Hardening Off Plants: Ready for the Big Outdoors

What is hardening off & why it matters

Indoors, light is filtered and air is still; outdoors, sunlight is stronger, humidity lower and breezes constant. Hardening off lets plants thicken cuticles, increase protective pigments and adjust to transpiration rates. Skipping it often results in pallid, crispy patches on leaves, droop, and a two–three week growth stall. Do it right and you’ll see compact, sturdy growth and faster flowering or cropping.

When to start (UK timings)

Begin 10–14 days before your target planting date, aligning with your local last frost. As a rule of thumb:

Hardy annuals & brassicas: March–April (autumn hardening for Sept plantings).
Half-hardy annuals (cosmos, marigolds) & tender veg (tomatoes, squash, peppers): late April–May in the south; early–mid May further north/exposed sites.
Perennials & shrubs from indoors: 5–7 days usually suffices if they were cool-grown.

The 7–10 day hardening-off plan

Use bright shade first, then gentle morning sun, then longer exposures. If a day is unusually cold or windy, repeat the previous step.

Days 1–2: 2–3 hours outside in bright shade, sheltered from wind. Bring in at night.
Days 3–4: 4–5 hours; introduce early sun before 11am. Avoid midday glare.
Days 5–6: 6–8 hours with some direct sun and light breeze. Check moisture midday.
Days 7–8: Full day out. Use a cold frame or lee of a wall; close frame at night.
Days 9–10: Leave out overnight if frost-free. Reduce watering slightly to firm growth.

If plants look stressed (wilting, paling, crispy patches), step back a day, increase shade, and resume once recovered.

Seedlings vs. bought plug plants

Windowsill-raised seedlings are softest — give the full 10 days. Greenhouse-grown seedlings usually manage 7–8 days. Garden-centre plugs are often semi-hardened; still give them 3–5 days in a sheltered spot before planting.

Reading the weather: sun, wind, cold

Sun: spring UV is stronger than it feels; introduce morning sun first.
Wind: breezes build sturdy stems but dehydrate fast. Use fences, hedges or a cold frame as windbreaks.
Cold: below ~5 °C, pause. If frost threatens, move plants in or cover with fleece/row cover.

Watering, feeding & potting on

Keep compost evenly moist; avoid waterlogging which makes sappy, weak growth. Feed lightly with a balanced liquid fertiliser if seedlings have been in the same cell for 4+ weeks. If roots circle the plug, pot on before hardening so growth isn’t checked during the process.

Common mistakes to avoid

Rushing the schedule: 2–3 days is rarely enough for tender plants.
Midday sun too soon: causes bleaching/scorch; favour morning light initially.
Unshielded wind: gusts desiccate soft foliage; provide a lee.
Overwatering: constant wetness elongates, weakens growth.
Planting before frost risk passes: keep fleece to hand through May in cooler areas.

Planting day & aftercare

Pick an overcast afternoon. Water the rootball, plant at the same depth, and water in to settle soil. Mulch 3–5 cm to steady moisture and temperature. For tall or leggy plants, stake lightly. Continue to watch forecasts for a fortnight and throw fleece over if a cold snap appears.

FAQs

Do hardy annuals need hardening off?

Direct-sown hardy annuals don’t. If raised indoors, give at least 4–5 days of acclimatisation.

Can I harden off in a greenhouse?

Yes — open vents and doors by day for airflow, then move plants outdoors for the final few days.

My seedlings flopped after day 3 — what now?

Bring them in overnight, give two easier days (shade, no midday sun), then resume the sequence.