Greenhouse planting is one of the best options for cultivating winter vegetables. Besides ensuring that your plants grow all winter without issues, greenhouse planting is also useful for extending their growing period. Thus, your vegetable garden can grow your favorite crops abundantly. Apart from that, choosing winter greenhouse plants is also very important considering that not all of them are frost-tolerant plants.
For that reason, we have compiled some winter greenhouse plants you can grow easily in your garden. They cope well in cold weather. Hence, you can leave your mini greenhouse unheated overnight in the deep winter.
Curious about what winter greenhouse flower or crops for a winter greenhouse you can grow in the winter season? Check them out below!
Table of Contents
- How Much Warmer Is An Unheated Greenhouse?
- How To Keep Plants In A Greenhouse In Winter?
- Plants Survive Winter In A Greenhouse
- 1. Beets (Beta Vulgaris)
- 2. Carrot (Daucus Carota)
- 3. Celery (Apium Graveolens)
- 4. Dill (Anethum Graveolens)
- 5. Kale (Brassica Oleracea)
- 6. Lettuce (Lactuca Sativa)
- 7. Mint (Mentha)
- 8. Parsley (Petroselinum Crispum)
- 9. Peas (Pisum Sativum)
- 10. Radish (Raphanus Sativus)
- 11. Swiss Chard (Beta Vulgaris Subsp. Vulgaris)
- Final Thought
- FAQ (Frequently Asked Questions)
How Much Warmer Is An Unheated Greenhouse?
Starting planting in the winter season is tricky. Lower lights of winter day means these plants only get a short period of sun exposure to thrive. In fact, it is crucial to conduct photosynthesis and keep your greenhouse warm.
Due to these conditions, it is not uncommon for farmers to install grow lights and heaters to keep the greenhouse temperature from freezing. But will the unheated greenhouse make a significant temperature difference from the outside?
Surprisingly, it’s not!
Your unheated greenhouse will stay warm even though it’s unheated. Especially if you install plastic wraps in the structure, place a thermal mass, and have a solar panel that keeps the temperature stable and controlled.
Broadly speaking, the temperature difference between the greenhouse and the surrounding area can reach 30°F. But of course, this difference depends on your heat insulation.
How To Keep Plants In A Greenhouse In Winter?
Having a greenhouse for winter growing is challenging. You must understand how to keep the warmth during harsh winters, as low temperatures can harm your crops. People usually will install a greenhouse heater. But there are more effective ways for those slashing their budget for greenhouse gardening.
Luckily, you can heat a greenhouse without electricity using some of the cheapest methods, including thermal mass. This method is so easy, simply by painting plastic bottles with half black and white colors. Then, fill them up with water. Next, stack them in the north area of your greenhouse and leave them be. They will absorb the heat at noon and release it during the freezing winter night to warm up the surroundings.
If you grow your plants in pots, wrapping them with plastic bubble wrap is the ideal practice. And, of course, they are affordable compared to installing a heater. In case those tips don’t work properly, you can install grow lights that will transmit the heat and light at the same time
Plants Survive Winter In A Greenhouse
Do you still need to decide which winter greenhouse plants to choose which can survive cold temperatures? Don’t worry! You are here to help you identify the best ones to grow in your greenhouse garden. They can tolerate an unheated greenhouse.
Thus, they won’t bother you with a complicated heating system that costs you a lot of money. Curious? Let’s get on the list!
1. Beets (Beta Vulgaris)
Characterized by bright magenta colors, beets are often used as a natural dye. They are also the raw material for the sugar you consume today. Besides that, this winter greenhouse vegetable is also a favorite to add to salads, smoothies, and even bakeries!
For this reason, many people are trying to plant it in their backyard greenhouses. And luckily, beets can thrive in the USDA hardiness zones 2 to 11, like in Columbia.
Furthermore, beets also withstand temperatures down to 50°F and hot summer months up to 95°F. However, they prefer cool weather, around 60°F – 70°F. To plant beets in a greenhouse, ensure the area has a similar temperature requirement.
You can install a heating system, like thermal curtains, thermal mass, or a heater, to say the least, to maintain cool and humid conditions. Remember to place your winter harvest crops facing the open side to absorb as much sun exposure as possible.
2. Carrot (Daucus Carota)
When you ask someone about root vegetables, their answers are not far from carrots. This cool-season veggie has a bright orange color which is its trademark. Not only that, the carrot is nutritious and loaded with pro-vitamin A that will help you get a clear vision.
People love to include it in their veggie salads, soups, cakes, and smoothies. Even cooler, they are easy to grow. The best time to sow them is between April and July in your greenhouse. Or by 10-12 weeks before the first fall frost.
But before that, you must prepare the greenhouse so that the temperature and environmental conditions follow the growing conditions for carrots. They love sandy, loamy, well-drained soil to grow in. You should also place them in the center of your greenhouse dome to get optimum sun exposure.
Remember to maintain the temperature around 50°F – 85°F to allow germination. Any temperature that exceeds the maximum will slow down the growth of foliage. And the great news is it thrives in the USDA hardiness zones 3 to 11, which includes California as the U.S. biggest producer of carrots, accounting for 85%.
3. Celery (Apium Graveolens)
While other vegetables are consumed for their high fiber, celery is more popular for its aromatic, peculiar scent to improve meals’ aroma and taste. Because of this feature, they are often mistaken as herbs. They are one of the favorite winter plants for greenhouse growing because they are easy to care for.
Additionally, this veggie copes well with low temperatures of about 36°F. So, growing them in winter is not a big deal. On the contrary, you must be aware when planting them in summer because they can only handle temperatures up to 86°F.
Considering such temperatures, they will happily thrive in the USDA hardiness zones 4 to 10, covering most of the United States except for Alaska and northern Minnesota, Wisconsin, Michigan, and Maine. To plant celery in the greenhouse, you need some extra preparation. Especially if you sow them in late winter to early spring.
The seeds require at least 16 hours of light exposure. So, you will need to install grow lights to meet this preferred condition. Besides providing their light requirement, these lights also transmit heat to make the celery frost-free.
4. Dill (Anethum Graveolens)
Like its fellow herbs, dill is cultivated for its aroma and strong flavor. Their small stems, immature edible flowers, and leaves are commonly dried. Then, people will add the dry parts into soups, fish, sauces, and pickles. If you like fresh dills, you can plant them in a greenhouse.
The best season to sow them is between May to July, when the outside temperature is pretty warm. Hence, there’s no doubt that they thrive in the USDA hardiness zones 8 to 11 that runs from South Carolina to Hawaii.
However, this herb tolerates low temperatures up to about 25°F, despite loving warm weather of about 60°F – 75°F. Considering its growing conditions that require cool temperatures, you must adjust the temperature of your greenhouse if you sow them in the winter. The seeds need full sun exposure with moist, well-drained soil. So, make sure to put them in the south-facing area in the greenhouse.
5. Kale (Brassica Oleracea)
Greens are one of the popular winter veggies you can grow in your greenhouse without complex issues. One of the most common greens is kale. They are also known as salad plants with a strong earthy flavor and crunchy texture that people love.
In terms of greenhouse gardening, they are easy to grow and care for. And most importantly, coping well with winter temperatures, reaching about 14°F and a maximum of 75°F. Depending on the variety, kale can grow in the USDA hardiness zones 2 to 11, for instance, Missouri and Wisconsin.
Since this cool-season veggie has a great tolerance to cold winters, you don’t need to set up a complex heating system for your greenhouse. It’s just that they need bright light to thrive best.
To fulfill the requirement, you must ensure that the walls of your greenhouse are made of transparent plastic or glass. This way, you can let the sun pass through to your crops. It also helps to heat up the greenhouse without electricity. Grow them in late winter to early spring if you want a summer harvest. And to harvest in winter, they are best planted in late summer.
6. Lettuce (Lactuca Sativa)
You can almost spot lettuce everywhere in any dish. People usually use this leafy green to decorate the main course and add to the burgers. Nowadays, they are favored to make lettuce wraps with meat!
Moreover, lettuce is also one of the famous cold-season crops to grow in the greenhouse because the structure can protect them from excessive heat exceeding 85°F. On the other hand, it can cope with winter temperatures as low as 20°F even though the ideal conditions range from 70°F – 75°F.
Despite being a cold hardy doesn’t mean your lettuce doesn’t need protection in the greenhouse. Adding a plastic wrap around the setting is recommended to insulate the heat and maintain the optimum temperature.
You also must ensure they get at least six hours of sunlight to boost their growth. Mixing loamy soil and compost for the beds is also a great idea to keep them thriving. You can grow them in the USDA 2a to 11b, covering Washington to Puerto Rico, during spring and fall.
7. Mint (Mentha)
An aromatic, intense aroma with a hint of cooling effect when added to beverages and cuisines is one of the main characteristics of mint. This aromatic herb is usually grown in pots near the windowsill, where you can pick them whenever you want to add the leaves to your foods.
However, if you want more spacious space, try to build a greenhouse in your backyard to accommodate their pots.
Moreover, they thrive in the USDA hardiness zones 3 to 8, which include New Mexico and Arizona, while the south is too warm for mint cultivation. This zone is famous for its winter temperatures which can fall far below 0°F. But amazingly, mint can survive extremely low temperatures down to around -20°F.
On the other hand, the optimum temperature to grow mint is between 55°F to 70°F. So, you don’t need to worry about planting it in an unheated greenhouse. In case you are interested in plant mint, do it in the frost-free spring or fall.
8. Parsley (Petroselinum Crispum)
We can safely say that parsley is one of the most popular culinary herbs in the world. It is cultivated for its fresh, grassy, and spicy aromatic leaves, commonly sold in dry form. You can easily find them infused in various cuisines.
Furthermore, parsley will thrive in the USDA hardiness zones 4 to 9, for example, Florida. You can grow them in your herb garden. But, controlling the growing conditions will be much more tricky than growing it in the greenhouse.
Looking at its growing zones, we’re not surprised that parsley is one of the cold hardy crops that keep growing all winter. It tolerates freezing temperatures down to 20°F. They will die if the surrounding temperature is above 90°F.
For greenhouse growing, you can plant them in lined pots, raised beds, or hang them on a roof structure. Make sure the soil is rich and moist. Since they cope well in winter, parsley won’t bother you with the heating system. Nonetheless, consider plastic wraps for the least protection from frost.
9. Peas (Pisum Sativum)
Some think that peas are beans. In fact, they are pod-shaped vegetables that belong to the Fabaceae family, bearing round seeds in the pods. They are commonly added to soups, toasts, salads, sautee, and pasta.
Not only that, you can also make peas as one of the raw materials for making hummus and puree. Because they can incorporate many dishes, not a few people try to grow peas to be able to harvest them for themselves. Try planting peas between February and April.
Luckily, peas can grow in USDA hardiness zones 3 to 11, which cover North America, Hawaii, and Puerto Rico. They can also survive light frost around 40°F, while the maximum temperature is around 70°F. However, they are not frost-hardy. Their roots will rot, and growth will be stunted if exposed to temperatures below the minimum temperature.
Therefore, you must set a natural heat absorbent, such as fill water bottles, aluminum roofing, or dark bark wood. A heater may be a quick fix, but the cost is not worth it.
10. Radish (Raphanus Sativus)
Similar to beets, radish is a root vegetable belonging to the Brassicaceae family. Like other brassicas, it has a crunchy texture. However, the taste is more spicy and peppery. Most people process it by sautéing and roasting. Some add them to toast with bacon or beef.
Moreover, this veggie is also a fast-growing winter vegetable that can withstand low temperatures of 27°F. In fact, it needs cold weather with a temperature range of 41°F for germination and a maximum of 86°F.
Regarding the temperature requirements, your greenhouse will require extra protection, especially in the harsh winter. The cheapest way to heat it is by placing thermal mass materials around your plants, such as dark bark wood.
Also, install heat insulation like plastic wrap to cover the greenhouse walls. You can start planting radishes in spring, from April to May. The next growing season is in the early fall, around September. It thrives in the wide range of USDA hardiness zones 2 to 11, including Illinois and Colorado.
11. Swiss Chard (Beta Vulgaris Subsp. Vulgaris)
From its shape, Swiss Chard is a leafy green with bright red veins against green colors. The thick texture of the stems is crispy, while the leaves are tender, like spinach. It also tastes like spinach. They include many in Mediterranean recipes, though some also simply saute them with proteins.
Apart from being popular because of its texture and taste, Swiss Chard is also a favorite among greenhouse gardeners because it has excellent tolerance to cold weather. Its lowest temperature can reach 15°F, while its highest is around 75°F.
You can grow this veggie as annuals in the USDA hardiness zones 3 to 10 (all parts except Puerto Rico), approximately 2 to 4 weeks before the last frost in spring.
Since it is a hardy veggie, you may only need to prepare a heating system in the form of thermal curtains made of fleece or thermal mass to cover your greenhouse. They are beneficial in terms of insulating heat. Thus, your plants will not suffer from freezing temperatures on winter nights.
Besides flowering plants, greenhouses are ideal for growing crops you can harvest during the season. Some of the most recommended winter greenhouse plants, because they can grow in almost all areas of the USDA hardiness zones, are beets, radishes, Swiss chard, and peas. They are also resistant to quite extreme cold temperatures.
Thus, it makes it easier for you to care for it during the winter. You can also cut the budget for heating installation in your greenhouse since most vegetables and herbs can survive unheated.
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FAQ (Frequently Asked Questions)
Will an unheated greenhouse keep plants from freezing?
Even if you leave it unheated overnight, your greenhouse can still keep vegetables and plants from freezing temperatures. Typically, the greenhouse environment can maintain a temperature of about 5°C warmer than the outside temperature.
This can happen because of layers of fleece or bubble wrap that cover your greenhouse. In addition, thermal mass materials such as water, wood, and soil in the area also help insulate heat as they are excellent heat absorbents.
What can you grow in an unheated greenhouse in the winter?
Cool-season crops are those that are best for growing in an unheated greenhouse. It includes greens like spinach, kale, and Swiss chard. In addition, some root vegetables, such as carrots, beets, and radishes, also have excellent tolerance to low winter temperatures.
If you are interested in herbs, try planting parsley, chives, and mint. In case you are worried that your plants will suffer, it will be better to cover the pots with bubble wrap to protect the roots.
What flowers can you grow in the greenhouse over winter?
Generally, flowers will go dormant when winter arrives due to extreme temperatures. However, there are some flowers you can try planting in the greenhouse during winter. But, you have to know when is the right time to grow it. For example, irises and calla lilies are best grown in December when the air temperature hasn’t fallen far below zero.
Meanwhile, hyacinths, tulips, and daffodils are flowers for January, when the height of winter arrives. In February, you can try planting orchids so that in summer, they can flourish.
What is the lowest temperature plants can survive in a greenhouse?
The greenhouse temperature at noon is usually in the range of 65°F – 70°F in winter. But at night, the temperature can drop drastically to 45°F. Farmers or gardeners often fear this temperature because it may harm the plants.
Therefore, you must create a proper greenhouse heating system, with or without electricity, so your plants can survive extreme temperatures. Also, choose winter plants to grow in a greenhouse because they tend to have a good tolerance for cold temperatures.