Growing vegetables in a greenhouse during winter can extend the production of cool-weather crops. As long as you can keep your greenhouse from dropping below temperatures where the plants you're growing are hardy, you'll be able to enjoy fresh vegetables long after the same crops would stop producing if they were growing outdoors without protection from the elements.
It's important to have realistic expectations for wintertime greenhouse gardening. You won't to able to produce vegetables that require a lot of sun and heat (such as tomatoes, eggplants, peppers, etc.) in an ordinary greenhouse over the winter, but you absolutely can grow - and harvest - quite a few cool-season veggies this way.
Best Greenhouse Vegetables to Grow in Winter
When growing cool weather vegetables in a greenhouse, your goal should be to keep the temperature above (or as close to) freezing as possible. The frost-resistant vegetables listed below can tolerate temperatures at - or even slightly below - freezing.
- Broccoli - Start your broccoli plants in fall, then keep them going all winter in your greenhouse so you can enjoy fresh broccoli during the coldest months of the year.
- Broad beans - Also known as fava beans, broad beans - unlike other beans - are cool-weather crops. Start them prior to first frost and keep them growing over winter for an early spring harvest.
- Cabbage - Cabbage is a great candidate to grow in winter greenhouses. It thrives in conditions as low as 27°F, and the cover of a greenhouse will continue to protect it from pests when spring arrives.
- Carrots - Carrots do well in temperatures as low as 20°F, so you can actually alternate them between outdoors and your greenhouse depending on the weather forecast.
- Cauliflower - Cauliflower grows well when it's cold (as low as 25°F), and it even tastes better after at least a light frost. Growing cauliflower in a greenhouse means you can enjoy it all winter.
- Green peas - Cool weather peas - which all green peas are - will keep growing when temps stay above 40°F and will survive light frosts. So, they're great candidates for winter greenhouse growing.
- Kale - Kale is extremely cold hardy, so you really only need to put it in a greenhouse if you frequently get temps below freezing. It'll do great even in cold and drafty greenhouses that dip down to well below freezing.
- Kohlrabi - Kohlrabi is a cool-weather crop that will grow well in a greenhouse during winter. It has similar needs as cabbage, so it can handle temperatures in the upper 20s (°F).
- Lettuce - A greenhouse makes it possible to grow and enjoy fresh lettuce all winter. Some varieties are more cold tolerant that others and can even survive temperatures in the teens (°F).
- Onions - You won't be able to harvest onions in the winter, but it's a good idea to start onion seeds in your greenhouse (or indoors) in mid-to late winter to get a head start on spring planting.
- Spinach - Spinach is one of the most cold-hardy crops. It doesn't like warm weather and thrives in cold conditions. You can keep it going all winter as long as the temperature in your greenhouse stays above 15°F.
- Swiss chard -Swiss chard can also survive temperatures as low as 15°F, so it's a great plant to grow in a greenhouse over winter. It also does okay in heat, so you can move it outside in spring or summer.
If you live in an area where it rarely freezes, you may be able to grow these vegetables outdoors all winter, as well as in a greenhouse. If your area gets hard freezes, you'll be able to keep them going much longer - maybe even until spring - by growing them in a greenhouse or a cold frame.
Using a Heater in Your Greenhouse
You can help keep things warmer in your winter growing shelter with a greenhouse heater, which can keep the temperature warmer than it'll stay naturally. While this won't make it possible for you to get summer plants to produce fruit during the winter in a backyard greenhouse, it will help keep your cool-season plants producing longer into the cold season, maybe even all winter (depending on your climate).
You won't need to keep heaters on all the time, but using them when the temperatures dip below freezing may help keep your frost-tender plants growing much longer. Keep in mind, though, that it costs money to operate a heater. You'll need to consider if you'll get enough boost in productivity to operate a heater in this manner.
Rather than using heaters to keep cool-season plants going all winter, you can use heaters in a greenhouse to protect frost-tender seedlings you start in early spring to plant outside after last frost. That way, you can start summer seedlings in a greenhouse as soon as it warms up, then protect them with a heater when a late cold snap comes.
Adding Grow Lights to Your Greenhouse
Grow lights can also be beneficial in a greenhouse in the winter. The short number of daylight hours during the winter causes cool season plants to grow very slowly. If you add grow lights and use them in a way that mimics spring or summer sun, they may grow faster - and produce more food - than they would without additional light. If you decide to do this, you'll need to keep the lights on between eight and 16 hours per day, every day.
What you shouldn't expect, though, is to be able to use grow lights in a greenhouse to get summer plants to produce during the winter, even if you combine them with a heater. Warm weather vegetables depend on a combination of summer soil and air temperature, as well as weather conditions and pollinators. These conditions cannot be reproduced in a backyard greenhouse.
Starting Spring Seeds in a Greenhouse
You can also start spring seeds in a greenhouse in late winter if you live in a mild climate, or in early spring if you live elsewhere - but only if your greenhouse can provide springlike growing conditions. You may even be able to overwinter dormant eggplant or pepper plants in a greenhouse until it's warm enough to move them outside for the growing season. However, you won't be able to produce warm weather crops in an ordinary greenhouse during the winter. They need way more light than is available in winter, as well as much higher air and soil temperatures in order to produce fruit.
Ready to Grow Vegetables in a Greenhouse?
Now that you know what vegetables grow well in a greenhouse, are you ready to get started? If you don't already have a greenhouse, your next step will be to decide if you are going to build your own greenhouse or purchase one, then to learn more about how to use a greenhouse. If you're not yet ready to commit to a greenhouse, consider starting with a cold frame. It'll make it possible to grow the crops listed above during winter, though on a smaller scale than in a greenhouse.