How do I know when it is safe to plant out tender plants?

Almost everyone plants out too early. The impulse is understandable — the plants are outgrowing their pots and the weather has turned pleasant — but a courgette or a tomato set into cold ground stops growing, becomes vulnerable to rot and slugs, and takes weeks to recover if it recovers at all.

Step by step

1. Check your last frost date has actually passed.Not the national one, not the one on the seed packet, yours — corrected for your site. This is the first gate and it is not negotiable for anything genuinely tender.
2. Read the ten-day forecast for frost conditions rather than for numbers.You are looking for clear, calm nights with minima within a few degrees of freezing. If any of the next ten nights looks like that, either wait or plan to cover.
3. Take a soil temperature at planting depth.Tender plants need warm soil, not merely frost-free soil. Roots in cold ground cannot take up water or nutrients properly, which is why a plant in cold soil wilts in sunshine despite wet ground beneath it.
4. Confirm the plant is hardened off.A plant raised indoors or under glass has soft tissue and no wind conditioning. It needs a week or more of graduated exposure before it goes out permanently, and skipping this scorches and checks it even without a frost.
5. Plant in the evening or on a dull day.Planting into full afternoon sun asks a root system that has just been disturbed to supply a full transpiration load immediately. Evening or overcast planting gives it hours to settle.
6. Water in properly and keep fleece to hand.A thorough soak settles soil around the roots and removes air pockets. Then keep fleece within reach for a fortnight — the plants are at their most vulnerable in the first two weeks and a single unforecast frost undoes everything.
7. If a frost is coming and they are already out, cover before dark.Fleece over hoops or canes so the material is clear of the leaves, taken right down to soil level and weighted all round, so warmth rising out of the ground is trapped underneath.

Tips

• There is nothing lost by waiting. A tender plant set out a fortnight later into warm soil routinely catches up and passes one planted early, because it never stops growing.
• Pot on rather than plant out if the plants are outgrowing their containers. A bigger pot buys you two or three weeks of safety at almost no cost.
• The first-frost end of the season is the other half of this decision. Check that what you are planting has time to crop before your first autumn frost, or you are planting for nothing.
• Plant into your warmest microclimate first and the open ground a week later. That is a free week and it costs only the effort of thinking about the map.

Common mistakes

• Planting out because a bank holiday or a free weekend has arrived — The calendar is not the constraint. Some of the most reliably damaging frosts in Britain arrive after the traditional planting weekends, and a whole garden centre's worth of bedding goes out the day before them every year.
• Planting out plants straight from a greenhouse or a windowsill — Untreated soft growth scorches in wind and sun even without frost. Hardening off is a separate requirement from frost risk and skipping it checks the plant badly.
• Judging by air temperature alone — Soil lags air by weeks. Warm air over cold soil is the classic late-spring pattern and it is exactly the condition in which tender plants sit and sulk.

Questions people ask