The short version: you plant tomatoes outside once nighttime temperatures stay reliably above 10°C and soil temperature has hit 15°C, not on some fixed date from a seed packet. In the UK that's usually late May to early June, in much of the US it's after your local last frost date plus a week or two of buffer, and if you jump the gun you'll spend three weeks watching sulky, purple-tinged plants that never quite recover.
Forget the calendar, read the soil
Every gardening forum tells you a date. "Plant after the last frost." "Wait until mid-May." That's lazy advice dressed up as helpful advice, because frost dates are averages, not promises, and two gardens ten miles apart can have completely different microclimates.
What matters is soil temperature, measured at about 10cm deep, ideally first thing in the morning before the sun has warmed the top layer artificially. Tomatoes are tropical plants pretending to be annuals in temperate gardens. Their roots stop functioning below 10°C and real growth doesn't start until the soil is sitting at 15°C or above, consistently, not for one warm afternoon.
You can buy a cheap soil thermometer for under a tenner. Stick it in the bed where you're planning to plant, check it for four or five mornings running, and if it's holding at 15°C or higher, you're in business. If it's bouncing between 11°C and 16°C depending on the day, wait.
A worked example: the "it's sunny, let's go" mistake
Say you run a small kitchen garden in the Midlands and you've got six tomato seedlings sitting on a windowsill, already leggy and desperate to get outside. The first week of May gives you three glorious sunny days, 19°C in the afternoon, and you decide that's your sign. You plant them out on day one.
By day four the nights have dropped back to 4°C, which is entirely normal for early May in the UK. The plants don't die outright, but their growth stalls for two to three weeks while the cold-damaged roots try to recover, the leaves turn a dull purple-blue from phosphorus uptake being blocked by the cold, and you end up with plants that are smaller and later-fruiting than if you'd simply waited until the end of the month and planted into warmer soil.
That's the uncomfortable truth most planting guides skip over: an early start almost never gives you an earlier harvest. A tomato plant set back by cold soil in May will frequently be outgrown by one planted three weeks later into warm soil, because the warm-soil plant never stops growing while the early one is busy recovering. Patience beats enthusiasm here, every time.
Regional timing, roughly
- UK, most regions: last week of May into the first week of June. Scotland and northern England, push that to mid-June.
- US, northern states (Minnesota, Maine, Montana): early to mid-June, after soil has had time to warm.
- US, mid-latitude (Ohio, Pennsylvania, Virginia): early to mid-May.
- US, southern states (Georgia, Texas, Florida): March into early April, and sometimes a second planting in late summer for a autumn crop.
- Australia, temperate zones like Melbourne and Sydney: mid-October to November, after the last frost risk has passed in spring.
These are starting points, not promises. Always check your local frost date and your own soil, because a sheltered south-facing bed against a brick wall will be weeks ahead of an exposed allotment plot a mile away.
Harden off or lose the plant anyway
Even if your timing is right, a seedling raised indoors or in a greenhouse will scorch, wilt or snap in the wind if you move it straight outside. This is the step people skip because it's boring and takes a week.
Want AI doing the heavy lifting in your marketing?
I build the systems that handle the boring 80 percent, so you get your week back. Done properly, with the human kept in.
- Day 1 to 2: put plants outside in a sheltered, shaded spot for two to three hours, then bring them back in.
- Day 3 to 4: extend to four to five hours, introduce some direct morning sun.
- Day 5 to 6: leave them out most of the day, bring in overnight if temperatures are forecast below 10°C.
- Day 7: leave them outside overnight if nights are mild, then plant into the ground the next day.
Skip this and even perfectly timed planting will set the plants back, because the leaves haven't built up the waxy cuticle and the stems haven't toughened enough to cope with wind and direct UV.
What to check before you plant, in order
- Soil temperature at 15°C or above for at least four consecutive mornings.
- No frost forecast for the next ten days, including a buffer for a late cold snap.
- Plants hardened off for a full week.
- Planting hole prepared with compost and, ideally, a handful of crushed eggshell or a slow-release feed worked in, since tomatoes are hungry feeders.
- Support stakes or cages already in the ground before you plant, not added later when roots are established and easily damaged.
Plant deep too. Bury two-thirds of the stem, up to the first set of true leaves, because tomato stems grow roots along their length when buried, giving you a stronger root system than planting shallow ever will.
What if a cold snap hits after you've planted
It happens, even with good timing. Keep fleece or an old bedsheet on hand and cover plants overnight if frost is forecast, removing the cover in the morning once temperatures rise. A plastic cloche or even an upturned bucket works for individual plants in a pinch. Don't rely on this as your primary plan though, it's a backup for the odd rogue cold night, not a reason to plant three weeks early.
Frequently asked questions
Can I plant tomatoes outside in April?
In most of the UK and the northern half of the US, no, soil and night temperatures are still too low in April and you'll set your plants back. Southern US states are the exception, where April planting is normal.
What temperature is too cold for tomato plants?
Growth slows noticeably below 15°C and roots struggle below 10°C. A frost at 0°C or below will kill an unprotected tomato plant outright.
Is it better to plant tomatoes early in pots and move them later?
Yes, raising seedlings indoors or in pots and only moving them outside once soil and air temperatures are right gives you the strongest plants, which is exactly why nurseries do it this way rather than direct-sowing outdoors.
Do cherry tomatoes need different timing to beefsteak tomatoes?
No, the soil temperature rule applies the same across varieties, though cherry tomatoes tend to tolerate a slightly earlier, cooler start better than large beefsteak types because they're generally hardier plants overall.
