How do I harden off tomato plants before planting outside?

Hardening off takes 10–14 days. Start by putting plants outside in a sheltered spot for a few hours each day, gradually increasing their exposure. Bring them in at night until overnight temperatures are consistently above 10°C.

When to Plant Tomatoes Outside in the UK — Frost Dates & Hardening Off

Q: When can I plant tomatoes outside in the UK?

Most UK growers plant tomatoes outside after the last frost — typically late May in southern England and early June further north. Night temperatures should consistently stay above 10°C before planting out.

Q: Can tomatoes go outside in April in the UK?

No — April is too early for outdoor tomatoes in most of the UK. A single frost will kill an unprotected plant. Wait until after your last frost date, which in most areas falls between mid-May and early June.

Regional planting dates

Region	Typical last frost	Safe to plant outside
South West England, South Wales	Late April	Mid to late May
South East England, Midlands	Early May	Late May
North England, North Wales	Mid May	Early June
Scotland, Northern Ireland	Late May / June	Mid June
Ireland (south)	Late April / Early May	Late May

These are averages. Check your local forecast in the week before planting — a surprise late frost in late May is not uncommon, even in the south.

The two conditions to meet before planting out

❄️ No frost risk

Check the 10-day forecast. If there's any risk of frost in the next two weeks, wait. One frost will kill an unprotected tomato plant overnight.

🌡️ Nights above 10°C

Tomatoes stop growing and become stressed when night temperatures drop below 10°C. Even without frost, cold nights slow plants significantly and invite disease.

How to harden off tomatoes properly

Plants grown indoors under glass or in a heated greenhouse are not used to outdoor conditions — wind, direct sun, and fluctuating temperatures. Moving them straight outside causes transplant shock and sets them back weeks.

Hardening off takes 10–14 days and it's worth doing properly:

Week 1

Move plants outside to a sheltered, partially shaded spot for 2–3 hours each afternoon. Bring in before evening.

Week 2

Increase outdoor time to 4–6 hours, including some direct sun. Still bring in at night if temperatures drop below 10°C.

Final days

Leave outside all day, including overnight if the forecast is clear and above 10°C. They're ready to plant.

Outdoor vs greenhouse tomatoes

For outdoor growing in the UK, stick to varieties bred for British conditions. Greenhouse varieties often fail outdoors — they need the extra warmth and are more susceptible to blight in wet conditions.

Good outdoor varieties for UK conditions

Gardener's DelightTumbling TomOutdoor GirlSweet MillionTigerellaFerline (blight resistant)

Frequently asked questions

When can I plant tomatoes outside in the UK?

After the last frost — typically late May in southern England and early June further north. Night temperatures should consistently stay above 10°C.

Can tomatoes go outside in April in the UK?

No — April is too early. A single frost will kill an unprotected plant. Wait until after your last frost date, which falls between mid-May and early June for most areas.

How do I harden off tomato plants?

10–14 days of gradual outdoor exposure. Start with 2–3 hours in a sheltered spot, increase daily, and bring in at night until overnight temperatures stay above 10°C.

Get frost alerts and planting reminders

Vercro monitors your local forecast and tells you exactly when it's safe to plant your tomatoes out — and when to bring them back in.

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When to Plant Tomatoes Outside in the UK