Every vegetable has a temperature range in which it grows well, and outside that range it sulks, bolts, or dies. Tomatoes sulk in cold soil and stop setting fruit in extreme heat. Lettuce bolts the moment summer arrives. Peas thrive in cool spring weather and collapse in a heatwave. The single most useful skill in vegetable gardening is knowing which crops are cool-season and which are warm-season, and timing your planting so each crop gets the weather it wants.
Cool-season vegetables grow best in cool weather and tolerate, or even need, a light frost. Warm-season vegetables need warm soil and warm air, and they are killed or badly checked by frost. Plant a cool-season crop in summer and it will bolt or turn bitter; plant a warm-season crop too early and a single cold night can set it back weeks or kill it outright. This guide explains the two groups, how soil temperature matters as much as air temperature, and how to use frost dates, covers and succession sowing to keep the beds full all year. For your local frost dates, use our frost date calculator and check your local extension service, which publishes planting dates for your area.
Cool-season vegetables
Cool-season vegetables are the spring and autumn crops. Lettuce, spinach, kale, cabbage, broccoli, cauliflower, peas, carrots, beets, radishes, onions, leeks and garlic all grow best when daytime temperatures are roughly 15 to 20 degrees Celsius, and most of them tolerate a light frost once established. Some, such as kale, Brussels sprouts and leeks, actually taste sweeter after a frost, because the cold triggers the plant to store sugars in its leaves. Sow them in early spring as soon as the soil can be worked, and again in late summer for an autumn harvest.
Cool-season crops have their limits too. A hard freeze will damage even hardy crops, and a hot spell will make lettuce and spinach bolt — send up a flower stalk and turn bitter — almost overnight. In warm climates, cool-season vegetables are winter crops, grown from autumn to spring, and summer is the fallow season for them. If your spring warms up fast, sow cool-season crops early and choose bolt-resistant varieties, and give lettuce and spinach a little afternoon shade to buy them a few extra weeks. See zone 5 lettuce and zone 3 lettuce for timing in colder zones.
Warm-season vegetables
Warm-season vegetables are the summer crops, and they are the ones most gardeners look forward to. Tomatoes, bell peppers, cucumbers, eggplant, zucchini, okra, green beans and sweet potatoes all need warm soil and warm nights, and they are killed by frost. They grow best when daytime temperatures are roughly 20 to 30 degrees Celsius, and they sulk badly below about 10 degrees at night — a tomato plant set out in cold soil will sit still for weeks, and may never catch up with one planted two weeks later into warm soil. Wait until the soil is warm and all danger of frost has passed before planting them out.
Warm-season crops also have an upper limit. When daytime temperatures stay very high, tomatoes and peppers drop their flowers and stop setting fruit, and cucumbers and zucchini wilt in the afternoon heat even in moist soil. In hot climates, give warm-season crops a little afternoon shade, mulch thickly to keep the roots cool, and water deeply in the morning. Planting a second round of beans and cucumbers in midsummer, timed to fruit as the worst heat eases, often gives a better late harvest than the first sowing. For timing, see zone 7 tomatoes.
Soil temperature matters as much as air temperature
Gardeners watch the air temperature, but seeds care about the soil. A seed sown into cold soil may sit for weeks without germinating, and in wet, cold soil it may rot before it ever sprouts. Peas, spinach, lettuce and radishes will germinate in cool soil, which is why they are the first crops to sow in spring. Beans, cucumbers, zucchini and sweetcorn need warm soil — sown too early, bean seed often rots in the ground, and the gardener blames the seed when the real problem was the soil. A simple soil thermometer, pushed a few centimetres into the bed in the morning, tells you more than a calendar date.
Warm the soil before planting warm-season crops by covering the bed with black plastic or a cloche for a week or two beforehand, which can raise the soil temperature noticeably. Cold frames and cloches do the same job for individual plants. Conversely, cool the soil for late-sown cool-season crops by mulching with straw and watering well, and by sowing into a bed that gets afternoon shade. If you are unsure when your soil is warm enough for beans or tomatoes, check your local extension service — they publish soil-temperature guidance for your area, and it is more reliable than a date on a seed packet written for a different climate.
Frost: light, hard, and how to protect plants
Not all frosts are equal. A light frost, just below freezing for a short time, will damage tender crops such as tomatoes, peppers, cucumbers and zucchini, but hardy crops such as kale, cabbage, leeks and Brussels sprouts will shrug it off. A hard frost, several degrees below freezing for hours, will kill tender crops outright and damage even semi-hardy ones such as lettuce and broccoli. Know your average last frost date in spring and first frost date in autumn — our frost date calculator gives them for your location — and treat them as averages, not guarantees: frost can come a week or two either side.
Protection is straightforward. Cover tender plants with fleece, old sheets, or a cloche on nights when frost is forecast, and remove the cover in the morning so the plants do not overheat. Water the soil before a frosty night — moist soil holds heat better than dry soil. Plant tender crops in the warmest spot, such as against a south-facing wall, where the wall releases stored heat overnight. And keep a few spare plants in pots under cover, so a late frost that kills the planted-out crop is a setback rather than a disaster. Our guide to damping-off seedlings covers the cold, damp conditions that also trouble young plants in spring.
Heat stress and bolting
Heat is the other end of the problem. When temperatures climb, cool-season crops bolt: lettuce, spinach, radishes and cilantro send up a flower stalk, the leaves turn bitter, and the crop is over. There is no way to un-bolt a plant, so prevention is everything: sow early, choose bolt-resistant varieties, give afternoon shade, mulch to keep the soil cool, and water consistently so the soil never dries out completely. Succession sowing — a small sowing every two weeks — means that even if one sowing bolts, the next is coming along behind it.
Warm-season crops suffer differently in heat: they wilt, drop flowers, and stop setting fruit. Tomatoes are the classic example — in very hot weather the flowers fall without setting, and fruit that does set may develop sunscald on the exposed side. Shade cloth, mulch, and deep morning watering help, and so does choosing varieties bred for heat tolerance. Do not feed a heat-stressed plant in the hope of reviving it — fertiliser on a stressed plant can burn the roots. Wait for cooler weather, keep the water up, and the plant will usually recover. Our fertilizing vegetables guide explains when feeding helps and when it hurts.
Timing your planting through the year
The practical answer to temperature is a planting calendar built around your frost dates. In early spring, sow the hardy cool-season crops — peas, spinach, lettuce, radishes, onions — as soon as the soil can be worked. A few weeks before the last frost, start the warm-season crops indoors: tomatoes, peppers, eggplant and cucumbers. After the last frost, when the soil is warm, plant them out. Through summer, sow more beans, cucumbers and zucchini for succession, and in late summer sow the autumn cool-season crops — lettuce, spinach, kale, radishes and garlic — so they mature in the cooling weather of autumn.
In mild climates with no hard frost, the calendar shifts: cool-season crops become the winter garden, and warm-season crops grow nearly all year. In cold climates, the season is short and every week counts, so starting warm-season crops indoors is not optional. Whatever your climate, write down your frost dates, your sowing dates and what worked each year — after two or three seasons you will have a calendar tuned to your own garden, which is worth more than any general guide. Check your local extension service for the planting dates they recommend for your area, and adjust from there.
Common mistakes
The most common mistake is planting warm-season crops too early. A tomato set out into cold soil a month before the last frost will not grow faster — it will sit, sulk, and may be killed by a single frosty night, while a plant set out two weeks later into warm soil overtakes it within a month. The second mistake is sowing cool-season crops too late in spring, so they bolt before they are worth picking. The third is ignoring soil temperature and blaming the seed when beans rot in cold, wet soil. The fourth is leaving covers on through a hot day, so plants cook under fleece or a cloche. And the fifth is treating frost dates as fixed — they are averages, and a late frost is always possible, so keep covers and spare plants handy until the risk has truly passed.
| Group | Examples | Best growing weather | Frost tolerance |
|---|---|---|---|
| Hardy cool-season | Kale, leeks, Brussels sprouts, parsnips | Cool, 15–20°C | Tolerate light frost; sweeter after frost |
| Cool-season | Lettuce, spinach, peas, carrots, broccoli, cabbage | Cool, 15–20°C | Light frost once established; bolt in heat |
| Warm-season | Tomatoes, peppers, cucumbers, zucchini, beans | Warm, 20–30°C | Killed by frost; sulk in cold soil |
| Heat-loving | Okra, sweet potatoes, eggplant | Hot, 25–30°C | Killed by frost; need the warmest spot |
Frequently asked questions
What vegetables can survive frost?
Hardy cool-season vegetables such as kale, leeks, Brussels sprouts, cabbage and parsnips tolerate light frost, and kale and leeks often taste sweeter after a frost. Tender warm-season crops such as tomatoes, peppers, cucumbers and zucchini are damaged or killed by even a light frost.
When should I plant tomatoes outside?
Plant tomatoes out only after all danger of frost has passed and the soil is warm — they sulk badly in cold soil and may never catch up. In most temperate areas that means late spring; start them indoors a few weeks earlier and harden them off before planting out.
Why did my lettuce bolt?
Lettuce bolts when it gets hot — it sends up a flower stalk and the leaves turn bitter. Sow early, choose bolt-resistant varieties, give afternoon shade, mulch to keep the soil cool, and sow a little every two weeks so there is always a fresh sowing coming along.
Can I grow vegetables in winter?
In mild climates, yes — cool-season crops such as kale, leeks, spinach and winter lettuce grow through winter, and garlic is planted in autumn for a summer harvest. In cold climates, a cold frame or unheated greenhouse extends the season, but most vegetables need protection from hard frost.
What is the best temperature for growing vegetables?
It depends on the crop. Cool-season vegetables grow best at roughly 15 to 20 degrees Celsius, and warm-season vegetables at roughly 20 to 30 degrees. Match the crop to the season and check your local extension service for planting dates for your area.