In most regions you can start using an unheated greenhouse about 4 to 6 weeks before your last spring frost, and you can grow cold-hardy crops right through winter in USDA zone 6 and up. An unheated structure trades climate control for timing and plant selection. Pick the right crops, watch your overnight lows, and the season stretches at both ends.
TL;DR: Begin using an unheated greenhouse 4 to 6 weeks before your average last spring frost to start seedlings, and grow cold-tolerant greens and roots through winter in USDA zone 6 and warmer. Keep nights above 45 to 55F, ventilate sunny days, and protect plants when lows fall under 20F.
Start in late winter to early spring, roughly 4 to 6 weeks before your average last frost date. If your last frost lands near May 1, begin in mid to late March. The USDA Plant Hardiness Zone Map sets your baseline, since it represents the average annual extreme minimum winter temperature over a 30-year period.
A greenhouse buys you a head start at the front of the season and a buffer at the back. In those late winter weeks you can start warm-season seedlings like tomatoes, peppers, and marigolds for transplanting once frost danger passes, and you can sow cold-tolerant spinach, lettuce, peas, and brassicas directly to harvest weeks earlier than an outdoor bed allows.
In USDA Hardiness Zones 6 and up, an unheated greenhouse gives cold-tolerant vegetables enough protection to keep producing through winter with little or no added heat. University of Georgia Extension notes that many protective structures “do not require supplemental heat,” and that simple covers add roughly 8F, often the margin between a crop surviving and failing.
Even in zones 4 and 5, the structure pushes the green season into late fall and early winter. Full-season growing stalls without heat, but you can still:
Set the goal as safely overwintering plants, not pushing them to grow fast. Growth slows in cold, and that is fine. A spell of cold actually helps many vegetables and perennials, nudging them into dormancy so they bank energy for spring. Do not expect lush fruiting from warm-season annuals in the cold months. Lean into cold-tolerant greens and root crops that handle cooler conditions, and you will get a steady, if slower, supply.
Keep the interior no lower than 45 to 55F (7 to 12C) on cold nights and days, since prolonged exposure below that range damages or kills many plants. If outside lows are forecast under 20F (-7C), you will need a little added heat to hold the line. Daytime passive solar gain does much of the warming work on its own.
Hardiness zones tell you what your greenhouse is up against. USDA’s Plant Hardiness Zone Map is “the standard by which gardeners and growers can determine which plants are most likely to thrive at a location,” and it is built on the average annual extreme minimum winter temperature, divided into 10F zones (source: USDA ARS, planthardiness.ars.usda.gov).
As a rule, hold the interior at 45 to 55F (7 to 12C) or warmer on cold nights. Below that threshold for long stretches, cold injury sets in. A minimum-reading thermometer placed at plant level shows you the actual overnight low your plants felt, so you know when supplemental heat is genuinely needed rather than guessing from the daytime reading.
During daylight, most unheated greenhouse plants want 55 to 75F (13 to 24C), with some variation by crop:
Ventilation matters here. On a sunny winter day the interior can spike fast, so open vents and doors to keep the range from overshooting and cooking your plants.
Ideally the interior stays above 45 to 50F (7 to 10C) overnight to prevent cold damage. Some semi-hardy plants tolerate lower nights if the days run warm enough, but most vegetables, annuals, and tropicals suffer once nights drop below 40F (4C). To hold heat after dark, you can reference our guide to winterizing a greenhouse for sealing and insulation steps that protect plants on the coldest nights.
A surprising range of plants handle cool conditions when chosen and cared for well. University of Georgia Extension confirms that protective structures let growers “increase the yield of hardy crops and grow a few additional plants that would not otherwise survive the winter,” and many of those structures need no supplemental heat at all.
The easiest winter crops include:
Many culinary herbs hold through the cold months in an unheated greenhouse:
Get a jump on the outdoor planting season by starting seeds as early as late winter, depending on your climate:
Many semi-hardy potted perennials like fuchsias, geraniums, and lemon trees overwinter well in an unheated greenhouse. It saves you the chore of hauling them indoors and back out each spring, and they wake up ready to grow once light and warmth return.
Three habits carry plants through winter without a heater: steady ventilation, restrained watering, and frost protection on hard-freeze nights. Get those right and the structure does the rest of the work. Plants in a sheltered, unheated greenhouse need far less day-to-day intervention than most gardeners expect, as long as you respond to the forecast. In the coldest zones, a thicker-walled model from our roundup of the best greenhouses for cold climates makes that easier.
Ventilation is critical in a greenhouse without heat, since it clears the excess heat and humidity that build up on sunny days and breed disease.
Plants drink less in the cold, so check soil moisture often and avoid overwatering, which invites fungal disease.
When a hard freeze threatens, add a layer:
Three levers raise output without a heater: cold-hardy varieties, passive solar capture, and insulation. None of them runs up an energy bill, and together they widen the range of what survives. The aim is steady survival and slow growth, not a forced harvest, so stack these passive methods before you reach for any heat source.
Pick vegetables and flowers bred for cool weather:
Passive solar gain can lift daytime temperatures sharply.
Stop conductive heat loss through the frame and foundation after dark:
Yes. An unheated greenhouse is well suited to starting hardy greens, root vegetables, and cool-season crops, as long as it holds enough warmth on cold nights. Start cold-tolerant seedlings 4 to 6 weeks before your last frost, and protect them when overnight lows threaten.
Most vegetables, annuals, and tropicals begin to suffer once interior nights drop below 40F (4C), and prolonged exposure under 45F damages many plants. Cold-hardy greens and root crops tolerate brief dips lower, especially if the days run warm enough to recover.
In USDA zone 6 and warmer, an unheated greenhouse keeps cold-tolerant crops producing through winter with little or no added heat. In zones 4 and 5 it extends the season into late fall and early winter and shelters overwintering plants, though full-season growing stalls without supplemental heat.
When outside lows are forecast below 20F (-7C), an unheated greenhouse usually needs a small added heat source to keep the interior above the 45 to 55F safe range. Below that point, passive solar gain alone rarely keeps tender plants out of danger overnight.
Yes. Even in winter, sunny days can spike interior temperatures and humidity, which invites fungal disease. Open vents or doors on mild days above 50F to move air, then close everything before nightfall to conserve the warmth the structure gathered during the day.
You can start tomato seedlings 8 to 10 weeks before last frost, but you should not expect heavy fruiting in the cold months. Tomatoes are warm-season plants, so move transplants outdoors or into a heated space once they are established and frost danger has passed.
Ready to extend your season at both ends? The right structure does most of the work, holding daytime solar warmth and sheltering crops long after the outdoor garden gives up. Browse our greenhouse kits for sale to find a build matched to your climate and growing goals.
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