Hardening Off Plants: The Step You Can’t Skip

Hardening off is the step that gardeners most often skip — and the one they most often regret. You’ve spent weeks nurturing seedlings on a warm windowsill or in a heated greenhouse. They look strong, lush, and ready. You plant them outside on a mild May afternoon, and two days later they’re wilted, yellowed, and shocked into a standstill. This is transplant shock, and almost all of it is preventable.

Hardening off — gradually acclimatising indoor-grown plants to outdoor conditions before planting them in the ground — takes one to two weeks and requires almost no effort. It just requires that you don’t rush.

Why Plants Need Hardening Off

Indoor-grown seedlings are adapted to a stable, sheltered environment: constant warmth, still air, consistent moisture, and filtered light. Outside, conditions are dramatically different — fluctuating temperatures, wind, direct sun, and rain. The difference isn’t just about cold: wind causes physical stress that indoor-grown stems aren’t built for, and direct sun can scorch leaves that have developed in lower light intensity. Even a hardy plant, raised indoors, needs time to adjust.

During hardening off, plants physically change: leaf cuticles thicken, stems become stronger, and root systems adjust. Plants that are properly hardened off establish faster and grow more vigorously than those planted out in a rush.

How to Harden Off: Step by Step

Week One: Daytime Only

Move plants outside to a sheltered spot — out of direct midday sun and wind — for a few hours each day. A porch, the base of a wall, or a cold frame with the lid propped open are all ideal. Start with two to three hours and increase daily. Bring plants back inside in the afternoon, well before temperatures drop in the evening.

I harden mine off for ten days against the south-facing wall of my 6×4 shed.

If the weather is particularly cold, wet, or windy in the first few days, it’s fine to skip a day. The goal is gradual exposure, not forcing plants through bad conditions.

Week Two: Longer Days and Light Nights

By the second week, plants can stay outside for most of the day. From mid-week, leave them out overnight — but only if no frost is forecast. A cold frame with the lid closed handles overnight protection well. Alternatively, cover with a layer of fleece if temperatures are expected to drop below 5°C.

By the end of week two, plants that have experienced a few nights outside and are showing no signs of stress — no yellowing, no wilting, no leaf scorch — are ready to plant out.

Using a Cold Frame

A cold frame is the most useful hardening off tool available to home gardeners. A simple wooden box with a glass or polycarbonate lid, it creates a microclimate that’s several degrees warmer than the open garden while still exposing plants to outdoor light levels and air. Open the lid progressively wider across the hardening off period — fully open on warm days, half open or closed on cold nights — to manage the transition smoothly.

Cold frames don’t need to be expensive. A wooden frame with an old window glass on top works perfectly well. The key is that the lid can be propped at different heights and removed entirely when the time comes.

Crops That Need Careful Hardening Off

All tender crops need proper hardening off before planting outside, but some are more sensitive than others:

• Tomatoes — sensitive to cold and wind; take the full two weeks
• Courgettes and squash — fast growers that put on size quickly once established, but very susceptible to cold check
• Basil — the most cold-sensitive of all common crops; even a chilly night can blacken leaves and set the plant back weeks
• Cucumbers — hate cold and wind; harden off thoroughly and plant in the warmest, most sheltered spot available
• Sweetcorn — not quite as tender as tomatoes but dislikes check; a full two-week hardening off produces noticeably stronger plants

Crops That Are More Forgiving

Brassicas (cabbages, kale, Brussels sprouts, calabrese) are hardy and can manage a shorter hardening off — five to seven days is often enough. Leeks, onions, and lettuces are similarly robust.

What to Do If You Haven’t Got Time

If you’re short of time and need to plant out without a full two-week hardening off, minimise the damage: choose a mild, overcast day rather than a bright windy one; water plants well before planting; plant in the evening so plants have the night to adjust before facing direct sun; and cover with fleece for the first week. It’s not ideal, but it’s significantly better than planting out in full sun on a breezy day with no preparation.

For the full spring growing picture — what to sow when and how — see our guides to sowing seeds indoors, April garden jobs, and May garden jobs. All spring content lives in the Spring Garden hub.

You’ll find more growing guides and seasonal advice in our Garden & Outdoor.

Last reviewed: April 2026

🛡 Our standards: How we research · Editorial policy · Affiliate disclosure · Corrections · Last reviewed: April 2026

About Gareth

Gareth Isherwood is the editor of Danbury & Lovejoy. He has baked bread at home for 25+ years and has 15 years of growing experience across gardens, containers and other growing spaces, including an East Sussex allotment for the past three years. Read his full story →