What to Grow in a Greenhouse by Season: A Temperature Calendar

Q: Can I grow tomatoes in a greenhouse year round?

Only with heat and, in most of the country, supplemental winter light. Tomatoes need 65-85F and long days to fruit well, so year-round means real running costs. Spring through fall a greenhouse grows excellent tomatoes; deep winter is where it gets expensive.

Q: How hot is too hot inside a greenhouse?

Sustained temperatures above roughly 90F stress most crops, and above 95-100F they stop growing and can suffer damage, with pollination failing in fruiting plants. Even in cool weather a closed greenhouse climbs past that fast, so ventilation and shade cloth matter as much as heating.

Updated July 2026 · Editorial team · Topic: greenhouses / planting calendar

A new greenhouse owner does one predictable thing in March: plants tomatoes, because tomatoes are what greenhouses are for. Then the tomatoes sit there, sulking, refusing to set fruit, while the spinach nobody thought to plant would have been thriving. A greenhouse does not suspend the seasons — it shifts them a few weeks and buffers the extremes. Growing well inside one is less about the structure and more about matching the crop to the temperature the glass can actually hold that month.

Short answer: Grow to the temperature, not the calendar. Cool-season crops (spinach, lettuce, kale, radish) thrive at 45–65°F and carry the greenhouse through fall, winter, and early spring. Warm-season crops (tomato, pepper, cucumber, basil) need 65–85°F and belong to late spring through summer. An unheated greenhouse mostly runs cool-season crops plus seed-starting; a heated one adds a warm-season winter, at a cost. The single most common failure is planting a warm crop below 60°F, where it stalls and rots instead of growing.

Reviewed by the BackyardStead Lab editorial team. We publish real ROI, plain numbers and USDA/extension data so you can judge for yourself — we run the math, not a farm. Educational information only: backyard-chicken and livestock rules vary by city, home canning must follow USDA/NCHFP-tested methods (botulism risk), and mushrooms should be grown only from a known-species kit — never foraged on our word.

The temperature calendar

This is the table to actually use. It maps what grows to the greenhouse temperature each season tends to hold, so you plant what the month can support rather than what you wish it could.

Season	Typical GH temp	Grow this	Role
Late winter	40–60°F	Seed-start tomatoes/peppers; spinach, lettuce, mache	Propagation + hardy greens
Spring	55–75°F	Transplant starts; radish, peas, chard, brassicas	Head start on the whole garden
Summer	75–90°F+ (needs venting/shade)	Tomatoes, peppers, cucumbers, basil, eggplant	Peak warm-season fruiting
Fall	50–70°F	Lettuce, spinach, kale, scallions, cilantro	Extend harvest past frost
Winter (unheated)	32–50°F	Cold-hardy greens: mache, kale, spinach, claytonia	Survival growing, slow
Winter (heated 55–65°F)	55–65°F	Salad greens, herbs, some tomatoes	True year-round, at a cost

Cool-season vs warm-season: the only division that matters

Nearly every greenhouse mistake traces back to blurring these two groups. Cool-season crops germinate and grow in the 40s and 50s, bolt and turn bitter in high heat, and are exactly what a greenhouse protects best in the off-season. Warm-season crops do nothing below about 55–60°F — they do not die, they simply sit, and cold, wet, stalled plants invite rot and disease. Below is the sorting that saves a season.

Cool-season (45–65°F)	Warm-season (65–85°F)
Spinach, lettuce, arugula	Tomato, pepper, eggplant
Kale, chard, mache	Cucumber, zucchini, melon
Radish, peas, scallion	Basil, okra
Cilantro, parsley	Beans (warm side)

Field note: The overlooked variable in winter is not temperature but daylight. Below roughly 10 hours of light a day — a stretch centered on the winter solstice across most of the U.S. — plant growth nearly stops regardless of how warm you keep the greenhouse. Growers call it the "Persephone period." Heating through it keeps plants alive and holds them ready, but it will not make them grow fast; expect near-standstill until the days lengthen again in late January and February.

Working from data, not our own beds

We do not harvest from a personal greenhouse each season — this planting calendar is assembled from extension planting guides and grower temperature data rather than our own rows — but the crop-temperature relationships are well established and do not depend on any single garden. A few principles hold everywhere:

Match crop to held temperature, not to the date. A warm February greenhouse in Arizona and a cold one in Minnesota grow completely different things the same week.
Succession-sow greens. Cool-season crops mature in weeks; sowing a small batch every two to three weeks keeps a steady harvest instead of one glut.
Vent hard in summer. A closed greenhouse hits plant-lethal temperatures fast; shade cloth and open vents turn July from a hazard into a harvest.
Pollinate warm crops by hand under cover. Enclosed tomatoes and cucumbers may need a tap or a brush to set fruit, since the wind and bees that do it outdoors are absent.

If holding a warm winter temperature is your goal, the running costs and cheaper alternatives are in how to heat a greenhouse in winter.

Common mistakes

Planting warm crops too early. Tomatoes set into a 50°F greenhouse in March stall for weeks. Wait for a reliable 60°F+ or start them for transplant, not for fruit.
Ignoring winter daylight. Warmth without light will not grow much near the solstice. Plan for holding, not harvesting, in the darkest six weeks.
Letting summer cook everything. The same glass that saves March seedlings roasts July tomatoes. Shade and ventilation are not optional in the warm months.
Growing one big planting. A single sowing of lettuce all matures at once. Succession-sow small amounts for a continuous supply.
Forgetting hand pollination. Fruiting crops under cover often need help setting fruit, or you get flowers and no harvest.

FAQ

What can I grow in an unheated greenhouse in winter?

Cold-hardy greens: spinach, kale, mache, claytonia, arugula, and scallions all tolerate near-freezing nights and keep producing slowly. Growth is minimal in the darkest weeks around the solstice, but the crops hold and resume as daylight returns, giving you fresh salad through winter with no heating bill in mild and moderate climates.

What is the best crop for a beginner greenhouse?

Leafy greens and salad crops. Lettuce and spinach germinate readily, tolerate a wide temperature range, mature in weeks, and forgive mistakes, which makes them ideal for learning how your particular greenhouse behaves before you risk a season on fussy warm-season fruit.

Can I grow tomatoes in a greenhouse year round?

Only with heat and, in most of the country, supplemental light through winter. Tomatoes need 65–85°F and long days to fruit well, so a year-round tomato greenhouse means real running costs. Through spring, summer, and fall a greenhouse grows excellent tomatoes; deep winter is where it gets expensive.

How hot is too hot inside a greenhouse?

Sustained temperatures above roughly 90°F stress most crops, and above 95–100°F they stop growing and can suffer damage, with pollination failing in fruiting plants. On sunny days even in cool weather a closed greenhouse can climb past that quickly, which is why ventilation and shade cloth matter as much as heating does.

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General educational information, not professional horticultural advice. Prices, energy costs, plant hardiness and local climate vary by region and season; check figures against current listings and your local extension office before spending.