What to Grow in Greenhouse in Winter | Cold-Hardy Crops That Keep Producing

A winter greenhouse in USDA Zones 5-7 can keep producing leafy greens, root vegetables, alliums, brassicas, and herbs that tolerate air temperatures down to 28°F and soil temperatures down to 32°F without supplemental heat.

Most gardeners shutter their greenhouse in November, but the right crops keep it running through January and February. The trick is matching the plant to the environment: an unheated structure with short days and cold soil. Below are the specific varieties that thrive there, the planting schedule that works, and the winter maintenance habits that prevent rot, mold, and failure.

Which Crops Survive an Unheated Winter Greenhouse?

Five crop categories reliably produce through winter in Zones 5-7: leafy greens, root vegetables, alliums, brassicas, and several hardy herbs. Each tolerates temperatures that would kill a tomato or pepper plant outright.

Leafy Greens

Kale, spinach, Swiss chard, arugula, collards, and mustard greens all handle air temperatures down to 28°F. They need a full-sun spot (six-plus hours daily) and soil pH between 6.0 and 7.0. Water them about once a week in winter — far less than summer’s schedule — because the plants are barely growing and soggy soil invites rot.

Root Vegetables

Carrots, beets, radishes, turnips, parsnips, and rutabagas are natural winter survivors. They need full sun and the same soil pH range. One critical timing rule: don’t plant carrots or beets in the fall in short-day regions like Zone 6b — the low light prevents them from forming a proper root. Instead, plant them in late winter for a harvest two months ahead of your outdoor garden.

Alliums

Garlic, onions, leeks, and scallions sail through winter cold. They’re full-sun crops with the same 6.0–7.0 pH preference. Leeks are especially forgiving; they stand up to repeated frosts and keep growing slowly whenever the greenhouse warms above freezing.

Brassicas

Brussels sprouts, broccoli, cabbage, kohlrabi, cauliflower, and Asian greens all work. Cold weather actually sweetens them — the plants convert starches to sugars as a natural antifreeze. Head-forming types like cabbage and broccoli develop better when grown for late-winter harvest than for fall harvest.

Herbs

Parsley, cilantro, dill, chives, rosemary, thyme, and oregano tolerate winter conditions in an unheated greenhouse. Rosemary is a tender perennial in Zone 6b and might not survive in every microclimate, so test it in your setup. Parsley is more dependable and keeps producing leaves all winter if you pick them one at a time instead of chopping the whole plant.

What’s the Right Planting Schedule for a Winter Greenhouse?

The ideal planting window runs from mid-summer to mid-autumn. The crop must reach maturity before day length drops below 10 hours. Count backward from your first frost date using the days-to-maturity number on the seed packet: a 58-day crop goes in the ground roughly 58 days before frost. For spinach, start transplants with successive sowings so you have young plants ready to move into the greenhouse as space opens up.

Table: Best Winter Greenhouse Crops at a Glance

Winter Greenhouse Maintenance That Makes or Breaks the Harvest

Keeping an unheated greenhouse productive takes different habits than summer growing. The big shift is water: cut back to once or twice a week in fall, then water only occasionally on warm sunny winter days. The plant’s vascular system runs slow, and wet soil plus cold temperatures equals root rot. If you’re building a new winter greenhouse from scratch, our tested DIY greenhouse designs show insulated setups that hold heat better through January nights.

Vent the greenhouse on mild days — even 40°F is warm enough to open a vent for an hour. This cuts condensation, which is the main cause of fungal diseases in winter. Insulate the walls with bubble wrap attached to the plastic sheeting. A 55-gallon water barrel painted black and placed inside acts as thermal mass, absorbing heat during the day and releasing it at night.

Use frost blankets or low-tunnel hoops over individual crops for extra protection. Perennial herbs like rosemary may still fail in Zone 6b winters — if you lose one, treat it as a test that tells you to switch to a hardier variety next year.

Table: Watering and Temperature Ranges for Winter Greenhouse Crops

Five Common Mistakes That Wreck a Winter Greenhouse Harvest

An unheated greenhouse is forgiving, but these errors are the ones that cause the most losses.

Planting root crops in fall in short-day regions. Carrots and beets need long daylight to form roots. In Zone 6b and similar zones, planting them in late winter gives you a harvest two months early instead of a tray of leafy tops with nothing underground.

Harvesting frozen greens. Picking kale or spinach when the leaves are frozen damages the plant’s cells and often kills it. Wait until the leaves thaw naturally — usually by late morning on a sunny day.

Overwatering. Daily watering works in July. In December it drowns the roots. Cut back to an occasional deep soak when the soil feels dry at a knuckle’s depth.

Ignoring condensation. A sealed greenhouse looks efficient, but humidity trapped inside breeds gray mold and powdery mildew. Venting for even 10 minutes on a 40°F day drops the humidity enough to stay safe.

Choosing the wrong varieties. Basil, beans, sunflowers, and tomatoes have no place in a winter greenhouse. Stick with cool-tolerant crops — kale, arugula, spinach, and the others on the table above.

Winter Harvest Checklist: What to Do Now

Start succession planting in early summer, late summer, and fall so crops are near maturity when the cold hits. Hang yellow sticky traps near the vents to catch early pest warnings. Keep a frost blanket or low-tunnel kit on hand for the coldest nights. Harvest leafy greens by removing individual leaves from the outside of the plant rather than taking whole heads — this keeps the plant producing for months. If you’re still building your setup, aim for insulation first (bubble wrap and thermal mass) and worry about supplemental lighting only if you plan to start seeds indoors.