How To Prepare Your Greenhouse For Winter

Winterizing Your Growing Dome Greenhouse

When the nights start to get cold, and all those fans and vents are working against your best greenhouse gardening efforts, it’s time to adjust your dome for the changing weather. Of course, it’s never a seamless shift from summer to winter, and temperatures can fluctuate quite a bit. We want to share some tips that will help you in winterizing your greenhouse. Helping you adapt to your specific environment and enjoy a flourishing winter garden.

Autumn in Zone 5 and Under or Winters in Zone 6 or 7

Adjusting Ventilation

upper vents in a Growing Dome greenhouse

When the days are warm and nights are 25-35°F, basically freezing but not far from it, it is time to start winterizing. We suggest unscrewing your roof and wall vents (counter-clockwise) so that they require more heat to open. They can be manually propped open for a short time during the day. By unscrewing the pistons, you allow the dome to accumulate more heat before venting, therefore reducing heat loss and storing more heat for the evenings.

During this time, we don’t suggest closing your vents fully as ventilation, and fresh air in your dome will still help to prevent various pest problems and strengthen your plants. Make it a priority on days above 40°F to open your door for a couple of hours during the warmest part of the day. Maybe even prop a vent or two open, but be sure to close it all up at least 2 hours before sunset so your dome can build up heat for the evening.

preparing a greenhouse for winter by disengaging vents
Unscrew clock wise to allow the dome to accumulate more heat before venting

This practice is a little different in every environment. In humid, less windy places, the heat will hold better than in dry, windy areas. Then there is plenty of in-between. The size of your Growing Dome also makes a big difference. Smaller domes have less thermal mass and therefore lose stored heat faster than larger Domes. This means that you can leave the doors open on larger greenhouses for longer periods of time to allow more airflow, which results in healthier plants through the winter. Smaller domes have much more significant temperature fluctuations and, on occasion, can require a small backup heating system if it is below 20°F.

26' Growing Dome greenhouse in winter

It will take a little experimentation and attention, especially during your first fall and winter, to discover what is best for your dome greenhouse and environment. Remember optimizing ventilation during the day regulates humidity levels and encourages airflow for healthy gardens.

Winter in Zone 5 and Under:

Disabling Your Vents

It’s cold, and things need to be sealed up. Now is when we disable the vents. It’s very simple to engage and re-engage pistons, and you might do it several times a winter if your environment is anything like Colorado, where it often can’t decide between winter and spring for much of the season.

disengaging a univent opener

Univent® openers have a smooth black piston. The Univent® piston can be removed for long term disabling, or just with a little pinch at the base of the piston.

Bayliss® openers have a small aluminum knurled nut. To disable a Bayliss® remove the knurled nut.

Gigavent® openers have a large black piston with grooves. To disable a Gigavent® remove the piston and install the relief bolt that came with the opener.

If you do disable the vent openers to fully close your vents, make sure to use the safety cord to secure the vent and prevent damage caused by high winds.

Insulating Your Greenhouse

Modifications are made quickly, but it will require a little bit of attention to enjoy a long growing season through these peaks and valleys. Cover your fans from the outside using a piece of tarp, blue board, or greenhouse plastic. Feel free to get creative with it. I prefer something easy to remove and replace when temperatures in Colorado fluctuate drastically.

Insulating from the inside is also an option. You can utilize a couple of screws and a bungee cord to hold rigid foam insulation over your fans shutters from the inside, or use greenhouse plastic and/or tarp to seal everything off nicely.

Seal Windows and Doors

Check around door frames for gaps as well. Although Growing Domes have been known to last well over 30 years, the greenhouse frame can settle over time, and it doesn’t take a big gap to freeze things when it gets frigid. Seal all your gaps up with backer-rod, spray foam specifically for windows and doors, or whatever means necessary. Remember to check it through the winter to make sure everything is holding up.

Growing Dome greenhouse with snow piled up

Thermal Mass

Your pond is the Growing Dome’s main source of thermal mass, so it is best to make sure your pond is exposed to the most sunlight possible. So, grow those giant kale and cabbage plants away from your pond and choose shorter plants to be near it, if possible. Keeping your pond full and water covered with healthy pond plants will help it achieve its intended purpose more efficiently. If you don’t have plants, you can cover it with large pieces of polycarbonate, insulation, thermal pad or a solar pool cover.

pond plants in a greenhouse

If the pond is not full, the battery bank cannot properly charge. The tank needs to be kept topped off to within 2-3” of the underside of crossbar or, in the case of the circular tank, the top of the tank wall.

Extra thermal mass can be helpful. Adding rocks or gallon jugs of water around the perimeter beds near the polycarbonate to collect extra heat is great. Saltwater holds heat longer, and painting the water containers black will attract more heat. Some people like to use huge rocks or large drums of water as well.

Heavy Snow Areas

Unlike the average greenhouse or polycarbonate tunnel, the Growing Dome Greenhouse actually benefits from snow loads. The layer of snow acts as an insulating layer and you will never have to worry about the integrity of your greenhouse. The geodesic structure is immensely strong, and it allows snow to slide off naturally, so there’s rarely, if ever, that much snow pushing off. Keep in mind however if you are reaching 20+ inches of snow with a crazy freak winter storm you may benefit from clearing some of your Dome’s south side in order to assure sunlight is reaching your plants and your pond. You can build up snow around your foundation wall and the north side of the dome for added insulation.

greenhouse preparing for heavy snow
The Growing Dome right after a snow storm The snow will sluff off naturally as the dome warms up

Solar Attic Fan

If your Growing Dome is equipped with a solar attic fan, then you may also want to add insulation to prevent heat loss through the fan. This also depends on your climate, as you may still want to let some heat escape from the top of your dome. Cut a scrap piece of insulation to size (approx 12″ X 12″), and install four pieces of 3M Dual Lock or Command Strips to each corner. Dual Lock comes in two pieces, so it must also be installed underneath the solar attic fan. Keep it applied to the mating strip on the insulation, and remove the backing. You can then apply the insulation underneath the fan to ensure proper placement of the dual lock strips. A hook can be added to grab and remove it with a pole, and even poke a hole in the blue board so you can install it from the ground. Make sure to disable the fan at the thermostat so it does not run.

Beginning in 2022, our solar attic fans come standard with a winter cover or louvers depending on the Dome size. Due to these improvements, you might find adding extra insulation unnecessary.

Don’t Forget These Other Crucial Tips

Any plants touching polycarbonate walls are subject to freezing temps, make sure to give them adequate space to protect them when it gets below freezing. If leaves do freeze by accident, do not let them rot on the walls of your Dome. That will breed disease and hurt your growing space for the long run. As always, in winter, remember to keep your garden free of debris and clear out dead material. This includes all those fig or grape leaves that fell.

gardener watering plants in winter

Winter watering in your dome is just as critical as always. Your soil is alive, whether there are plants in it or not. It requires water to stay viable and ready to grow your garden when desired. Depending on weather and soil type, your soil will need varying amounts of water, but still far less often than summer. We recommend checking soil weekly to assess its needs.

Water and moisture will also help protect your plants if they do freeze. It creates a barrier to protect the plants and root structure.

Remember to change the direction of the undersoil fan such that it is pulling air in from the south side of your dome, and exhausting the warm air behind the water tank.

Please feel free to contact us with any questions or comments. If you are interested in heating, we recommend checking out our article To Heat or Not to Heat Your Greenhouse. We also recommend viewing our Heat Loss Spreadsheet to calculate exactly how much heat is required to reach the desired temperature inside your Growing Dome.

We are always happy to hear from you and wish you an abundant winter gardening experience.

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Growing Spaces
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