What Actually Grows in an Unheated Greenhouse Down to 10°F: The Cut-and-Come-Again Cold List

Most “what to grow in an unheated greenhouse” lists give you the same five names — spinach, kale, lettuce, chard, brassicas — and call them “cold tolerant” without ever saying how cold. That vagueness is the problem. An unheated greenhouse isn’t one environment; it’s a range from a mild 25°F night to a brutal single-digit one, and which crops make it through depends on real numbers most articles skip. This one doesn’t. Below is a crop-by-crop cold list built from university hoop-house trials and multi-year market-farm data, the sowing-date math that gets each crop full-sized before winter growth stalls, and a cut-and-come-again harvest rotation that actually accounts for how little plants grow once the days get short.

Why “Unheated” Doesn’t Mean “Unprotected”

A bare unheated greenhouse is a poor insulator after dark. During the day, sunlight passes through the glazing and warms the air, soil, and plants inside — classic passive solar heating. But the same glazing that let heat in has almost no ability to hold it once the sun sets, especially under a clear winter sky. Colorado State University Extension puts it plainly: a greenhouse “can absorb heat during a sunny winter day,” but “it will lose heat quickly when the sun goes down,” and by the coldest part of the night the inside temperature is close to whatever it is outside [1]. In a cold, dry, clear-sky climate, that extension office notes, an uninsulated greenhouse alone may do more for early spring seed-starting than for carrying crops through a hard winter.

That’s why every crop on the list below survived because of a second layer of protection, not the glazing alone. New Mexico State University’s hoop house guide is explicit that “without supplemental heat, plants within the hoop house can still freeze if outside temperatures drop too low,” and recommends floating row cover draped directly over the beds, inside the structure, for exactly this reason [2]. Read “unheated greenhouse” in this article as “greenhouse plus a row cover you’re willing to throw on before a hard freeze,” not glass or plastic on its own.

Do the Sowing-Date Math Before You Plant Anything

The number that actually determines your harvest isn’t the greenhouse’s minimum temperature — it’s whether your plants are full-grown before the days get too short to grow at all. University of California Cooperative Extension calls this window the “Persephone period”: the stretch of the year when daylight drops to 10 hours or fewer, during which “most plants need at least 10 hours of daylight for active growth,” so growth essentially stops [3]. In much of the continental US that window runs roughly from mid-November to late January, though the exact dates shift with latitude. Whatever you haven’t grown to size by the time your area crosses that 10-hour threshold, you won’t finish growing until it crosses back — the extension office’s own guidance is to have transplants in the ground 60 to 90 days ahead of it.

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In my own zone 6 hoophouse, a bed of spinach sown in early September was still full-leafed and sweet in January. A second bed I sowed in mid-October, thinking I’d stagger the harvest, never got past baby-leaf size before growth flatlined — it simply ran out of daylight before it ran out of time. That’s the whole lesson of the Persephone period in one comparison.

To work out your own sow-by date: take the crop’s days-to-maturity from the seed packet, add roughly 10 to 14 days to cover slowing autumn light, then count backward from either your first fall frost date or the date your area drops below 10 hours of daylight — whichever comes first. NMSU’s hoop house data puts spinach at 37-45 days to maturity and leaf lettuce at 40-50 days, which is why spinach needs sowing about 6-8 weeks out and carrots — at 50-95 days — need a head start closer to 10-14 weeks [2]. Our frost date calculator will give you the first number; count backward from there.

For crops beyond the greenhouse walls, our full winter planting list covers cold-season timing for the rest of the garden.

The Cold List: What Survives an Unheated Greenhouse, and to What Temperature

The most useful cold-hardiness numbers come from paired trials — the same crops, the same structure, the same night — rather than generic per-crop averages pulled from different climates. Market farmer Pam Dawling’s multi-year data from Twin Oaks Community (zone 7a, Virginia) includes exactly that: a set of crops that rode out one documented night at 10.4°F unprotected inside a double-layer unheated hoophouse, and a colder night at -2.2°F once a floating row cover went on over the beds inside that same structure[4].

That -2.2°F figure is a documented record from one well-managed setup, not a guarantee for every greenhouse — treat it as proof of what’s possible with two layers of protection, not a baseline to plan around. A handful of other reliable cold-list crops come from separate single-crop reports rather than that same side-by-side trial, so their numbers are listed on their own:

The carrot number comes from NMSU’s hoop house guide, which lists a lethal minimum of 12°F for the root itself [2] — worth knowing before you plan an unheated greenhouse around root crops the way you would leafy ones.

What Not to Try in an Unheated Greenhouse

None of this works for warm-season crops, and it’s worth saying plainly because “greenhouse” makes people assume otherwise. Tomatoes, peppers, basil, cucumbers, and squash aren’t just less cold-tolerant than spinach or kale — they’re a different category entirely. As a general guideline, tomato fruit and foliage start suffering chilling injury somewhere in the 40s Fahrenheit, long before ice ever forms in the tissue, so an unheated structure that keeps kale alive at 10°F won’t keep a pepper plant alive at 40°F for long. If you’re set on winter tomatoes, that’s a heated-greenhouse project with grow lights, not an unheated cold-list one. Dwarf potted citrus is the one common exception worth knowing about: brought into a cool but frost-free greenhouse for winter, acclimated citrus like Meyer lemon or satsuma mandarin can generally tolerate brief dips into the mid-to-high 20s, though it still needs protection from the same hard freezes that the leafy crops on this list shrug off.

Stack Your Protection Instead of Reaching for a Heater

Since the greenhouse structure loses most of its daytime heat gain by the coldest part of the night, the fix isn’t a bigger structure — it’s a second barrier right at the plants. A single layer of medium-weight row cover fabric traps a layer of still air against the leaves, which is a meaningfully different job than the greenhouse glazing is doing: the glazing blocks wind and holds in daytime warmth, while the fabric slows the plant’s own heat loss to the cold air immediately around it. Dawling’s data puts a single row-cover layer at roughly 6°F of added protection on its own, and the combined greenhouse-plus-row-cover effect at up to 21°F warmer than bare outdoor conditions on the same night[4]. Our row cover vs. greenhouse plastic comparison breaks down which material to use where.

Before reaching for a heater instead, it’s worth knowing the economics don’t favor it for most of this list. In a Cornell-monitored trial at the Poughkeepsie Farm Project, heating a high tunnel to 40°F instead of 33°F used 2.14 times more propane, and researchers found the extra cost was justified for winter lettuce but not for spinach or kale — the added yield didn’t cover the fuel [5]. If your cold list is mostly spinach, kale, tatsoi, and mizuna, a roll of row cover fabric is doing more for less money than a propane heater would. If lettuce is the priority, that’s the one crop where the Cornell data says heat actually pays for itself. This two-layer approach — structure plus a second barrier over the crop — is the core trick behind most successful season extension in genuinely cold climates, greenhouse or not. If you haven’t built the structure yet, our greenhouse build guide covers framing and siting for exactly this kind of unheated setup.

The Cut-and-Come-Again Harvest Rotation That Gets You to February

Cut-and-come-again harvesting works because of where the cut lands, not how often you do it. Every one of these leafy crops carries its active growing point — the apical meristem — at the center of the plant, and that’s where new leaf tissue is generated [6]. Cut an inch above that crown and take only the outer leaves, and the plant still has both its growing point and enough leaf area left to keep photosynthesizing; cut into the crown itself and you’ve removed the one thing the plant regrows from.

The rotation cadence should track the Persephone period, not fight it. Before your area drops below 10 hours of daylight, a bed can often handle cutting every 7-10 days and bounce back quickly. Once you’re inside that window, expect regrowth to nearly stop — this is the point where the greenhouse becomes a cold-storage room with a pulse more than an active growing space. Treat it as a rotation of standing stock rather than a fast-regrowing crop: harvest a different bed or row each week and give each one 3-4 weeks to rest before you cut it again, rather than expecting the 7-10 day pace to hold through the darkest weeks. Once daylight climbs back past that 10-hour mark, the faster rotation returns on its own, usually with a noticeable jump in regrowth within days.

When a Bare Unheated Greenhouse Won’t Be Enough

Everything above assumes at least a greenhouse plus a row cover inside it. In a genuinely cold, dry, high-elevation climate with clear winter skies, Colorado State University Extension’s guidance is more cautious: without added protection — row cover, thermal mass like water barrels, or both — an unheated greenhouse in that kind of climate may not reliably outperform an outdoor cold frame, and might be more useful for getting a jump on spring seedlings than for carrying crops through winter [1]. Gardeners in milder, more humid winter climates — the Pacific Northwest, the mid-Atlantic, much of the Southeast — tend to get away with less added protection because their coldest nights are less extreme and their cloud cover holds in more of the day’s heat. If you’re in a continental climate with frequent sub-zero, clear-sky nights, budget for row cover and a thermal mass source from the start rather than treating them as optional extras. Thermal mass works on the same day/night cycle as the greenhouse itself, just slower: dark-colored water barrels or jugs placed inside absorb solar heat during the day and release it gradually overnight, buying you a few extra degrees right when the row cover’s insulation alone runs out. It’s not a substitute for row cover — it’s a third layer for the coldest nights, not a first line of defense.

Frequently Asked Questions

What’s the coldest temperature an unheated greenhouse can handle in winter?

With the right crops and a row cover added inside, documented survivals go down to -2.2°F for a group that includes spinach, kale, tatsoi, and mizuna[4]. Without that extra layer, the structure alone is doing far less on a clear, cold night than most gardeners expect.

Can you grow lettuce in an unheated greenhouse all winter?

Cold-hardy leaf types can survive into the single digits with protection, but treat lettuce as a standing crop to pick from during the darkest weeks rather than one that keeps growing quickly — growth largely stalls once your area drops below 10 hours of daylight.

Do you need to heat a greenhouse at all in winter?

Not for most of this list. Cornell’s trial data found the extra propane cost to heat a tunnel from 33°F to 40°F wasn’t justified by the yield gain for spinach or kale, though it did pay off for lettuce [5].

How early do I need to plant for a winter greenhouse harvest?

Six to eight weeks before frost for fast leafy greens like spinach and lettuce, and up to 10-14 weeks for slower crops like carrots — see the sowing-date table above for specifics by crop.

Can I grow tomatoes or peppers in an unheated greenhouse in winter?

No — warm-season crops like tomatoes, peppers, and basil suffer chilling injury well above freezing and won’t survive the temperatures this cold list is built around. Save the unheated greenhouse for the crops on this page and treat tomatoes as a separate, heated setup.

Key Takeaways

Pick crops from the cold list based on your actual coldest nights, not a generic “cold tolerant” label. Then work backward from your frost date or your area’s Persephone period using each crop’s real days-to-maturity, because a plant that isn’t full-sized before the dark weeks hit won’t finish growing until they’re over. Put your money into a row cover before a heater — for spinach, kale, tatsoi, and mizuna, it’s doing more of the work anyway. And once you’re deep in the Persephone period, stop expecting a fast cut-and-come-again cycle; rotate through your beds slowly and let the calendar, not the calendar you wish you had, set the pace.

Sources

[1] Ask Extension (Cooperative Extension System) — “What to grow in an unheated greenhouse in the winter?”
[2] New Mexico State University Extension — “Hoop House Vegetable Production,” Guide H-252
[3] UC Agriculture and Natural Resources, Napa Master Gardener Column — “The Persephone Period”
[4] Pam Dawling, Sustainable Market Farming — “Winter-Kill Temperatures of Cold-Hardy Vegetables” (2025 revision)
[5] Cornell Small Farms Program — “High Tunnel Research to Support Winter Production”
[6] LibreTexts Biology (Kimball) — “16.4A: Plant Growth”
[7] Long Season Farm — “Cold Hardy Greens: Claytonia, Mâche, and Minutina”