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How to Winterize a Greenhouse [Step-by-Step Guide]

Whether you’ve been a long-time summer garden enthusiast, or you’re just dipping your toes into the waters to become a proud plant parent, keeping a winter greenhouse is next-level when you’re flexing your green thumb. 

They’re a verdant paradise, standing defiantly against the gray, gloomy days of winter and offering outdoor enthusiasts a place to reap the (literal) fruits of their labor without limiting their options to cold-hardy evergreen perennials. 

If this is your first year working with a winter garden, you’re likely wondering how to winterize a greenhouse. Check out our step-by-step guide to learn how to clean up, protect your plants, and winterize your greenhouse insulation, so that your leafy progeny thrives all year long. 

Why Is It Important to Prepare an Outdoor Greenhouse for Winter?

With all the tender, loving care you show your plants, you want them to last through the winter. Only by preparing your greenhouse for the wind and chill of winter can you hope to keep your veggies, fruit, flowers, and herbs alive until spring replanting. 

As temperatures begin to dip, you need to ensure that your greenhouse is ready before the first frost. For many plant species, it’s like a signal that it’s time to pack up their metaphorical bags. 

Perennials begin preparing their roots for a season-long hibernation by keeping nutrients in their roots. At the same time, annuals accept that they’ll be taking a permanent dirt nap, making room for next year’s floral delights. 

A winterized greenhouse is a safe haven for plants needing a temperature, humidity, and light-controlled environment to continue growing. 

How to Winterize Your Nursery or Greenhouse

When you establish your first winter garden, you may focus primarily on how to winterize your greenhouse insulation. However, the real secret to success is a bit more complicated than that. 

Whether you’re working with a full-blown, self-sustaining system or smaller, portable greenhouses for winter, it’s essential to clean, inspect, and monitor the environment for plants to genuinely thrive.

Pay attention to detail, and sow the seeds of success! Ready to winterize? Grab your gardening gloves, and let’s get started!

Step 1: Inventory Plants and Their Winter Needs

Woman Checking Plants on a Greenhouse

As part of your checklist, make a care list for all your plants. This is especially important if you are housing plants new to your winter rotation. Do your research and make a calendar for when you need to feed, water, and prune each piece of vegetation. 

If possible, laminate your list. Keep one copy in your greenhouse, and one copy hung prominently in your home, then note when you complete each task.

This is also helpful if you plan to go away for the winter holidays. Find someone who you trust to keep up with the calendar while you’re away, and you’ll come home to greenery as lush as when you left. 

See Related: Best Mini Greenhouses

Step 2: Empty the Greenhouse

Man Carrying a Pot of Plant

To understand the full extent of the job ahead of you, start by temporarily rehoming all of your greenhouse tenants to another safe location.

Then, you’ll be able to see every crack, crevice, and cranny that requires attention.

See Related: Best Portable Greenhouses to Buy Today

Step 3: Make a Detailed Checklist

Pen and a Paper with To Do List Note

The first step to taking on any major chore is getting organized! Take a walk through your greenhouse and note any obvious issues, like major repairs or replacement parts you’ve been putting off. 

Then, as you work through the rest of the process, continue adding to the list as you go.

Doing so ensures that you won’t miss anything, and your plants will have safe, warm accommodations throughout the cold winter months.

See Related: Best Off-Grid Greenhouse Heating Systems

Step 4: Clean and Sanitize

Spray Bottle and Gloves for Cleaning

To start winterizing your greenhouse, make it like you’re a teenager whose parents are on their way home after you threw a weekend rager.

Gather up all of your cleaning supplies, plenty of clean towels, and a shop vacuum. It’s time to roll up your sleeves and bust some dust. 

Start on the inside, wiping each panel with either a plastic-safe cleaner, like Plexus, or a biodegradable glass detergent, depending on what your greenhouse is made from.

A pack of lint-free, microfiber cleaning cloths is invaluable for achieving a streak-free shine without wasting roll after roll of paper towel. 

Vacuum along with corners and edges, keeping an eye out for insect nests and seal cracks. We’ll deal with those a little later, but it’s good to note them now. That way, you’ll remember to show them a little TLC before the cold settles in. 

Once the inside is sparkling clean, you’ll want to whip up a batch of something more substantial to handle mold, mildew, and algae on the outside of your greenhouse.

Mix a few drops of dish soap, a couple of capfuls of bleach, and water into a spray bottle, then wipe down exterior panels. 

See Related: Best Greenhouse Plastic Sheets for Protection

Step 5: Caulk All Seals

A Caulk on the Top of a Paint Bucket

Cracks in your caulk can be a death sentence for cold-sensitive foliage. 

When you’re pumping heat into your greenhouse all winter, the last thing you want is that glorious warmth leaching out into the frosty outdoor air. Not only will it drastically increase your heating costs, but it takes away precious degrees your plants need to thrive. 

Start on all fours, checking the foundation for gaps.

Then, move on to windows, doors, and vents.

Even if the crack isn’t a full-blown hole yet, go ahead and fill it in regardless. Depending on how cold and dry winter is in your area, it can make old caulk brittle, causing it to worsen before warmer days have a chance to arrive. 

If you notice too large cracks for caulk to handle, try spray foam insulation. If you choose one with a built-in pesticide, it does the double duty of keeping the warm air in and creepy-crawlies out. 

Great Stuff Pestblock can expand up to 1-inch, forming an airtight barrier to keep insects from turning your greenhouse vegetable patch into their personal Thanksgiving feast.

It adheres to just about any, cures rigid in an hour, and comes with a dispenser that prevents drips and ensures precision threading. 

See Related: Best Portable Greenhouses to Buy Today

Step 6: Ensure Structural Integrity

Strong Wooden House Structure Frame

It’s every gardener’s worst nightmare to make up to a mound of snow slapped right in the middle of your greenhouse. Winter precipitation is weighty, and loose trusses can cave to the pressure, dumping buildup and turning your hothouse ice cold. 

Grab your tool kit and tighten up all bolts, screws, and fasteners. Even if it doesn’t feel terribly loose, it never hurts to give your connectors another half-turn, just in case.

If you use rigid cables, ensure they are nice and tight.

Once you’ve gone over everything once, do it again. Sometimes, tightening one component in a system puts too much or too little tension on another. Invest the time to ensure everything is strong enough to withstand inclement weather and high winds. 

See Related: Best Glass Greenhouses in 2023

Step 7: Drain and Winterize Pipes

Set of Pipes

Depending on where you live and whether you have automatic watering systems, you should hand-water your plants while the weather is chilly enough to freeze pipes. Otherwise, you risk costly repairs. 

Greenery needs less water in the wintertime because the cooler weather doesn’t evaporate moisture as quickly. Many plants also go dormant when the temperature hits extreme lows, putting them into “survival mode,” during which they conserve all water and nutrients already in their system. 

That’s good news! It means that you can safely reduce watering and winterize your plumbing to prevent burst pipes.

If you don’t plan to shut down your pipes because the weather you live rarely dips below freezing, you should at least keep a close watch on the weather. Wrap each line in ½” pipe insulating cases or foiled fiberglass wrap to keep ice crystals from building up at night. If there’s a cold snap looming, empty the system temporarily and switch to manual watering until it passes. 

Step 8: Check Your Water Tanks

Red Water Tank

Once the pipes are empty, take the opportunity to give your water tanks some attention. Drain any reservoirs you keep around your greenhouse to release natural sediment that can build up in the bottoms. 

While you’re at it, check for leaks and cracks. It’s easier to replace components when the entire system is dry, and you can be back in action with repaired tanks before springtime. 

See Related: Best Aquaponics Greenhouse Kits

Step 9: Insulate from Top to Bottom

Man Adding an Insulation

The key to keeping plants alive through the winter is excellent insulation, especially if it gets frigid or the electricity goes out, causing your heaters to stop working. It’s also helpful if you missed a spot during your caulk and seal step to help mitigate the effects of cold air finding its way inside. 

Many gardening enthusiasts rely on large-cell, translucent bubble wrap as an inexpensive option because it traps geat inside while still allowing plenty of sunshine.

Air is an excellent insulator, and it’s used in double-paned windows to help regulate the temperatures inside our homes.

Duct tape the bubble wrap to the interior walls, making sure that you keep the tape relegated to the outer margins of each sheet. If you’re careful, you can reuse the pieces for several years before replacing them. 

Step 10: Create Healthy Air Flow

Mini Fan

Clean, oxygenated air is essential for plants to stay fresh and green in the winter, but greenhouses can get pretty stale when they’re shut all winter long. Circulating air also keeps temperature and humidity consistent throughout the entire structure. 

Perhaps most importantly, plants use the carbon dioxide in the air to make their food through a process called cellular respiration. It’s a little complicated on a chemical level, but vegetation requires CO2 to combine with water and energy from the sun to create oxygen and glucose molecules. 

Stale greenhouse air is overly oxygenated, leaving plants starved for CO2 necessary for photosynthesis. But, growers can create the proper balance of gases by simply keeping the air moving all winter. 

Start by closing all of your vents for the season, making it easier to regulate the temperature. If you have warm, sunny weather between cold snaps, you can open them back up for a few hours, as long as you remember to shut them again at night. 

Inexpensive box fans are popular greenhouse accessories for winter because they keep things moving without the cost of specialized greenhouse fans. A good rule of thumb is to have one low-speed box fan for every 5,000 square feet of space inside the greenhouse

See Related: Best Greenhouse Fans to Buy Today

Step 11: Inspect Heater System

Red Orange Light from a Heater

Most overwinter plants won’t survive on insulation alone. They require a reliable heating system to keep their roots comfy and warm. An integrated heating system is the most reliable method of keeping your herbs, veggies, and fruits alive, but a temporary setup is better than nothing at all. 

No matter which method you’re choosing to keep plants comfortably snuggled into their soil, be very thorough in your inspection. Always cut the power to your greenhouse before you start touching electrical components to avoid injury. 

Replace or repair wires before they have a chance to cause a short. Rodents love to get their sharp little chompers around electrical cables, and you may not have noticed the damage in the line if the heater hasn’t run since last year. 

Then, clean the heater of all dust and debris. Dust buildup can catch fire, devastating the plants and destroying your greenhouse. It’s also hazardous if it has the chance to spread out of the structure, setting the dry winter grass in your yard alight. 

See Related: Best Greenhouse Lights for Plant Growth

Step 12: Monitor the Thermostat

Smart Digital Thermostat on a Wall

A broken thermostat gives gardeners the wrong impression of how warm their greenhouse actually is. It’s a good idea to use a reliable mercury thermometer year-round to ensure the thermostats are honest in their ambient temperature evaluation. 

Set up several around the greenhouse, as different “zones” may be colder or warmer depending on which side of the yard they’re facing if there are other structures or shade trees nearby, and countless other factors. 

As soon as it’s cool enough outside to feasibly do so, run your heaters for a few hours, monitoring the temperature on your mercury thermometers. If you notice that particular zones stay consistently colder, move hardier plants into those areas or set up smaller space heaters to keep that area adequately warm. 

Step 13: Make Sure Your Plants are Cozy 

Arranged Plants Inside a Greenhouse

Finally, it’s time to let your plants move back into their winter home. You can also establish some lovely winter greens, like leeks and spinach. Both do fabulously even when the temperature is less-than-ideal for other veggies. 

For flora with tender roots, invest in insulated pots with excellent drainage. Ice crystal formation in the soil or plant can physically pierce through the sensitive “skin,” allowing rot to set in. Get familiar with what you’re growing, so you can provide a level of care that results in happy, healthy greenery. 

See Related: Best Paint for Greenhouses

Frequently Asked Questions

Do you still have a few questions about winterizing a greenhouse? The following answers should help clear up any doubts so you can start preparing your greenhouse for the colder months.

Do greenhouses stay warm in the winter?

Greenhouses can stay warm in the winter with the right combination of insulation, heating systems, and attention to detail.

Otherwise, you risk losing your plants to the chill. Most growing zones also need heaters to help maintain the temperature, especially at the peak of cold weather. 

How do I winterize a small greenhouse?

You can winterize a miniature greenhouse by ensuring that it is well-insulated. One inexpensive option for home gardeners is bubble wrap, which allows the sun to shine through the plastic or glass walls while keeping heat trapped inside. 

Because most small greenhouses don’t have automatic piping systems, you won’t have to worry about insulating your pipes. Instead, reduce manual watering, especially for dormant plants.

Does a tiny wooden greenhouse help me winterize my plants?

Any greenhouse, even a tiny wooden one, is better for plants than leaving them outside to survive the cold. Small greenhouses are ideal if your garden is just a little too big to comfortably re-pot inside of your home.

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