When it comes to greenhouse gardening, many people assume that you need an expensive heating system to keep plants alive during the colder months. But I’m here to tell you that it’s absolutely possible to grow cold-weather crops in your greenhouse or high tunnel without the need for supplemental heat. With the right strategies, tools, and crop selection, you can extend your growing season into the fall and winter, even if you live in a region with freezing temperatures. In fact, I’ve successfully grown several types of cold-hardy vegetables without ever needing to plug in a heater.
So, if you’re interested in keeping your greenhouse or high tunnel productive year-round without breaking the bank on energy bills, this post is for you. I’ll walk you through the types of crops that thrive in cold weather, the techniques you can use to trap and hold heat in your greenhouse, and a few simple tools to protect your plants when the mercury really drops. Let’s get started!
Cold-Hardy Crops: What to Grow in a Winter Greenhouse
First, let’s talk about crop selection. Not every plant is cut out for cold weather. If you try to grow heat-loving plants like tomatoes or peppers in an unheated greenhouse during the winter, they’ll likely struggle or fail. However, there are plenty of crops that not only survive but thrive in cooler conditions. These cold-hardy crops are your best bet for a productive winter greenhouse.
Here are some of the best cold-weather crops you can grow:
1. Leafy Greens
Leafy greens are the champions of cold-weather gardening. Many varieties can withstand frost and even improve in flavor after a light freeze. Some of my go-to leafy greens for winter growing include:
-Kale: One of the most cold-hardy crops out there, kale can handle temperatures down to around 20°F (-6°C) and often tastes sweeter after a frost.
-Spinach: Another frost-tolerant crop, spinach will continue growing in temperatures as low as 25°F (-4°C). It’s a fast grower, making it a great choice for succession planting throughout the season.
-Lettuce: While not as cold-tolerant as kale or spinach, many lettuce varieties can still handle light frosts. Look for cold-hardy varieties like “Winter Density” or “Arctic King.”
-Swiss Chard: While it prefers slightly warmer temperatures than kale or spinach, Swiss chard can still handle cool weather and even recover from frost damage.
2. Root Vegetables
Root vegetables are also well-suited to cold-weather growing. Many of them can be stored in the ground for a longer harvest window, and their growth tends to slow down rather than stop completely when temperatures drop.
-Carrots: Carrots can survive frost and even improve in sweetness after being exposed to cold temperatures. In an unheated greenhouse, you can continue to harvest them deep into the winter.
-Beets: Another hardy root crop, beets tolerate cooler temperatures well. Plus, you can harvest both the roots and the greens for added variety.
-Radishes: One of the fastest-growing crops, radishes can go from seed to harvest in about 30 days, making them a perfect choice for winter gardening.
-Turnips: Turnips are extremely cold-hardy and can grow in temperatures as low as 25°F (-4°C).
3. Brassicas (Cabbage Family Vegetables)
Brassicas are a family of vegetables that are well-known for their cold tolerance. These include crops like:
-Broccoli: Broccoli can tolerate light frost and will continue to grow in temperatures as low as 25°F (-4°C).
-Cauliflower: While slightly less cold-tolerant than broccoli, cauliflower can still survive light frost with proper protection.
-Brussels Sprouts: Brussels sprouts are one of the hardiest brassicas, withstanding temperatures down to 20°F (-6°C).
-Cabbage: Cabbage is incredibly cold-hardy and can be harvested well into the winter.
By choosing crops that naturally thrive in cooler weather, you’ll reduce the need for additional heating and increase your chances of a successful winter harvest.
Simple Tools to Keep Your Plants Warm
Once you’ve selected the right crops, the next step is to focus on keeping your plants as warm as possible without the use of a heating system. Fortunately, there are several simple and inexpensive tools that can make a big difference in maintaining a consistent temperature inside your greenhouse.
1. Frost Cloth
Frost cloth (also known as row cover or garden fleece) is one of the most effective tools for protecting plants from cold temperatures. It’s a lightweight fabric that you drape over your plants at night or during particularly cold spells. The material creates a barrier that traps heat from the ground, raising the temperature around your plants by several degrees.
I use frost cloth regularly in my greenhouse during the winter months, and it’s amazing how much of a difference it makes. On particularly cold nights, I’ll double up the layers for extra protection. The best part is that frost cloth is breathable, so you can leave it on during the day without worrying about your plants overheating or lacking sunlight.
Frost cloth comes in different thicknesses, with heavier fabrics providing more protection against frost. Depending on the severity of your winters, you can choose the thickness that’s appropriate for your growing conditions.
2. Black Barrels of Water
One of my favorite low-tech ways to keep a greenhouse warm is by using thermal mass. Thermal mass refers to materials that can absorb, store, and slowly release heat over time. One of the simplest and most effective ways to do this is by using black barrels of water.
Here’s how it works: During the day, the sun heats up the water inside the barrels, and because the barrels are black, they absorb even more heat. As the temperature drops at night, the barrels slowly release that stored heat, helping to keep the greenhouse temperature from plummeting.
I’ve found that placing a few 55-gallon black barrels filled with water along the north wall of the greenhouse can make a noticeable difference in nighttime temperatures. You can even line the entire perimeter of your greenhouse with smaller water containers for added heat retention. This method works particularly well when paired with other heat-trapping strategies like using frost cloth.
3. Black Ground Cover
Another excellent way to trap heat inside your greenhouse is by using black ground cover. Just like black barrels of water, black ground cover absorbs heat from the sun during the day and radiates it back into the greenhouse at night.
Black ground cover serves a dual purpose: it helps warm the soil, which is especially important for root development in cold weather, and it suppresses weeds, which can be a bigger problem in the winter when you’re not tending your garden as frequently.
I like to lay down black ground cover around my root crops and leafy greens in the winter. Not only does it help keep the soil warmer, but it also gives the entire greenhouse an extra boost in temperature during cold nights.
4. Second Layer Inflated System
Adding a second layer of top film and a small inflator fan can really help to keep the heat that builds up during the day in your greenhouse as well. Although it will require electricity, the amount is very minimal (typically about 1 amp), and the benefits are well worth it. The small fan will create enough air to separate the two layers of film and create a dead air space. This will work as both insulation and a means of keeping your environment drier, which can be a problem in the colder months. The Four Season Kit at Greenhousekits1 comes with this second layer of film and small inflator included.
Maximize Solar Gain
Since you’re not using a heating system, it’s crucial to maximize the amount of natural heat your greenhouse can collect during the day. There are a few key strategies for ensuring that you’re making the most of the available sunlight, even during the shorter days of winter.
1. Proper Orientation
One of the first things to consider is your greenhouse’s orientation. If you haven’t yet set up your greenhouse, make sure it’s positioned to capture the most sunlight possible. In most cases, this means orienting the longer side of the greenhouse east-west and endwalls to north and south to maximize sun exposure throughout the day.
For those of us already working with an established greenhouse, it’s all about making sure there’s nothing blocking the sun. Trim back any trees, shrubs, or other obstructions that could cast shade over the greenhouse during peak sunlight hours.
2. Keep the Greenhouse Clean
It might sound simple, but keeping your greenhouse clean can have a big impact on solar gain. Dirt, dust, and algae can accumulate on the greenhouse’s walls and roof over time, reducing the amount of sunlight that gets through.
I make it a habit to clean the outside of my greenhouse regularly, especially during the fall and winter months. This helps ensure that I’m getting as much sunlight as possible, which translates to more heat for the plants inside.
3. Insulate the North Wall in extreme climates
The north side endwall of the greenhouse gets the least sunlight during the winter and usually a north wind, which means it’s the coldest part of the structure. To combat this, I like to insulate the north wall.
Mulching for Extra Insulation
Another technique that I’ve found useful in cold-weather greenhouse growing is mulching. By adding a thick layer of mulch around the base of your plants, you can help insulate the soil and protect the roots from freezing temperatures.
I typically use straw or shredded leaves as mulch in the winter. These materials provide good insulation and are readily available. Just be sure not to pile the mulch too close to the stems of your plants, as this can encourage rot.
Ventilation: Don’t Forget About Airflow
While it’s tempting to seal up your greenhouse as tightly as possible during the winter, it’s important not to forget about ventilation. Good airflow is essential for preventing mold, mildew, and disease, which can be more of a problem in the colder months when humidity levels are higher.
I recommend opening your greenhouse on sunny winter days to allow fresh air to circulate. Just be sure to close them up before the temperature drops in the late afternoon to avoid losing too much heat.
Conclusion: Growing Cold Weather Crops Without Heat
Growing cold-weather crops in a greenhouse without a heating system is not only possible, but it can also be incredibly rewarding. By selecting the right crops, using simple tools like frost cloth and thermal mass, and maximizing your greenhouse’s solar gain, you can keep your garden productive all winter long. Whether you’re growing leafy greens, root vegetables, or brassicas, there’s something deeply satisfying about harvesting fresh produce in the middle of winter, knowing that you’ve made the most of the natural elements around you.
With a bit of planning and creativity, you can keep your greenhouse thriving without the need for heating systems. So, bundle up, grab your frost cloth, and enjoy the fruits (or rather, vegetables) of your labor—even when the temperatures drop. Happy winter gardening!
If you’re interested in a greenhouse, at Greenhousekits1 we’re happy to help! Whether you are looking to grow year round without heat, or simply extend your season, we have a kit that will fit your growing needs!
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