I grab my basket of vibrantly green kale, lettuce, baby spinach, and carrots and step out of my hoop house into a foot of snow. It’s January. Lows have dipped below -10 degrees Fahrenheit. And I am harvesting from my garden.
Fresh-from-the-garden vegetables seem almost magical when the world is covered in snow. But the real magic is in the flavor: when exposed to freezing temperatures, plants protect themselves by pumping out extra sugar. My family now anticipates winter carrots with as much zeal as summer heirloom tomatoes.
Here’s how to add a winter harvest to your garden.
Step 1: Look for Winter Sun.
Hoop houses as described here are most useful in garden zones 4 through 6 (find your garden zones here). Gardeners in warmer zones (7 and above) don’t need a hoop house to overwinter cold-hardy plants, although cold-sensitive plants may thrive in a winter hoop house there. In zones 3 and below, unheated hoop houses extend the season but likely won’t overwinter your plants. My garden is in zone 5; our lowest low was around -10 degrees Fahrenheit this winter and the plants in the hoop house all survived.
Observe how many hours of direct sunlight your garden gets in December: the more winter sun the better. The sun is obviously at a much lower angle in the winter than the summer, which translates to more potential sources of shade.
At our first house, the only place with anything approaching full winter sun was a rectangle between our garage and a parking lot, so we built a hoop house there. Be creative with your placement!
Step 2: Build a Sturdy Frame for your Hoop House.
I’ve built two frames to fit two very different gardens. The first was constructed over a raised bed wedged between our garage and a parking lot, which was the sunniest spot available at our previous house. The space was narrow enough (24 by 6 feet) that I used PVC pipes.
PVC pipes are easy to use. They are affordable, flexible, you can fit them together in almost any shape imaginable with connecting pieces, and you can attach plastic film using snap clamps. However, PVC pipes may be too flimsy for wider structures.
My second hoop house is more than twice as wide as the first and gets blasted with wind. I chose 1/2″ EMT conduit for the ribs, used a conduit bender to shape it, and sunk the ends into the ground.The doors (one on each side) are framed by wooden 4 x 4s sunk into the ground.
After a year, we noticed that the wind was gradually pushing one of the doors inward. We reset the 4 x 4 posts and placed a 2 x 4 to brace the tops of the two door frames against each other for extra stability.
Home improvement stores and greenhouse supply websites sell hoop house kits, simplifying the building process considerably.
Step 3: Cover your Frame with Plastic Film.
Build your frame during the summer or early fall so that you can quickly cover it when temperatures dip into the high twenties. Look for clear plastic film from a greenhouse supply company; I prefer 6 mm thick film with an expected life of 4 years.
Remove the plastic film in early summer and reattach it in mid fall: this prevents overheating in the summer and encourages beneficial insects to find your plants. I attached PVC pipes to the wooden door frame so that I can clip the plastic to the doors with snap clamps for ease of assembly each year. I’m sure there is a more elegant solution, but this was cheap and easy.
If you are constructing your own hoop house, carefully calculate the dimensions of your frame before ordering greenhouse plastic and add extra length around the base to anchor the plastic to the ground.
Attach the plastic to the ground using poly latch wire, or roll the bottom few feet of plastic around 2 x 4s and anchor the roll with heavy rocks. Last year I weighted the plastic directly with heavy rocks without rolling it in 2 x 4s first. I don’t recommend this method: the plastic was blown completely off the frame in February and I had to bury my greens under straw so that they would survive the spring. Several years ago we rolled the plastic around 2 x 2s which we screwed to 2 x 4s framing the base. I don’t recommend this method either: the plastic was quite solidly attached, but it took too much time for a twice-yearly chore, and the screws left lots of holes that weakened the plastic.
Step 4: Choose Cold-Hardy Plants for your Winter Season.
Anticipate freezing temperatures by planting frost-tolerant species. Possibilities include staples like carrots and lettuce; ‘superfoods’ like kale, arugula, beet greens, chard, mustard greens, and spinach; and Asian varieties like bok choy and mizuna.
When should you start your seeds for the winter harvest? If you start too early in the summer, the plants may ‘bolt’ (send up flower stalks) and become tough and bitter. If you start too late, your plants may not be big enough to provide a satisfying winter harvest. If you time it perfectly, your plants will be ready to harvest by the time your daytime temperatures regularly stay below freezing.
Cold temperatures suspend your plants’ growth, so the goal is to pack your hoop house with ready-to-harvest vegetables before winter really takes hold. Plant a larger area with greens than you would for a summer harvest if space allows.
Estimate the correct planting date for each vegetable variety by subtracting the ‘days to maturity’ (usually found on the seed packet) from the date that your temperatures regularly dip below freezing. Keep a journal of when you planted and harvested each plant variety so you can remember what worked and what didn’t.
With a little planning, the part of your garden that is covered with a hoop house will grow two crops per year. Inconveniently, the two crops tend to overlap in August and September when the summer garden is at its peak and the winter plants need to be started.
There are two solutions: transplanting and under-seeding. You can transplant lettuce, kale, and chard (started in pots or in other areas of your garden) into the hoop house after frost kills your summer plants. But this technique doesn’t work well for plants like spinach and arugula that tend to bolt when transplanted or for tiny seedlings like carrots. Instead, ‘under-seed’ these plants (as described in Four-Season Harvest by Eliot Coleman): spread some compost underneath upright or trellised summer vegetables and plant seeds there, working around the larger plants. Under-seeding does not work underneath sprawling plants such as squash or where an excess of rotting plant matter has attracted lots of slugs or other pests that will eat your seedlings.
Shade from taller plants likely helps my seeds germinate during the hottest part of the summer, although the same shade can later slow young seedlings’ growth until the taller plants are removed. Cut summer plants’ stems at ground level when their season is done; don’t pull them up by the roots or you’ll risk disturbing the seedlings growing around them.
Practice standard organic garden practices like crop rotation (see my post on crop rotation here) for your winter and summer plants to discourage pests and avoid depleting your soil. Replace nutrients in your hoop house soil by adding a thick layer of compost each spring and fall before you plant a new crop (see my post on compost here). Soil arthropods will mix nutrients from the compost throughout soil layers; tilling is not necessary, although loosening garden soil with a large garden fork once a year is helpful, especially before planting root crops.
Step 5: Use Row Covers to Insulate your Hoop House during the Coldest Months.
Row covers inside your hoop house are key to winter success. In Four-Season Harvest, Eliot Coleman speculates that this double-layer system warms his garden from zone 5 to 8. My garden thermometer agrees. While the hoop house traps the sun’s warmth during the day, there is not enough insulation to keep that heat from leaking out at night. But when row covers are added to this windless, humid environment, they can keep plants 10 to 15 degrees (Fahrenheit) warmer than outside temperatures through the night.
When lows drop below the mid-twenties, it’s time to add row covers inside to your hoop house to protect your plants from frost damage.
Buy spun-bonded, breathable row covers, choosing the thinnest fabric available. While it may seem counter-intuitive, thinner fabric keeps the plants warmer. It lets in more sunlight than heavier fabric and provides almost equivalent insulation after water condenses or freezes on it. I use 0.55 ounce spun-bonded row covers that transmit 85% light. Leave your plants covered day and night, moving the fabric aside only to harvest and water.
Suspend the row covers above the leaves with wire supports to avoid frost damage. Bend the wire into hoops and poke them into the ground every 4 feet along your beds. Use clothespins to hold the row covers to the wires, making sure that the row covers reach down to the soil on all sides.
Remove the row covers in the spring when lows regularly stay above 28 degrees Fahrenheit.
Step 6: Harvest Fresh Vegetables through the Winter.
Although your plants will stop growing in the darkest months of winter, they are still alive and you can continue to eat them. Cold-tolerant plants use sugar as antifreeze, and when temperatures drop, they become deliciously sweet.
Make sure that leaves are thawed when you cut them: if they are cut while frozen they will be a mushy mess. When harvesting spinach, arugula, bok choy, or similar greens, cut individual leaves from the outside of the plant, leaving the roots and inner leaves intact: when spring finally comes, overwintered greens grow like crazy even if only a few small leaves are left. Lettuce can be cut horizontally a few inches above the soil. Let the lettuce leaves grow back and harvest multiple times.
Carrots are our favorite winter crop. Dig up carrots before they start to grow again in the spring or they will be too tough to eat. If needed, carrots can be stored for months in a cold cellar or the vegetable drawer of your refrigerator. They keep best if you cut the greens an inch above the root and brush off the dirt. Avoid washing them until just before you eat them. (Yes, there is always dirt in our refrigerator.)
Periodically check that the soil is damp inside your hoop house. Your plants will probably not need water more than once every week or two during the winter.
Step 7: Jump Start your Summer Garden.
Heat-loving vegetables like tomatoes, peppers, cucumbers, and melons will love the warm conditions in your hoop house in the spring. But while sunny days get toasty under plastic, your hoop house is vulnerable to freezing temperatures that devastate tender seedlings. Consider surrounding your frost-sensitive plants with ‘walls-o-water‘ (shown in the photo below) to prevent frost damage in the early spring. Or, if you anticipate only a few freezing nights, you can temporarily cover your baby plants with row covers just before a freeze.
When your winter greens are exposed to warm temperatures they will bolt (send up flower stalks) and become tough and bitter. Remove them from your garden to make room for summer plants. If your winter greens are still providing quality harvests when it’s time to plant summer plants, cut out circles of winter greens, spread some compost, and plant your summer plants in the newly vacated spots (see photo below). Expand these circles slowly as your summer plants grow (and as you need to harvest more greens) until all the winter plants are gone.
In the fall, you’ll likely be planting at least some of your winter crops underneath your summer vegetables (see ‘Step 4’). Plan ahead and consider trellising your summer plants to keep them off the ground, or plant early-maturing summer varieties in your hoop house to avoid overlap with winter vegetables (for more on trellising, see my post here).
Once constructed, a hoop house can provide garden-fresh veggies year round and keep your winter gardening yearnings at bay.