How to Harden Off Young Plants Before Planting Out

Why Young Plants Need a Transition Period

A seedling raised on a windowsill or in a heated greenhouse has lived a cosseted life. It has known steady warmth, still air, gentle light and a cosy root run that never dries out. The open plot is nothing like that. Out there it will meet gusting wind, hard rain, blazing sun, cool nights and temperature swings of ten degrees or more between afternoon and dawn.

Plunge that pampered plant straight into a bed and the shock is real. Leaves that have never felt a breeze can scorch and crisp. Stems that grew soft and fast in shelter may snap or flop. Growth stalls for a fortnight while the plant rebuilds itself, and in that pause slugs, pigeons and weeds all get a head start. Hardening off is simply the art of introducing change slowly, so the plant has time to toughen up.

What actually happens is fascinating. Exposure to wind and cooler air prompts the plant to lay down thicker cell walls and a waxy cuticle on the leaves, and to build sturdier stems. The whole process takes a week or so. Rush it and you lose the advantage of all those weeks of raising your own stock.

When to Start Hardening Off

Begin about seven to ten days before you intend to plant out. The timing should be driven by the plant, not the calendar. A good rule is that seedlings are ready to start when they have three or four true leaves and a sturdy little stem, and when the roots have filled their pot enough to hold the compost together but not so much that they are circling.

Check the forecast before you begin. A cold snap, a late frost or a spell of violent wind will undo your good work, so start when a settled, mild week is on the horizon. In most of the UK that means late April into May for hardy sorts, and late May into June for tender crops such as courgettes, runner beans, sweetcorn and tomatoes.

Plants that will be planted under cloche, fleece or in a cold frame need less hardening than those going straight into open ground, but they still need some. A little toughening never goes to waste.

The Day-by-Day Routine

Consistency matters far more than perfection. Move plants out in the morning and back in at night, extending the time outdoors a little each day.

• Days 1 and 2: Two or three hours in a sheltered, bright spot out of direct sun and wind. A shaded patio, a north-facing wall or the lee of a shed is ideal. Bring them in before dusk.
• Days 3 and 4: Four to six hours, still sheltered, with an hour or two of gentle morning sun. Keep the compost watered, as pots dry out quickly in a breeze.
• Days 5 and 6: Leave them out for most of the day in a spot with more sun and a bit more air movement. By now they should be standing up to a light breeze without wilting.
• Days 7 and 8: Overnight outdoors, if nights are mild. If frost is forecast, bring them in or throw fleece over them.
• Days 9 and 10: Full days and nights outdoors, in the position they will occupy, with only a little protection if the weather turns.

If you have a cold frame, the job is easier. Leave the lid propped open a little more each day and close it at night, then leave it fully open for the last two nights before planting. A cloche or a length of horticultural fleece laid over plants in their final position works in much the same way.

Hardening Off Different Crops

Not everything needs the same treatment. Tender crops raised from seed indoors need the full routine, and the slowest, gentlest version of it. Tomatoes, peppers, aubergines, courgettes, squashes, cucumbers and sweetcorn all resent cold air and should not go out until night temperatures are reliably above about 10C.

Half-hardy bedding and brassicas sit in the middle. They will take a light frost once hardened, but they still need a week of gradual introduction. Hardy vegetables such as leeks, onions, lettuce, peas, broad beans and most root crops need only three or four days of easing, and some, like peas sown in modules, barely need it at all.

Watch for the difference between a sulk and a problem. Slight wilting in the afternoon heat is normal and the plant will perk up by evening. Leaves that go pale, papery and crisp at the edges have been scorched by sun and need more shade, not more time outside.

Common Mistakes to Avoid

• Forgetting to water. Wind dries compost far faster than indoor warmth. Check pots morning and evening.
• Skipping the return indoors. One cold night can set a tender plant back by weeks, or finish it off.
• Going from windowsill to open bed in one move. This is the classic error and the reason so many home-raised seedlings look poor next to bought ones.
• Hardening off in full sun. Bright sun on soft leaves causes scorch. Start in shade and build up.
• Letting plants get pot-bound during the process. If roots are circling badly, pot them on or plant them out sooner rather than later.

Planting Out Day and the First Week After

Choose a cool, cloudy afternoon or early evening for the move. Water the pots well a few hours beforehand so the rootball holds together, and water the planting hole too. Set each plant at the same depth it grew in the pot, firm gently and give a final drink to settle the soil around the roots.

For the first week, keep an eye on the weather and be ready with fleece, a cloche or a windbreak of netting. Shade tender leaves from strong midday sun for a day or two with a sheet of newspaper or a propped-up board. A short length of cane beside each plant gives support and marks the spot when you are weeding.

Do this patiently and you will barely notice the transplant at all. The plants carry on growing without a pause, roots reach down into cool soil, and by mid-summer you have the sort of sturdy, productive plot that makes all that early sowing worthwhile.