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

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. 

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

Greenhouses and polytunnels aren’t just for farmers. There are many sizes, shapes, and types of walk-in structures that can be used to winter harvest cold season vegetables and herbs from a backyard garden. 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

Selecting a winter greenhouse

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. 

Consider your climate and extreme weather

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 sturdy structures and excellent at shedding snow and wind.

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). I have an unheated greenhouse and don’t use a heater, like a propane heater, but if you did minimally heat your greenhouse you could grow less hardy crops. 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. Those in milder climates, such as those in zones 7 and above, can grow an even wider selection of winter vegetables and herbs. Many hardy herbs like chives, thyme, and parsley can also be winter harvested from a greenhouse. I dig these up from my raised beds in early autumn and transplant them inside the structure.

By late winter most of the crops inside my greenhouse have been harvested. Any empty growing space is amended with compost and seeded with fresh greens and root crops for early spring harvesting.

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

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 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, chard, and spinach grow faster than root crops and are sown in late summer to early autumn. These are 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. Green onions are also a favorite vegetable to harvest in winter. They need about 55 to 70 days to go from seed to harvest.

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, apply a deep straw or leaf mulch 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 create thermal mass or 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.

Caring for vegetables in a winter greenhouse

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

Watering

The big question is how often do I need to water during the cold stretch from December through February? 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. I water my greenhouse almost daily from late spring through late summer. Watering is reduced to once or twice a week in early to mid-autumn when the days get shorter and the temperatures begin to drop. 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

Ensuring proper ventilation 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 early spring and late autumn, I roll up the sides of my tunnel by a few inches. This permits good air flow, especially if the weather is forecast to be warmer than 40 F (4 C). 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. I love picking veggies from the cold frames and mini hoop tunnels in my raised bed garden, but it’s pretty cold work. When harvesting in my greenhouse it’s far more comfortable. This is because the inside temperature is typically warmer than the outside temperature and I’m sheltered from winter winds.

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. 

If you don’t have room for a large greenhouse, consider using mini hoop tunnels to create a small-scale greenhouse. In my online course on using mini hoop tunnels you’ll get the scoop on how to use these amazing tools to grow more food than ever before. The video below is a sneak peek into the course.

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

• Row cover hoops for frost and pest protection
• 8 vegetables to grow for winter
• My online course: How to Build & Use Mini Hoop Tunnels in the Vegetable Garden
• Corn mache: the perfect vegetable for the winter garden
• 5 tips to successful cold frame gardening
• My conversation on winter gardening for the Joe Gardener podcast

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