Overwintering brassicas—or at least growing through fall and into the winter as far as you can—is a great way to prolong harvests, keep some fresh, vitamin-laden produce on the table, and give you a chance to do some gardening even after most of our other plants have gone away. Going outside for a fresh basket of greens at Thanksgiving isn't just for those in the deep South. With proper selection and a few season-extending tips, even New England gardeners can have fresh produce at turkey time and beyond. 

Many brassicas are extremely cold hardy, and can be coaxed to continue providing fresh greens and vegetables after the frosts have set in. Not all brassicas will provide a second or subsequent harvest, of course. Cauliflower, for example, makes one head and then is done. A turnip, once harvested, won't sprout another turnip. However, a continued harvest of turnip greens is possible if you are growing them for the leaves and not the root. A couple rows of kale will keep you in fresh greens for most of the winter, even in cold climates. 

Of the cruciferous vegetables, kale, brussels sprouts, and collards are the most cold hardy. They are good candidates for overwintering even in harsh climates. Cabbage and broccoli are also tolerant of frosts. Mustard greens are at the more sensitive end of the range, and can be grown into cold weather, but are harder to overwinter. 

Choose leafy brassicas for cut-and-come-again winter harvesting. They'll continue to provide picking opportunities all winter with proper care. Kale, collard, and turnips (for the greens) are easier, and mustard greens brighten up a winter meal if you protect them from the bitter cold snaps. 

General Overwintering Tips

While most brassicas are tolerant of some cold weather, that doesn't mean they enjoy living in snow. Photosynthesis slows as the temperature drops, which means growth slows, too. If you'd like to harvest more than a leaf or two from your kale or collards, a little help is needed. 

Successful overwintering and harvesting begins with healthy, vigorous plants. Pay attention to recommended fall planting times in your area—some fall crops are planted as early as the end of summer. You'll also need a way to provide water even though your hose might be freezing up. Warm sunny days will still dry the soil, and the plants need water even if temperatures drop below freezing at night. 

Don't overlook the local herbivores (besides yourself). Deer and rabbits especially will find any greens you are nursing through the winter to be a rare treat. A cold frame or low tunnel is usually sufficient to protect them, but brassicas in the regular garden need some fencing. 

Protecting Your Brassicas from the Cold

When winter comes for good, even cold hardy brassicas will do better with a bit of protection. But, you don't need a fancy heated greenhouse—far from it. Here are a few ideas to try that are simple and easy on the budget.

Make a ten-minute cold frame

A cold frame is essentially a wall, a transparent covering to allow sunlight in and keep heat inside, and some soil. Try using straw bales for an easy cold frame that you can remove in spring.

You'll need a few straw bales (likely six), an old window or two from a flea market, or the back of the shed, and that's it! If your cold frame will rest against a south-facing house wall, you'll need fewer bales.

Arrange the straw bales around your cold-hardy brassicas like a pillow fort, taking care to plug any gaps with a bit of loose straw pulled from the bale. Set the window on top. You may need to adjust the bales a bit to get a decent draft-free design. Add a thermometer and slide the window off to the side a bit if the inside gets too warm on sunny days. This simple cold frame will provide protection throughout the entire winter in mild climates, and in spring, the old straw bales can be used for garden mulch. 

Quick and Easy Low Tunnels 

A low tunnel, also called a low hoop house, is a quick season-extending solution that can be built with no experience or fancy tools. The idea is to provide a few hoops, usually made of plastic pipe, and cover them with greenhouse plastic to make a low tunnel over your garden bed. 

When shopping for PVC piping, I've found that ½" pipe is easier to bend but gets overcome by even a light snow load. The ¾" diameter PVC (inside diameter) is still easy to bend and is much more capable of withstanding a bit of wind or snow.

A piece of greenhouse plastic 8 x 25 feet should run you around thirty dollars and provide enough plastic to cover a 4-foot wide by 15-foot long bed (lots of winter greens!) with a bit left for tucking in and anchoring. PVC pipes are commonly available in 10-foot lengths. Go to the home improvement store for the best prices, and they'll be about 6 bucks a piece. 

Place a pipe at each end and one every 3–4 feet. I just push them right into the ground, but you can also drive a piece of rebar into the ground and then slip the pipe over the top for a more secure build. Bend the pipe over and push it into the soil on the other side of the bed to make an arch. A few one-inch spring clamps to hold the plastic to the PVC and a few rocks or bricks to hold the edges down are all you need for a reusable but removable low tunnel that will grow more winter greens than you likely know what to do with. I've been reusing the same plastic sheeting and pipes for years. Don’t forget to water!

Row covers

Spun row covers (frost cloth) are commonly used as frost protection in autumn, but in mild climates, they may be sufficient to protect brassica crops all winter long. They can be held off the plants by wire hoops or small PVC pipes or even allowed to rest on the plants. Tuck the edges in to prevent drafts and remove them during the day. 

Tips for Overwintering Broccoli

Broccoli heads often get damaged when the temps drop below about 25℉. The plants themselves can survive into the teens. Harvest large heads before a cold spell, and cover plants as above. After the main heads of your broccoli are harvested, they will usually put out side shoots, which form miniature broccoli heads just as tasty as the larger ones. They won't get as large, but keep harvesting them, and don't forget the leaves are edible, too.

Frost cloth will protect broccoli for short cold periods, but try a low tunnel for the deeper winter freeze. Mulching around the base of the plant helps as well. Succession planting of indoor started broccoli can continue all winter in a low tunnel or cold frame in moderate climates.

Tips for Overwintering Kale

This is one of my favorites to overwinter, mainly because it doesn't need much from me. I've had kale survive Zone 4 winters, although I'll admit it looked a bit cranky. Kale is super cold hardy and is often poking out through the snow in my garden. But, while it may survive, you'll get more tasty leaves to eat if you provide a cold frame or low tunnel and water as needed.

Keep harvesting single leaves from the outside as the plant grows, starting near the base. Either snip or snap off the leaves, taking one or two from each plant. Leave the smaller, higher leaves to grow. Kale is often sweeter in cold weather, although it may be tougher and take longer to cook.