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Moving plants outdoors in spring after a winter under a lamp

A plant that has spent the winter under a lamp is used to a very particular life: steady light from above, still air, a warm room and no rain. Spring sunshine, wind and a cold night are all new to it. Moving it outside in one go is the commonest way to undo a winter of careful growing. This page covers that transition, for seedlings raised indoors, kitchen herbs going out to a pot by the door, and houseplants that like a summer on a balcony.

Why lamp-grown plants need hardening off

Hardening off means getting a plant used to outdoor conditions gradually. Lamp-grown plants need it as much as greenhouse plants do, and often more:

• Sun. Direct spring sun is far more intense than a houseplant lamp at its recommended distance. Leaves that grew indoors can scorch, turning white or brown in patches, within a day of full sun.
• Wind. Stems that never moved indoors are soft. Wind bends and snaps them, and dries leaves faster than the roots can replace the water.
• Cold. Nights outdoors fall well below a heated room. Tender plants can be checked or killed by cold well before any frost.
• Rain and pests. Heavy rain flattens soft seedlings, and slugs find them quickly.

The propagation page makes the same warning briefly. A seedling that has only seen LEDs needs that week or more of introduction.

A gradual week or two

There is no single correct timetable, and plants vary. The usual pattern is gentle exposure that increases day by day:

1. Start with a short spell outside in a sheltered, lightly shaded spot on a mild day, then bring the plants back in.
2. Increase the time outside over the following days, and move them gradually into more sun.
3. Leave them out overnight only once nights are mild and no frost is forecast, and keep fleece or a cold frame handy.
4. Plant out, or leave pots outside full time, once they cope with a full day and a night outside without wilting or scorching.

A cold frame, a porch or a sheltered corner against a wall makes the process easier, because you can open and close it rather than carrying trays in and out. If the weather turns cold or windy partway through, take a step back for a day or two. Hardening off is a direction of travel, not a fixed number of days.

Watch the plants rather than the plan. Pale or bleached patches on leaves mean too much sun too soon. Wilting in moist compost often means wind or sun. A purple or blue tinge on some seedlings can mean they are cold.

Frost risk where you live

The UK does not have one last-frost date. Frost risk in spring depends on where you are, how high, how close to the coast or a town, and whether your garden sits in a frost pocket at the bottom of a slope. A sheltered city courtyard and an exposed garden on a hill in the same county can behave very differently, and late frosts can still arrive after a run of warm days.

So this page does not give a date. Use these instead:

• Your local forecast in the days before planting out, especially the overnight minimum.
• Seed packets and plant labels, which say whether a plant is hardy or tender and when to plant it out.
• A local sowing calendar, which the propagation page links to.
• Your own garden. Note when the last frost actually happened this year, and next spring you will have a better guess.

Tender plants, such as basil, chillies, tomatoes and most houseplants, are the ones to keep in until the risk of frost has passed. Hardy herbs such as parsley cope with cold better, but still need hardening off after a winter indoors. The herb page covers what each one wants indoors: grow lights for kitchen herbs.

When to stop using the lamp

The lamp's job shrinks as the days lengthen. In late winter it may be most of the plant's day. By spring, a bright window does much of the work again. Two approaches work:

• Shorten the hours gradually. Reduce the timer setting as the window gets brighter, keeping the dark period every night. The reasoning behind daily hours is in hours and a dark period.
• Switch it off once the plant moves. When a plant goes outside or onto a bright summer sill, it no longer needs the lamp at all.

Seedlings are the exception that needs more care. Keep them under the lamp until they start hardening off. Seedlings moved from a lamp to a dim sill in early spring stretch again. Raising them is covered in raising seedlings on a windowsill.

The plants that stay indoors all year, such as the fern in a north room, may still benefit from the lamp in summer on a reduced day. Whether a plant needs a lamp at all is in do I need a grow light for a UK winter.

Storing the lamp for summer

Unplug it, take it off the timer, dust it, and store it somewhere dry. Wind the lead loosely rather than tightly round the fitting. If the lead, plug or fitting has cracked or discoloured, replace it before the autumn rather than discovering it when you need it. This site does not give repair or wiring instructions.

Bringing plants back in autumn

The same logic runs in reverse. A houseplant that summered outside brings in pests and is used to brighter light. Check leaves and the pot rim, give it a few days in a bright spot, and expect a little leaf drop as it adjusts. Then start the lamp again once the days shorten.

Questions

How long does hardening off take?

There is no fixed number. A week or two of gradually increasing time outdoors is a common pattern, but weather and the plant decide. Move faster in mild, still weather and slow down in cold or wind.

When is it safe to plant out tender plants in the UK?

When the risk of frost has passed where you live. That varies a lot around the country and even between neighbouring gardens. Check your local forecast and the plant label rather than relying on a single national date.

Should I keep using the grow light while plants are hardening off?

Yes, while they are still indoors overnight or for part of the day, keep their normal lamp hours and dark period. Once they are outside full time, switch the lamp off.