Greenhouse Overwintering Tips: Keeping Your Plants Protected

A greenhouse is a great place to keep frost-tender plants for the winter months. You can take some steps to make sure they stay safe and sound in there until spring. Join gardening expert Melissa Strauss to talk about things you can do to make sure your plants make it through the winter in the greenhouse.

Various potted plants, including Pothos, Caladium, Silver Satin Pothos, and Piper ornatum, in hanging, plastic, and clay pots, overwintering in a greenhouse.

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Preparing your potted plants to overwinter can be stressful, even if you have a greenhouse. Thankfully, with a protected space, it is decidedly less so. However, you shouldn’t just put them in there and forget them until the spring. If you do that, you may have disappointing results. 

A greenhouse is a wonderfully useful tool for gardeners. One of the main benefits I have found is that they make it possible to overwinter tender perennials. You can keep your lovely tropicals alive and thriving without crowding your home and desperately searching for window space. 

While it’s certainly easier than bringing them indoors, there are still some special considerations to successfully overwinter your plants in a greenhouse. Otherwise, you could end up with severe pest, disease, and stress issues. It’s important to prepare your space and your plants and keep up with them for the duration. Here are some ways you can prepare and maintain your space and the plants you store in it for the winter. 

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Make Repairs Ahead of Time

Close-up of male hands with a red screwdriver repairing panels in a greenhouse.
Inspect your greenhouse for repairs before bringing in plants.

Before you start moving things in, give your greenhouse a once-over to make sure that things are in tip-top shape. I know that I often have repairs to make after the fall storms that come through my area. 

If you have any loose or missing panels, get them repaired ahead of time. It’s easier to do this when you’re not trying to work around a space crowded with plants. However, you may need to work on things from the inside, and this is tough when there are things to climb over or move out of the way.

Check that your panels are secure and sealed up. Caulk over or seal up any gaps or spots that have materialized. This is especially important in colder climates. You want to hold as much heat as possible in there. Drafty spots can result in damage to the plants that receive that frosty air. 

Over time, your panels can become cloudy or dirty. This impedes light from passing through and decreases the temperature as a result. Give the exterior of your panels a good cleaning to clear away anything that could get in the way. 

Check and Treat For Pests and Diseases

An Asian woman in a checkered shirt, overalls, straw hat, and blue gloves kneels down, spraying pest control solution on various potted plants in the garden.
Check plants for infestations before putting them together indoors.

Before you start moving plants into your greenhouse to overwinter, it’s important to make sure they aren’t suffering from any diseases or pest infestations. Moving them into an enclosed space is a sure way to make sure that they all share any issues. 

Many of the pathogens that affect plants thrive in cool, moist environments. This is precisely the type of environment they will spend the winter in if you place them in a greenhouse. Give your foliage a once over, and make sure to treat or remove any that appears affected

Pest infestation is a bit trickier, as you can’t always see them. Since you’ll be putting them in an enclosed space, you don’t need to worry about poisoning pollinators. It’s a good idea to treat everything for pests before you move them in together. Insects will thrive and reproduce in your greenhouse, and you could end up with a much greater problem. 

Regulate the Environment

Close-up of a thermometer in a greenhouse attached to the trunk of a potted tree surrounded by various other potted plants.
Regulate temperature to mimic natural conditions for plants.

Depending on your climate, it may be necessary to take additional steps toward regulating the temperature in your greenhouse while you overwinter certain species. In warmer climates, this probably won’t be an issue, but in cold climates, it can get pretty chilly in there at times. The ideal situation is to keep the temperature as consistent as possible. 

Don’t focus on heating it up to summer temperatures. Most of your plants are tolerant of cool weather. In fact, a period of cool weather is vital for many tropicals to initiate blooming. This is especially true of orchids. Maintaining a temperature between 40-50°F (4-10°C) is enough for most. This keeps them alive but not in an active growth phase. Essentially, it gives them the break they would get in their native environment. 

Insulate if Necessary

Young seedlings in trays and peat pots sit on a white metal shelf wrapped with bubble wrap inside a greenhouse.
Use bubble wrap for better light and root insulation.

If you want to insulate your space, you can purchase a specialty bubble wrap made for this purpose. Apply it to the interior walls of the greenhouse to ensure a cozy overwinter temperature. The size of the bubbles will affect the amount of light that passes through. Larger bubbles will allow more light to pass through. Smaller bubbles will act as more of a filter. 

You can also wrap this insulating wrap around the containers of more vulnerable plants. Preserving the roots is the most important factor in keeping them alive through the winter. Since there isn’t much soil to insulate the roots in containers, wrapping them with an insulating material can make a significant difference. 

Ventilation is Important

A large glass greenhouse with an open ventilation window allowing fresh air for the plants inside.
Keep air moving to avoid fungal and bacterial problems.

Moist, stagnant air is the best way to make sure you end up with bacterial and fungal problems that overwinter with your greenhouse plants. For this reason, ventilation is important. This will look different depending on your climate and will be more complicated in colder regions. 

Here in Zone 9, it rarely freezes for an extended time, so I can use the vents in the roof and open the door a bit on most days to improve air circulation. This won’t work in cold climates where you experience long periods of freezing temperatures. 

In cold climates, a temperature-controlled ventilation system is ideal. These can be costly, though. Another option is to place fans on either end of the space, which will create a circular airflow pattern. This prevents a situation where warm air rises to the top, and cold air settles at the bottom. Basically, it helps regulate not only the moisture level but also the temperature. 

Adjust the Light With Shade Cloth

Potted Clivia miniata plants with long, green, arching leaves are lined up on a shelf in a greenhouse with green shade cloth regulating the temperature.
Adjust shade cloth based on season for optimal plant health.

Shade cloth can serve more than one purpose in your greenhouse. It helps to regulate the temperature in the summer, and protects plants that would wither in the direct exposure. In winter, it can do the opposite and create a problem. 

If you use shade cloth to cool your space in the summer, it would make sense to remove it for the colder month. There are fewer daylight hours in winter, so shade cloth may become unnecessary. However, it can also act as insulation on colder days. 

A lightweight, dark-colored shade cloth can absorb and trap heat, which is good during cold spells. During warmer periods, make sure to remove the shade cloth to avoid raising the temperature. Remember, you want to provide as consistent of an environment as possible. 

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Don’t Forget to Water

A woman in a striped apron waters potted plants on a wooden shelf with a blue watering can in a bright greenhouse.
Keep watering balanced to prevent root freezing and fungus.

Watering the greenhouse can be a balancing act. On one hand, you don’t want to create an overly humid environment where fungal pathogens can run wild. You also don’t want your soil to dry out because that makes those roots more susceptible to freezing

Water no more than once weekly but at least twice a month to strike a healthy balance. Always do your watering in the morning, when there is plenty of time for the plant to absorb what it needs, and when the heating action of the sun will cause some evaporation. 

Keep It Clean

A smiling woman in a brown apron sweeps the floor with a broom in a wooden greenhouse filled with potted plants and large glass windows.
Regular cleaning and ventilation prevent fungal issues in winter.

As part of your preparation to overwinter crops and ornamentals, cleaning up the interior of your greenhouse is an important step. Clear out anything that doesn’t need to be in there. Clean your tools, and just declutter in general. 

Over the season, maintain a clean space by regularly checking up. if you want to get things spic and span, choose a mild day to remove everything. Give a thorough once over, and then put them back into the clean space.

Proper ventilation will go a long way toward keeping things in your greenhouse in good condition. The idea is to prevent fungus as much as possible because that is a major killer in cool, moist environments. 

Transition Slowly

A woman in a large green sweater carries several potted plants from a greenhouse into a sunny garden.
Gradually transition plants outside for a smoother acclimation process.

At the end of the season, make sure to transition your plants slowly back outside. You can do this by opening up the greenhouse as soon as the outdoor temperatures are warm enough. 

Taking the time to create a smooth transition will minimize stress on your plants. It prepares them for the outdoors without a jolt and allows them to adapt to the natural pattern of warming weather. 

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