leaving spent plants in place invites disease and gives pests a place to settle in for the winter. Clear out fallen leaves, dead foliage, and anything that could start to rot once the greenhouse cools down.
For plants that are sticking around, give them a proper check before the weather turns. Wash the leaves with a soft, soapy cloth and look closely for pests or signs of disease. Cool nights paired with warm days can encourage sooty mold to grow. If you spot it, a Q-tip dipped in alcohol will take care of small patches. Once your plants are clean, cover them with a protective fleece for extra frost protection over winter.
Many greenhouse owners do a full deep clean in mid-November, right before settling in for winter. Empty the greenhouse completely so you can get into every corner. A shop vacuum is your best friend here, and not just for the floor. Run it along the walls, interior bolt tracks, door thresholds, vent screens, and weather stripping, anywhere pests like to hide.
A pressure washer on a low setting works well for a deeper clean. If you have polycarbonate panels, take care around the silicone seals so water does not get into the flutes, and skip the acetone or paint thinners since they will damage most glazing materials. Naptha is the safe choice for spot cleaning polycarbonate.
Fall is the right time to make sure everything mechanical and structural is ready to handle colder weather.
If a roof vent is not opening the way it used to, the wax cylinder inside may just need a kickstart. Remove the cylinder, place it in the refrigerator (not the freezer) for 30 minutes, then run it under hot water for 5 to 10 minutes. If that doesn't do the trick, it is likely time for a replacement. Wax cylinders typically last 5 to 7 years, and if you would rather remove the guesswork from a windy winter, you can take the hinge off entirely and secure the vent closed with wire or zip straps.
Once your Greenhouse is clean and checked over, it is time to think about heat retention.
A layer of clear 4mm plastic or bubble wrap along the inside of the greenhouse can make a real difference in how much heat you retain. Insulating clips or short pieces of rope tucked into the bolt track will hold it in place.
If your greenhouse gets some solar gain even in the colder months, try placing black barrels filled with water inside. They soak up heat during the day and slowly release it overnight, acting as a simple thermal battery.
A layer of Agribon fleece adds meaningful frost protection, anywhere from 2 to 7° C or 35 to 44° F depending on the weight you choose. The higher the number, the more protection it offers, and it is a low-tech solution that goes a long way.
Sometimes the most efficient fix is warming the root zone directly. Waterproof soil warming cables laid under your pots give plants a cozy boost without heating the whole greenhouse.
Pairing a space heater with a thermostat means your greenhouse stays protected whether you are at your desk or asleep. The thermostat regulates the heater as needed and lets the sun do the work once solar gain kicks in during the day.
A circulating fan running around the clock, in both winter and summer, helps prevent the cold pockets where frost damage tends to start. Moving air is far less likely to freeze than air trapped in a still corner of the greenhouse.
As growth slows down, so should your watering. Sitting water in cooler soil created the perfect conditions for fungus gnats to move in. Most winter crops shift into a kind of holding mode, growing slowly and needing far less water than they did in summer. In some cases, it makes sense to stop watering certain plants altogether until spring.
If you are pulling out fabric row covers for the season, run them through a hot wash first. Starting clean helps keep pests and disease from getting a head start under the fabric.
Fall does not have to mean the end of your growing season. It just means switching to plants that can handle the cold.
A little preparation now means a greenhouse that keeps giving you fresh produce, warm air, and a reason to head outside, long after the rest of the garden has gone quiet for the year.