How to Know When to Plant
Soil temperature — not the calendar — is what tells you when to plant. Seed sown below its germination minimum sits and rots; the same seed in warm soil is up in days. Planting calendars are written for one place, so move a few hundred kilometres, change hemisphere or garden at altitude and the dates stop meaning anything. The three conditions a seed actually responds to — soil temperature, frost and day length — behave the same way everywhere. Learn to read them and you can plant correctly in any climate.
The method: observe, measure, decide
- Observe. Watch what your own garden does, not what a calendar says. When does standing water disappear in spring? Where does frost linger longest? Which corner warms first? A season of notes beats any regional average.
- Measure. Two instruments settle almost every timing question: a soil thermometer pushed 5-10cm (2-4in) into the bed at the same time each morning, and your own records of first and last frost. Soil temperature is the number that actually governs germination.
- Decide. Match the measurement to the crop's temperature class, then count backwards from your frost date using days-to-maturity. If the arithmetic does not fit your season, change the cultivar or the crop — not the date.
Step one: know your crop's temperature class
Every vegetable falls into one of four classes. The class tells you what soil temperature to wait for and how much frost the crop tolerates.
| Temperature class | What it means | How to time it |
|---|---|---|
| Hardy cool-season | Survives frost; germinates in soil from about 4-5C (40F). Kale, spinach, garlic, broad beans. | Sow as soon as soil is workable and above its germination minimum. Frost improves flavour in several of these. |
| Half-hardy cool-season | Tolerates light frost; prefers soil 7-15C (45-60F). Lettuce, carrot, beet, radish, pea, most brassicas. | Sow from a few weeks before your last frost. Bolts once sustained heat arrives, so the second sowing window after peak heat is often the better one. |
| Tender warm-season | Killed by frost; needs soil above 15-16C (60F). Tomato, pepper, eggplant, bean, cucumber, squash, sweet corn. | Transplant only after your last frost has passed AND night temperatures hold above about 10-13C (50-55F). Planting into cold soil stunts these permanently. |
| Heat-loving | Needs soil above 18-21C (65-70F) and long warmth. Sweet potato, okra, melon, some peppers. | Wait for settled warmth. These gain nothing from an early start and are damaged by cold soil. |
Soil temperature chart for planting: all 22 crops
The figure that matters most is the minimum — the point below which seed sits in the ground and rots rather than germinating. Sowing at the minimum works but is slow; sowing 10°C (18°F) above it is usually where germination is fastest and most even.
| Crop | Temperature class | Minimum soil temperature | Germination time | Notes |
|---|---|---|---|---|
| Beets | Cool-season, frost-tolerant | 7-10°C (45-50°F) | 5-10 days | Direct sow at or above this soil temperature |
| Broccoli | Cool-season | Not direct sown | 5-10 days | Started indoors and transplanted |
| Brussels Sprouts | Cold-hardy | Not direct sown | 5-10 days | Started indoors and transplanted |
| Bush Beans | Warm-season | 15°C (60°F) | 6-10 days | Direct sow at or above this soil temperature |
| Cabbage | Cool-season | Not direct sown | 5-10 days | Started indoors and transplanted |
| Carrots | Cool-season, frost-tolerant | 7-10°C (45-50°F) | 10-21 days | Direct sow at or above this soil temperature |
| Cucumbers | Warm-season | 18°C (65°F) | 5-10 days | Direct sow at or above this soil temperature |
| Eggplant | Heat-loving, frost-tender | Not direct sown | 7-14 days | Started indoors and transplanted |
| Garlic | Cold-hardy, planted in fall | Not direct sown | Sprouts after winter vernalization, not a spring germination | Grown from cloves, planted in autumn |
| Kale | Cold-hardy | 7-10°C (45-50°F) | 5-8 days | Direct sow at or above this soil temperature |
| Lettuce | Cool-season | 7-10°C (45-50°F) | 2-8 days | Direct sow at or above this soil temperature |
| Onions | Cool-season | Not direct sown | 7-14 days | Grown from sets or transplants |
| Peas | Cool-season, frost-tolerant | 4-5°C (40-41°F) | 7-14 days | Direct sow at or above this soil temperature |
| Peppers | Heat-loving, frost-tender | Not direct sown | 7-14 days | Started indoors and transplanted |
| Potatoes | Cool-season, frost-tender foliage | Not direct sown | Sprouts from seed pieces in 2-3 weeks | Grown from seed pieces, not seed |
| Radishes | Cool-season, fast | 7-10°C (45-50°F) | 3-7 days | Direct sow at or above this soil temperature |
| Spinach | Cold-hardy | 2-5°C (36-41°F) | 7-14 days | Direct sow at or above this soil temperature |
| Sweet Corn | Warm-season | 15°C (60°F) | 7-10 days | Direct sow at or above this soil temperature |
| Sweet Potatoes | Heat-loving, frost-tender | Not direct sown | Grown from slips, not seed | Grown from rooted slips, not seed |
| Tomatoes | Heat-loving, frost-tender | 15°C (60°F) | 6-14 days | Direct sow at or above this soil temperature |
| Winter Squash | Warm-season | 18°C (65°F) | 5-10 days | Direct sow at or above this soil temperature |
| Zucchini | Warm-season | 18°C (65°F) | 5-10 days | Direct sow at or above this soil temperature |
9 crops in this list show no soil minimum, and that is not a gap in the data — broccoli, Brussels sprouts, cabbage, eggplant, garlic, onions, peppers, potatoes, sweet potatoes. Some are started indoors and transplanted; others are grown from sets, cloves, seed pieces or rooted slips rather than sown as seed. For those, the frost date and the crop's own start method govern timing, not a direct-sow soil reading.
💡 Soil temperature and air temperature are not interchangeable
Some thresholds are about air, not soil — peppers want night air above 13°C (55°F) before transplanting, whatever the soil says. Measuring one and acting as though it were the other is a common way to lose a first sowing.
Step two: measure your soil
Push a soil thermometer 5-10 cm (2-4 in) into the bed and read it at the same time each morning — early readings are the honest ones, because afternoon sun flatters the surface. Take readings for several consecutive days; a single warm day means nothing.
💡 Soil lags air by weeks
In spring, air temperature climbs well before soil does. This is why so many first sowings fail: the day felt warm, but the seed went into cold ground and rotted. The thermometer settles the argument.
Step three: record your own frost dates
Regional frost averages are a starting point, not an answer. Elevation, slope, proximity to water and even a nearby wall shift frost timing by weeks within a single garden. One season of your own records beats any published average.
Step four: check the crop can finish
Add the crop's days to maturity to your intended planting date, then add two to three weeks for the slower growth that comes with shortening days. If that lands after your first frost, the crop will not finish. Change the cultivar or change the crop.
Day length matters too
Some crops respond to day length rather than temperature. Onions bulb according to hours of daylight, which is why a long-day cultivar fails near the equator and a short-day cultivar bolts at high latitude. Spinach and lettuce bolt as days lengthen past a threshold. Where a crop is day-length sensitive, the guides say so.
Southern hemisphere gardeners
Everything here applies unchanged. The conditions are identical; only the months differ. This is precisely why these guides avoid calendar months — a soil temperature threshold is true in October in Melbourne and in April in Madrid.
Turn this method into a written plan. The Global Vegetable Garden Planner (printable PDF) has the soil-temperature reference, crop rotation planner, sowing log and harvest tracker — 73 pages, no calendar required.
Keep reading
Frequently Asked Questions
How do I know when to plant without a calendar?
Measure soil temperature at 5-10 cm depth, know your own first and last frost dates, and match both against the crop's temperature class. That gives you a planting time specific to your garden rather than to a region.
What soil temperature should I plant at?
About 4-5C (40F) for hardy cool-season crops, 7-15C (45-60F) for half-hardy ones, above 15-16C (60F) for tender warm-season crops, and 18-21C (65-70F) for heat-loving crops such as sweet potato and okra.
Does this work in the southern hemisphere?
Yes, without modification. Soil temperature, frost tolerance and day length behave identically in both hemispheres — only the months differ, which is why this method avoids months entirely.
Why did my seeds rot instead of germinating?
Almost always because the soil was below the crop's germination minimum. Air temperature warms weeks before soil does, so a warm spell is not evidence that the ground is ready.
Do I really need a soil thermometer?
It is the single most useful instrument in a vegetable garden and costs very little. It answers the timing question that guesswork gets wrong most often.
Last updated 25 August 2026
Helpful Next Steps
Use these practical links to turn the guide into a clear garden plan and the right next action.
Plan the next step
Use the printable SoilCommander planner for your region to turn this guide into sowing dates, bed layouts and weekly garden tasks.