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Unheated Greenhouse Growing: How To Use An Unheated Greenhouse

In an unheated greenhouse, growing anything during the cold months of winter may seem impossible. Alas, it is not! Knowing how to use an unheated greenhouse and what plants are better suited is the key to success. Read on to learn more.

Using an Unheated Greenhouse in Winter

An unheated greenhouse in winter will not only allow you to grow hardy veggies, but you can start tender annuals, propagate perennials, and overwinter cold sensitive plants. Of course, it helps to know how to use an unheated greenhouse (or “cold house,” as it may also be called) effectively and what plants will be most suited to this cooler environment.

During the day, a typical greenhouse will trap heat from the sun, which allows the plants inside to stay warm at night. That said, when winter nights get really cold, frost damage in the greenhouse can occur without added protection.

What types of protection are there in lieu of greenhouse heaters? This can be as simple as the addition of one or two layers of horticultural fleece over your plants (Remember to remove coverings during the day so they do not overheat.), and place some bubble wrap around your pots to help insulate plant roots and prevent clay pots from cracking. Horticultural bubble wrap can also be used by layering the inside of your greenhouse. The much needed sunlight will still come through but the extra layer of protection will keep your plants safe at night.

Chances are good your unheated greenhouse is a simple cold frame or hoop type of structure. This structure is very simple to use in the winter and fairly low cost. It should be situated so it is getting the most natural sunlight possible, out of the way of winds, and as close to a water source as possible.

Keep an eye on the thermometer, especially when heading towards spring. In many regions, temperatures can be in the 30’s one day and in the 60’s the next (in a buttoned-up greenhouse it can be much higher). Plants don’t often recover from sudden overheat like that, so be sure to open the greenhouse if temperatures threaten to soar.

What to Grow in Unheated Greenhouses

When you have a temperature-controlled greenhouse, the sky’s the limit as to what can be grown through the winter months. However, if your greenhouse is a simple affair, lacking any heat, don’t despair. Using an unheated greenhouse can still provide you with plenty of options.

An unheated greenhouse can be used to grow greens during winter, start warm season annuals, propagate landscape perennials, and shelter frost tender plants through the winter chill.

Besides greens like spinach and lettuce, you can grow cold tolerant veggies such as cabbage and broccoli in your unheated greenhouse. Celery, peas, and the ever popular Brussel sprouts are also excellent cool weather veggie choices for unheated greenhouse growing.

Other winter greenhouse plants that thrive during winter months are root vegetables. Winter temperatures actually stimulate sugar production in some root veggies, so you end up with the sweetest carrots, beets, and turnips imaginable. Don’t stop there with your winter greenhouse gardening though.

Perennial herbs are another option – oregano, fennel, chives and parsley do well. Cool-hardy flowers, like calendula, chrysanthemum, and pansy, not only thrive in a cold house but will bloom through the winter. Many annual and perennials that may not be hardy in your climate outdoors will actually flourish in the greenhouse, even those that are seeded in fall will grow and produce a bounty of blooms in late winter to early spring.

In an unheated greenhouse, growing anything during the cold months of winter may seem impossible. Alas, it is not! Knowing how to use an unheated greenhouse and what plants are better suited is the key to success. Learn about using an unheated greenhouse here.

A winter greenhouse: A productive way to harvest vegetables all winter

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In my vegetable garden, a winter greenhouse has become the heart of our cold season garden, providing us with homegrown vegetables and herbs from December through March. This unheated structure, which is also featured in my book, Growing Under Cover: Techniques for a More Productive, Weather-Resistant, Pest-Free Vegetable Garden, captures solar energy and shelters a wide variety of cold tolerant crops like kale, carrots, leeks, scallions, carrots, and spinach.

My winter greenhouse grows organic vegetables 365 days a year. In winter, I harvest cold season salad greens, root crops, and stem crops like leeks.

I also use the greenhouse to extend the fall harvest, start seeds for the main garden, harden off transplants, and get a jump on spring. And when the weather warms up in late spring, the raised beds inside are planted with heat-loving crops like tomatoes, peppers, and cucumbers to provide an extra-early harvest.

Just because I use a winter greenhouse doesn’t mean that I don’t use other winter structures in my garden. I have a variety of smaller season extenders like cold frames and mini hoop tunnels, and also use techniques like deep mulching. But having a winter greenhouse has upped my garden game by providing a covered space for growing food. This makes tending and harvesting crops more comfortable, especially when the weather is cold and snowy, but it also gives me a much larger area for food production.

Types of winter greenhouses

There are many sizes, shapes, and types of greenhouses that can be used to winter harvest cold season vegetables and herbs. Some structures are sold in kits while others are DIY’d by handy gardeners.

A few examples of types of home greenhouses:

  • Metal-framed glass greenhouse
  • Metal-framed polycarbonate greenhouse
  • Metal-hooped polyethylene greenhouse
  • Wood-framed glass greenhouse
  • Wood-framed polycarbonate greenhouse
  • Wood-framed polyethylene greenhouse
  • PVC-framed polyethylene greenhouse
  • Metal-framed polycarbonate dome greenhouse
  • Wood-framed polyethylene dome greenhouse

Dome greenhouses are becoming very popular in home gardens. Structurally, they are very strong and they can be used to produce a winter crop of hardy vegetables and herbs.

Whatever type of greenhouse you decide to buy or build, they all have two main components: a frame and a transparent cover. My greenhouse is 14 by 24 feet and was purchased as a kit from a local greenhouse supply store. I wanted a structure that would be strong enough to stand up to our maritime weather. In winter, that weather includes frequent storms that bring heavy snow, freezing rain, and strong winds. Other times of the year we deal with extreme weather like hurricanes.

If you’re anything like me, when you dream of a greenhouse you picture a luxurious metal-framed, glass-glazed structure. Garden goals to be sure, but these types of structures come with a significant cost. And while, they’re a great for growing vegetables, you may be surprised to learn that even a DIY wood frame covered in 6 mil greenhouse polyethylene sheeting is also effective in sheltering winter crops.

When deciding on a type of greenhouse, look first at your site, space, and climate. Most urban yards won’t have space for a large hoop greenhouse, but a small glass or polycarbonate-glazed structure may fit. Also take a look at the grade. Is your site sloped? A slight slope can generally be worked around, but a steep grade can make it hard to erect a greenhouse. While you’re inspecting your yard, also remember that a greenhouse needs to be placed where it receives full sunlight. Look around for potential sources of shade – nearby trees and buildings, for example.

As for climate, I live on the east coast of Canada where snow and wind can be extreme. As noted above, my greenhouse had to be strong enough to withstand hurricanes and winter storms. If you live in a milder climate, you can likely get by with a greenhouse made from more lightweight materials.

Another type of structure to consider is a geodesic dome greenhouse. These dome-shaped, rounded greenhouses are becoming popular in home gardens due to their strength. They are excellent at shedding snow and wind.

I grow many types of cold hardy lettuces in my winter greenhouse including Salanova, which forms pretty rosettes of tender-crisp leaves.

What to grow in a winter greenhouse

There are a lot of crops that can be harvested from a winter greenhouse. The crops you choose to grow depend on your climate and what you like to eat. I garden in zone 5 and have winter temperatures that can go down to -4 F (-20 C) . We plant a wide selection of cold season vegetables in our winter structures. Root crops like carrots and beets, as well as salad greens like kale, winter lettuce, spinach, Asian greens, endive, and arugula.

When reading seed catalogs and selecting varieties to grow, read each description carefully. Certain varieties are hardier than others. For example, Winter Density and North Pole lettuces are among my favorite lettuces to grow for December through March harvesting. They stand up well to cold temperatures, easily out performing summer or spring lettuces by months.

Those who live in climates colder than zone 5 should stick to the most cold hardy crops. In my garden, the winter superstars include Winterbor kale, mache, tatsoi, and scallions. On the opposite end of the temperature spectrum, those in mild climates, such as those in zones 7 and above, can grow an even wider selection of winter vegetables and herbs, often with minimal need for season extenders.

Niki’s 10 favorite crops to harvest in winter:

    1. Carrots
    2. Beets
    3. Scallions
    4. Leeks
    5. Winter lettuces
    6. Spinach
    7. Arugula
    8. Mache
    9. Kale
    10. Parsley

Kale is one of the hardiest crops to harvest in winter and we grow several types inside our structure.

For more crops you can grow in fall and winter, check out this video:

When to plant winter vegetables

Most of the vegetables in my winter greenhouse are planted from mid-summer to mid-autumn. Ideally, the crop should be almost mature or ready to pick just as the weather turns cold and the day-length drops below ten hours a day. That is the point when most plant growth slows dramatically. In my northern climate, that date is in early November and the mature or almost-mature vegetables remain in the greenhouse until we are ready to harvest.

To figure out the right planting date, you need to look at the days to maturity for the individual crop or variety. This information is listed on the seed packet or in the seed catalog. My Napoli carrot crop, for example, takes about 58 days to go from seed to harvest. So, ideally I would count backwards 58 days from my first expected frost date and plant. However, as the day length shrinks in autumn, plant growth slows, so I always add on an additional 7-10 days when planting crops for late fall and winter harvesting. That means that I end up sowing Napoli carrots for winter in mid-summer.

Salad greens like arugula, leaf lettuce, and spinach grow faster than root crops and are sown in late summer to early autumn. They can be direct sown or given a head start indoors under grow lights. If you wish to have mature kale or collard plants for winter harvests, these take around 70 days from seeding, so plan accordingly.

To further insulate my winter crops, I often erect fabric covered mini hoop tunnels over the raised beds. It helps trap heat and protects the vegetables from frigid weather.

How to boost heat in an unheated winter greenhouse

On a winter day when the outside temperature is well below freezing, my greenhouse is typically mild inside, thanks to the sun. For example, when it’s 17 F (-8 C) outside, the inside temperature can reach 50 F (10 C). That said, once the sun goes down, the temperature quickly drops. However, there are a few sneaky ways you can boost heat retention and insulate your crops. To insulate, I use deep mulching, row cover fabrics, or polyethylene covers floated on mini hoops. You can make your own or buy fleece tunnel kits. For root crops like carrots and beets, a deep straw or leaf mulch is applied over the bed in late autumn before the soil inside the greenhouse freezes.

To use fabric or polyethylene covers over beds of greens, hardy herbs, scallions, and other vegetables, I float the covers on top of simple wire hoops.

Another way to slow heat loss in a winter greenhouse, is to add a heat sink like a few water-filled barrels. The water absorbs heat during the day and slowly releases it at night, slowing the cooling process. If the greenhouse is large enough, you could also put a compost pile inside to generate some heat.

Many salad greens can be sown in late summer and early autumn for winter harvesting. Spinach, arugula, mizuna, and mustards are both easy and quick to grow.

Caring for vegetables in a winter greenhouse

There are five main tasks to keep in mind when tending a winter greenhouse:

  • Watering. The question I’m asked most is about watering and how often I need to water during the cold stretch from December through February. Honestly? Not much! It will depend on the year as some years we get an early freeze-up and my watering comes to an end by late November. Other years, the weather can be mild into late December and I do irrigate a few times in late fall. I use a hose to water, but you can also use a watering can and fill it up from a rain barrel situated near the greenhouse or one that catches water from the roof of the greenhouse. It’s important to note that while I water my greenhouse almost daily from late spring through late summer, once the days shorten and the temperatures drop in early to mid-autumn, watering is reduced to once or twice a week. In winter, I don’t water unless we get a few days of thawing temperatures.
  • Fertilizing. Soil health is always top of my mind in my garden beds and structures and so I work in compost, aged manures, chopped leaves, and other amendments into the earth between crops. I also apply organic fertilizers – both granular and liquid to promote healthy plant growth and a bountiful winter harvest. Slow-release granular fertilizers are added at planting time, while liquid fertilizers, like fish and kelp emulsion, are applied monthly, depending on the product. Always follow application instructions on whatever type of fertilizer you buy.
  • Venting. Venting is one of the most important tasks in a greenhouse, especially when the weather is hot. I have roll-up sides, windows, and a door for venting. In late fall or early spring, if the weather is forecast to be warmer than 40 F (4 C), I roll up the sides a few inches to allow air to circulate. The inside of a structure heats up quickly, and it’s best to grow winter crops on the cool side to promote hardy growth. If you keep the inside temperature of your greenhouse too warm in mid to late autumn, soft tender growth emerges which can be damaged when the temperatures drop. Venting is also the best way to reduce condensation in a greenhouse. Condensation can encourage fungal diseases to grow and regular venting on mild days will reduce the amount of humidity in the air.
  • Harvesting. It’s so pleasant to winter harvest from a greenhouse. Sure, I love picking veggies from my cold frames and mini hoop tunnels, but I’m still stuck outside in the cold weather. When I’m harvesting in the greenhouse, the inside temperature is typically warmer than the outside temperature and I’m sheltered from winter winds.

A heavy load of snow can damage a greenhouse. Use a broom or another

  • Snow removal. I live in an area where deep snow is not uncommon and I need to keep an eye on the snow load on top of my structure. I did buy a greenhouse designed to withstand a heavy snow load, but if snow begins to accumulate on top of my structure, I take a soft-bristled broom to carefully brush it off from the outside or tap it off using the broom from the inside. This works because my structure is covered with polyethylene. With a polycarbonate or glass-covered greenhouse, you need to gently brush the snow off the panels from the outside.

For further reading on winter vegetable gardening, check out these articles:

Also be sure to check out my late book, Growing Under Cover and my award-winning book, The Year-Round Vegetable Gardener.

A winter greenhouse is an unheated structure that captures solar energy and provides shelter for cold tolerant crops like kale, carrots and spinach.