Hardening off and planting out: the two weeks that decide whether seedlings survive

01At a glance

• How long: the RHS says two to three weeks. Its quicker guides say 10 to 14 days. Longer for plants raised somewhere warm.
• When to plant out tender crops: from the first half of May on the south coast to mid-June in northern Scotland.
• The rule: hardening off toughens plants. It does not make frost-tender plants hardy.
• Kit: fleece, or a cold frame, or just your own two hands and a tray.

02What hardening off actually does

According to the RHS, gradual exposure lets a plant thicken and alter its leaf structure and build up more wax on the leaf surface. That is what lets it cope with wind, cooler nights and stronger light. Skip it and growth can be checked badly.

03Three ways to do it

Carrying trays in and out. No kit needed. Stand the plants outside by day in a sheltered spot with dappled sun, and bring them in at night. Lengthen the time outdoors as the days go by.

With fleece. The RHS schedule for anyone without a cold frame:

1. 1
   Week one: outside by day under two layers of fleece. Indoors at night.
2. 2
   Week two: one layer of fleece, by day.
3. 3
   Week three: no fleece by day. Cover at night if needed, then leave them uncovered before planting.

With a cold frame. Less carrying:

1. 1
   Week one: open the lid a little by day and close it at night.
2. 2
   The following two weeks: raise the lid a bit more each day until it is off altogether.

04When is it safe? Last frosts across the UK

The RHS gives these typical first safe dates for planting out tender plants:

• South-west England and the south coast: first to second half of May.
• Southern England and south Wales: second half of May.
• Midlands, north-west England, Yorkshire, north Wales: end of May.
• North-east England, southern Scotland, Northern Ireland: early June.
• Highlands and northern Scotland: mid-June.

Work back from your date and start hardening off two to three weeks before it. The RHS adds a useful temperature test: plant out when nights are consistently above 7°C, and wait for nights above 10°C for heat-lovers such as tomatoes and basil.

05Which crops are frost-tender

All of these are killed or badly set back by frost, so they wait for the dates above:

• Tomatoes
• Courgettes and pumpkins
• Sweetcorn
• Runner beans

Hardy seedlings still need hardening off if they were raised indoors. They just get through it faster.

06Planting out

1. 1
   Ease each plant out of its pot without disturbing the roots.
2. 2
   Plant tomatoes deep. Set them with the first leaves just above the soil. The buried stem makes extra roots.
3. 3
   Give them room. Courgettes at least 90cm apart. Tomatoes 45–60cm. Sweetcorn 34–45cm apart in a block, not a single row, so the wind can pollinate it.
4. 4
   Firm in gently.
5. 5
   Water in well, and keep watering through dry spells until the plants are growing strongly.
6. 6
   Check for slugs. Soft new transplants are exactly what they like.

07If a late frost is forecast

It happens, even in late May. Cover tender plants overnight with fleece or cloches. The RHS advises a double layer of fleece on the coldest nights. Plants still in trays can simply come back indoors. There is more on this in protecting plants from frost.

08Mistakes that cost you plants

• Planting out on the first warm weekend of April. The warm spell is not the last frost.
• Treating a hardened-off plant as hardy. It is tougher, but a frost will still kill a tender crop.
• Rushing plants from a heated room. The warmer they were raised, the longer they need.