You shouldn’t expect to grow tomatoes in winter unless you have a hothouse. That said, you can grow a wide variety of vegetables in winter, in nearly any climate. All you need is a simple, unheated greenhouse or hoop-house.
Of course, extending the growing season in the spring and fall is the most common reason for getting a greenhouse, but you don’t have to limit yourself to that. With the right choices of vegetables and strategies like adding heat-sinks, you can grow and harvest vegetables from your greenhouse all winter in any growing zone.
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Greenhouse Gardening in Winter
You should plant most winter seeds in late summer. By doing so, you will give them time to mature before temperatures drop, and you transplant them into the greenhouse.
Before growing your winter crop, integrate compost or fertilizer into your soil. By doing so, you’ll compensate for the nutrients consumed by your summer greenhouse crops.
You’ll also need to pay attention to the temperature, ventilation, heat, humidity, and pests. Still, it will be worth the effort when you enjoy eating greens, herbs, brassicas, and other vegetables all winter long.
In the late winter and early spring, you can use your greenhouse in another way as a protected location to give your spring seedlings a jumpstart.
A greenhouse can make all the difference in winter gardening. However, there is a learning curve. If you don’t use a winter greenhouse correctly, you will have a disappointing harvest. This guide will help you make the most of your greenhouse in winter.
What Should You Grow?
Greenhouses are an important tool, but they aren’t magic. For example, if you live in southern Maine, in USDA plant hardiness Zone 5, you can’t expect to grow tomatoes in the winter in an unheated greenhouse.
To determine what you can grow, start by counting how many layers of protection you are offering your plants. For example, a cold frame inside an unheated greenhouse, each with one pane of glass, counts as two layers.
In Four Season Harvest by Eliot Colemon, he suggests that for every layer of protection, you should add roughly 1.5 zones to your natural growing zone.
For example, in our cold frame inside a greenhouse, we multiply two layers by 1.5 to get a growing zone adjustment of 3. That means if you are gardening in growing Zone 5, this +3 setup will create a Zone 8 microclimate.
Your adjusted growing zone, based on the microclimate you create, determines what you can grow. In our example, you can grow vegetables in Zone 5 that would ordinarily only be suitable to grow outdoors in Zone 8+.
Remember that this scenario means you can grow Zone 8 winter crops in a two-layer greenhouse setup in Zone 5, not that you can grow Zone 8 summer crops. If you want to grow crops that cannot tolerate winter temperatures at all, you will need to add supplemental sources of heat and maybe light.
The best cold-hardy winter vegetables can survive a few light freezes. Dozens of cold-hardy vegetables are a good bet in most growing zones, even in an unheated greenhouse.
Many types of lettuce thrive in a cool environment, 45-65 degrees Fahrenheit. In fact, making sure your greenhouse is ventilated is as big a consideration as keeping it warm. Temperatures above 80 degrees can cause bolting or a bitter flavor.
Little gem, rocket, and lamb’s lettuce, for example, are fast growers that do well in winter. Endive, arugula, and radicchio are also excellent choices. Even romaine, butterhead, and other lettuces will grow in winter.
Cut-and-come-again lettuces are an ideal choice. You can harvest lettuce from those plants all winter. Otherwise, you can use succession sowing to ensure you never run out of lettuce.
Spinach, chard, kale, pak choi, and mustard greens can tolerate even lower winter temperatures than lettuce – down to 35 degrees Fahrenheit. Kale, in particular, will recover well, even after freezing.
Moreover, you can repeatedly harvest many greens like lettuces throughout winter.
If you have an unheated winter greenhouse, you can choose from the large plant family brassica (otherwise known as “cruciferous vegetables”). Broccoli, cabbage, turnip, brussel sprouts, collard greens, kohlrabi, and many Asian greens are all brassica.
When growing winter brassica, choose cold-hardy varieties. Remember, some types are only suited to summer crops.
Floating row covers are a good idea for crops like broccoli that can be cold-sensitive and vulnerable to aphids, which flourish in humid greenhouses.
Onions are easy to grow in winter greenhouses. They need little upkeep and are not very susceptible to pests. You can grow and harvest scallions all winter long or harvest onion bulbs in the spring or early summer. If you only plan to use them as scallions, you can plant them densely to get bigger harvests per square foot.
The earth is typically warmer than the air in winter, so it makes sense that root vegetables would thrive in a winter greenhouse. Many of them even become sweeter after a frost. Keep in mind that even if the tops die, the roots may still be salvageable.
For root vegetables, you should consider carrots, beets, radishes, turnips, and parsnips. Make sure to look for winter-hardy varieties.
Carrots and radishes will germinate at particularly low temperatures, as low as 35-45 degrees. That means they can be succession sowed all winter, even in an unheated greenhouse.
Many annual and perennial herbs can be overwintered in a greenhouse. Cilantro, dill, and some other herbs need moderate temperatures to germinate but grow well in cool temperatures.
Mint, thyme, rosemary, and oregano are all perennials that may survive even without a greenhouse. They are easy to grow year-round in pots if moved into the greenhouse for winter.
Spring Harvest Vegetables
While all of the other vegetables listed can be harvested and consumed all winter, there are other vegetables you can grow in your greenhouse over the winter to harvest in early spring. These include peas, broad beans, and potatoes.
Garlic is another crop that should be planted in the fall for a harvest the following year. However, you can harvest garlic greens from your greenhouse much sooner if you choose. Hard-neck varieties are more tolerant of cold than softneck. In fact, to flourish, they need low temperatures, between 30-50 degrees, so don’t grow them in a heated greenhouse.
Using a Winter Greenhouse – How to Grow Vegetables in Winter
Winter greenhouses are simple to operate, but they do require effort to maintain a healthy growing environment. If you are new to greenhouse gardening, check out our article on How a Greenhouse Works to learn more about the basic principles.
You can maximize your winter greenhouse yield with a few key strategies.
Choose the Right Location
Most gardeners locate their greenhouses with summer growing conditions in mind. That can mean a location with partial shade to avoid issues with overheating. For winter greenhouse growing, it’s different.
First of all, you want your winter greenhouse to get as much sun exposure as possible. The more sunlight you capture, the warmer a micro climate you can create. Aim for at least six hours of direct sunlight in the winter.
Pro tip: Use a sun path chart to determine the ideal site for your greenhouse.This is because the sunniest spot in your yard in the summer may not be the brightest spot during winter.
You may have unexpected areas of good sun due to deciduous leaves. Simultaneously, the sun’s lower position in the sky means structures to your south will cast longer shadows. That means some places that get good summer sun will be shaded in winter.
If possible, take summer/winter variation into account when choosing a location for your greenhouse. Other options are to have two greenhouses or to move your greenhouse seasonally. The portability factor gives hoop houses an edge over glass greenhouses for gardeners who want a year-round harvest.
Orient your greenhouse on the east/west axis to maximize sun exposure.
Read more: How to heat a greenhouse
Add Water Barrels
Winter greenhouses do create microclimates warmer than the outside, but temperature control can be a challenge. You may find that your greenhouse becomes too hot during the day and still freezes at night.
There are various potential solutions, but an easy one is to put 50-gallon black plastic barrels inside your greenhouse (for example, this option). If the budget is limited, you can buy a secondhand or purchase cheaper barrels and paint them black.
Fill these barrels with water, and they become a source of passive heating. The water helps your greenhouse maintain a consistently warm temperature by absorbing heat during the day and releasing it slowly at night.
Add Heat, Light, and Ventilation
The sun may be enough to grow winter-hardy plants, but if you want to grow a wider variety of veggies in the winter, especially in northern climates, consider adding alternative sources of heat.
Here are 9 ways you can heat a greenhouse:
These could include heat from burning wood or space heaters (usually propane or natural gas, because electric space heaters are so expensive to operate). Other gardeners use more creative heat sources, such as compost piles, stored renewable energy, or geothermal heating tubes.
If you really want to grow warm-season vegetables, not just winter-hardy vegetables, you may also need to add grow lights to your greenhouse.
You will also need a strategy for ventilating your greenhouse to prevent overheating.
Plant Densely, Early, in Beds
During winter, it is ideal to plant directly into the ground or large raised beds. Avoid planting in containers if possible. The smaller amount of dirt in a pot will freeze much more quickly than the ground will.
You want to plant your winter veggies early. Even plants that grow happily at near-freezing temperatures will not germinate.
For example, beets will grow all winter but need at least 50 degrees Fahrenheit to germinate. If you start your seeds too late, you will need to use a heat mat like this one or another heat source to create a climate suitable for germination.
Here’s a handy video showing how to use a heat mat for starting seeds:
Time your planting so your vegetables are mature before either of two conditions occurs: the ground freezes or the daylight hours fall below 10 hours per day.
Keep in mind that they will mature slower in the fall than in the spring or summer. Give your plants plenty of time to grow strong before they face the challenges posed by winter.
You can plant your winter garden more densely (more plants per square foot) than your summer garden, especially lettuces and other crops that you can thin by harvesting baby veggies.
Watering plants during winter is entirely different than in summer.
Since the temperature is cooler and humidity levels are high, your plants or vegetables dry slower. With that in mind, you will need to observe the soil and wait until it dries out before watering.
When you do water, do it in the morning to give the plants a chance to dry off before temperatures drop.
At any time of year, crops are generally more tender when you harvest them young. Winter is perfect for harvesting baby vegetables because the plants tend to grow smaller and slower.
Pro tip: Keep in mind that even in a greenhouse, your vegetables may occasionally freeze. That isn’t necessarily a problem.
For example, if you find one morning that your lettuce is frozen, simply wait until a warmer time of day when it has defrosted to harvest it. Depending on how hard it froze, it may or may not be usable, but at least the plant itself is likely to survive, whereas harvesting from a frozen plant will kill it.
Some crops, like carrots, reportedly taste better if harvested after a freeze or two.
What Makes a Good Winter Greenhouse?
As Large as Possible
Install the largest greenhouse that you can fit in the available space and that you can afford. This gives you plenty of space for plants and enough room for water barrels, heating elements, and a chair to sit in when you want a warm place to absorb some Vitamin D. Keep in mind that size will affect heating costs in a hothouse.
For our recommended greenhouse kits, at a wide variety of price points, check out this guide.
Double-wall construction has walls made of two plastic layers, with a blower to circulate air between them. This trapped air can dramatically increase the temperature and humidity inside the greenhouse. Double walls create a microclimate that feels like you’ve relocated your garden three growing zones to the south.
It is 30-40% more efficient than glass. Also, the double thickness of plastic diffuses light so that you can use more vertical space without shading plants on the ground.
If double-wall construction isn’t an option, consider using a cold-frame or floating row covers inside your single-wall greenhouse for a similar effect.
Greenhouses can get surprisingly hot, even in the winter. One unexpectedly warm day and you could see inside temperatures near 100 degrees Fahrenheit. Your plants will suffer without proper ventilation.
To make ventilation easy, you can connect automatic ventilation to a thermometer. Otherwise, you’ll need to track the weather and your greenhouse temperature. You can use doors, vents, and fans to ensure proper circulation. This will keep your temperature and humidity within acceptable levels.
Heat sinks (heat-absorbing materials) are essential in a winter greenhouse. Filling your greenhouse with black water barrels, fish tanks, brick or concrete flooring, or other dense materials will help you maintain a stable warm temperature and keep your plants happy.
Just need a small greenhouse? Read our full buyers guide.
In an expensive, souped-up greenhouse — with multiple layers of wall, a heat source, and grow lights — you might be able to live the dream of growing tomatoes in the winter.
A more realistic goal is to grow winter-hardy vegetables, but that goal is within any gardener’s reach, even using a simple unheated greenhouse. Growing winter veggies may require time and effort, but the joy of gardening all winter and the delicious harvest make it well worth it for gardening lovers.