What Can I Grow in a Cold Frame in the Winter?
You probably know that a cold frame can help you extend the growing season into the winter, but do you know what plants will thrive in this environment? In this article, vegetable farmer Briana Yablonski covers what plants you can successfully grow in a winter cold frame.
If you’re looking to extend your growing season into the winter, a cold frame is a great tool to have on hand. It captures the sun’s energy, creating a warm environment and protecting plants from the harsh weather outdoors.
Sometimes, people think that you can grow warm-weather crops like basil. Unfortunately, that’s not the case in most areas. The best crops can tolerate a bit of cold, so hardy greens and roots are good options.
I will cover the factors that impact what you can grow in a winter cold frame so you can plant crops that work for you.
The Short Answer
Your hardiness zone and the size and location of the cold frame both impact the crops you can grow during winter. Generally, cold-hardy vegetables like spinach, tatsoi, carrots, beets, and lettuce will grow well. You shouldn’t grow frost-sensitive crops like tomatoes, cucumbers, and basil.
The Long Answer
I’ll explain the factors that impact what crops you can grow and then dive into some specific crops that will work in your area.
Consider Your Hardiness Zone
While a cold frame helps extend your growing season, it won’t turn a frigid New England garden into a tropical oasis. That means considering your USDA hardiness zone before deciding when to plant.
The lower your hardiness zone, the colder the minimum winter temperature. Since I live in zone 7b, I can expect a minimum winter temperature between 5–10°F. Remember, this is the lowest average temperature I can expect, so most nights remain warmer than this, and there may still be a few nights that get a bit lower!
If you live in zone 5b, temperatures will likely dip below -10°F at least once. And if you live in zone 9b, you may only experience below-freezing temperatures a handful of times.
This range of low temperatures explains why there isn’t a one-size-fits-all answer for what you can grow. Although cold frames increase air and soil temperature, they won’t cause drastic changes in outdoor environments. That means gardeners in warmer regions can grow a wider range of crops than gardeners in colder regions.
The Impact of Cold Frames
Cold frames work by trapping heat from the sun. If you think about how sealed cars become warm on sunny, cold days, you’ll understand how they function.
As heat is trapped during the day, the air and soil temperatures increase. But these temperatures drop once night arrives, and the air inside is typically only 5–10°F warmer than the outside temperature. That means if it’s 0°F outside, it will still be quite cold inside the frame!
The location also impacts its effect. Those in full sun will be warmer than those in partial shade. And since south-facing or west-facing brick walls trap heat, warmth increases when situated against one of these walls.
Covering your plants with a floating row cover can add an additional layer of cold protection, but it will only increase the air temperature by a few degrees. Covering the outside of the cold frame with blankets or towels will provide a similar effect.
Remember to Keep Planting Dates in Mind
When people ask me when I plant fall crops like cauliflower, kale, and carrots, they’re often shocked to hear I get these crops in the ground in early August. It may sound obvious, but you can’t plant crops in late September if you want to harvest them in the middle of October! The same logic applies to winter crops.
Just because plants will survive cold temperatures doesn’t mean they will grow rapidly during shorter days. Crops slow their growth as days grow shorter, and plant growth almost ceases when days dip below ten hours.
If you hope to harvest crops in December, January, and February, plant them in September or October. By the time your last ten-hour day arrives, the plants should be about 90% mature.
Crops for Extreme Cold
If you live in an area that regularly dips below 10°F, you’ll want to choose the most cold-tolerant crops. The following plants will survive supremely cold temperatures, but it’s always a good idea to cover them with a layer of row cover during cold nights.
Although these plants will survive cold, they happily grow in cold frames that reach 50°F or 60°F during the day. Just remember to vent your frame on warm, sunny days!
An Asian green similar to bok choy, tatsoi has thin stems topped with dark green leaves. You can plant tatsoi for baby greens, but I find that growing individual heads allows for better airflow. Varieties like “Rosette Tatsoi” can survive extremely cold temperatures.
Spinach can not only survive cold temperatures, but it becomes sweeter once frost arrives. Although short winter days will slow the plant’s growth, you can continue to harvest spinach throughout the winter.
Pick off larger outer greens and keep smaller inner leaves to grow. While all spinach is cold-hardy, ‘Bloomsdale’ is an excellent choice for cold climates.
If one veggie wins the title of winter king, it’s claytonia. Also known as miner’s lettuce, this succulent green can withstand temperatures below 0°F if it’s grown in a protected structure like a cold frame.
Most types of kale will survive temperatures down to 15°F. This applies to both mature plants and baby kale. ‘Dwarf Blue Curled Kale’ is especially cold-hardy. Like with all greens, let frozen leaves thaw before harvesting. Keep an eye out for aphids since they often flourish in protected structures during the winter.
The nutty cousin of the carrot, the parsnip is a cold-weather warrior. While the roots won’t grow much throughout the winter, they’ll remain cozy in the soil. Since parsnips like ‘All American’ take about three months to mature, plant the seeds in August for a winter harvest.
While all cabbage varieties can survive a light frost, varieties like ‘January King’ can tolerate temperatures near 0°F! Cabbages like ‘Copenhagen Market’ and ‘Red Acre’ can easily survive temperatures down to 20°F.
That means these cabbages are a great choice for cold frames in frigid areas. However, since cabbages take up so much space, I like to plant them outside if I can get away with it.
Crops for Moderate Cold
If you live in an area where winter temperatures often dip below freezing but rarely below 10°F, you can grow a wider variety of crops. Try out the following plants.
Most types of bok choy will survive a light frost, and some varieties will tolerate temperatures as low as 15°F. Outer leaves can become damaged by heavier frosts, but you can peel off the damaged leaves and enjoy the tender interior.
Both head lettuce and baby lettuce can survive below-freezing temperatures. When they’re tucked into a cold frame and under a few layers of row cover, they remain unscathed when the outdoor temperature drops below 15°F.
While you may think of cilantro as a summer crop due to its presence in salsa, this herb thrives in cool weather. It actually grows better in fall than summer, and it can survive temperatures below 15°F as long as it’s covered. Large outer leaves may die, but tender inner leaves will continue to grow after the cold spell.
Also known as rocket, arugula is one of my favorite greens to grow in winter. The cold weather causes the green to lose some of its sharpness, but it still remains delightfully spicy.
Since arugula is prone to develop fungal diseases in moist conditions, I keep the soil relatively dry and space the plants about an inch apart. While ‘Astro’ arugula is often my go-to option for arugula, I turn to the wild and cold-hardy ‘Rocky’ arugula in the winter.
Radishes can survive below-freezing temperatures, but the roots will become pithy and undesirable if they frequently freeze and thaw. And trust me, biting into a soft radish doesn’t provide the same satisfaction as chomping into a crisp root.
Since a cold frame traps heat, the soil stays warm and prevents frequent freezing. If you’re expecting a cold, cloudy period, you can also protect your radishes with a few layers of row cover. Large radishes like ‘Mantanghong Watermelon’ are especially good choices.
In case you haven’t heard, winter carrots are the best you’ll find. That’s because cold temperatures cause the plants to concentrate sugars in their roots, leading to a pleasantly sweet taste.
Although some carrot greens may die when temperatures dip below 15°F, the roots will persist. Just make sure to harvest your carrots before the ground fully freezes.
Also known as green onions, scallions are my favorite allium to grow in winter. The pungent whites and greens remain fresh throughout the winter.
While all types of scallions are frost-tolerant, ‘Evergreen Hardy’ is the most cold-tolerant variety I’ve found. I’ve grown it outdoors under row cover, and it has survived temperatures as low as 10°F.
Both salad turnips, like ‘Market Express’ and cooking turnips, like ‘Purple Top White Globe,’ grow well in winter cold frames. As long as the outdoor temperature remains above 15°F, your turnips will remain happy tucked inside the shelter of the frame.
Some leaves may die during a cold spell, but the roots will remain unaffected. You should only worry about the roots if it remains below 20°F for a few days.
Both flat-leaf and curly-leaf parsley can survive temperatures down to 20°F. When you add the protection of a cold frame (and a few layers of row cover), this herb can easily survive when outdoor temperatures drop below 15°F. The older outer leaves may die during periods below 10°F, but the new growth will survive and keep growing.
Crops for Warmer Areas
If you live where the temperature rarely drops below 25°F, you can successfully grow the following crops. Although these plants prefer above-freezing temperatures, they can withstand light frosts, especially if protected by a cold frame.
Red, golden, and Chioggia beets are all excellent candidates. They can easily survive temperatures in the mid to upper 20s, but they will require a layer or two of frost cloth if you want the roots and greens to survive below 20°F. Remember that vegetables won’t grow much during short winter days, so aim for mature beets by Thanksgiving.
The cold-hardiness of Swiss chard varies depending on the variety, but most types of Swiss chard tolerate less cold than greens like spinach and kale. These greens will survive below-freezing temperatures, but they’ll appreciate a blanket of row cover if hard freezes are in the forecast.
I love the sweet anise flavor of fennel, so it’s a must-have in the winter. Fennel can survive temperatures down to 25°F, but not much colder than that.
Another thing to keep in mind is fennel’s height. The feathery leaves can easily reach three feet tall, so make sure your cold frame is tall enough to accommodate them.
Napa cabbage is less cold-tolerant than green and red cabbage, so it’s better suited for areas where winter temperatures remain above 20°F. If your Napa becomes damaged by a freeze, peel off the damaged outer leaves until you find the undamaged heart.
With a cold frame and the knowledge to go with it, you’re ready to grow well into the winter! Remember that your hardiness zone impacts what winter crops you can grow, but even the coldest areas can grow through winter with the help of this method.