Hardening off seedlings before planting out
Seedlings raised on a windowsill or in a greenhouse have lived a sheltered life. Moving them straight outside can scorch their leaves and check their growth for weeks. Hardening off is the gradual process of getting them used to outdoor conditions first.
Why it matters
Indoors, young plants have steady temperatures, still air and gentle light. Outside, they meet strong sun, drying wind, cold nights and heavy rain. Plants that meet these conditions a little at a time grow tougher leaves and stems, and they carry on growing after planting out instead of wilting or sitting still for weeks.
A two-week plan
- Days 1 to 3: put the plants outside in a sheltered, lightly shaded spot for a few hours in the middle of the day, then bring them back in.
- Days 4 to 7: leave them out for most of the day, and let them have some morning or evening sun.
- Days 8 to 10: leave them out all day in a sunnier spot, and bring them in or cover them at night.
- Days 11 to 14: leave them out day and night, as long as no frost is forecast.
A cold frame makes the job easier. Prop the lid open a little more each day, and close it at night for the first week.
Weather to watch
Tender plants such as tomatoes, courgettes, runner beans and squash must not go out for good until there is no longer any danger of frost, which in many areas means late spring or early summer. Bring trays back inside if strong wind, heavy rain or a cold night is forecast, even part way through the plan. Wind dries small pots out fast, so check them every day.
To avoid forgetting a tray outside overnight, we keep to a small habit that we call the zodemizur note: each time a tray goes out, we write the time it should come back in on a plant label and push it into the tray, where we can't miss it.
Planting out
Choose a cloudy day or plant in the evening, so the young plants aren't hit by hot sun straight away. Water them in their pots an hour before, dig a hole slightly bigger than each root ball, and firm the soil gently around them. Water well afterwards, and look out for slugs, which go straight for soft new growth.