If you have ever grown tomatoes or courgettes on a windowsill and then put them straight outside in May, you will know what happens next. The plants stop growing. The leaves go pale. Sometimes they just give up altogether. It is incredibly frustrating, especially when you have looked after them for weeks.
The good news is that this is one of the easiest gardening problems to fix. The answer is hardening off, and it takes about two weeks of very little effort. Once you know how to do it properly, the results are dramatic. Your plants will settle into outdoor life within days rather than weeks.
This guide explains what hardening off is, how to do it properly in the UK, and what happens if you skip it. If you want the bigger picture of what to plant in May, What to Plant in May in the UK covers the full crop list.
What Is Hardening Off?
Hardening off is the process of getting indoor plants used to being outside before you plant them out for good. It really is that simple.
Plants grown indoors have only ever known warm, still air, gentle light, and steady temperatures. Outside the world is very different. Sunlight is much stronger. Wind is constant. Temperatures change a lot between day and night. If you take a plant from one environment and dump it into the other, it goes into shock.
The way to avoid this is to introduce them gradually. Spend a week or two letting them experience outdoor conditions for a few hours a day, then a bit longer, then overnight, then permanently. By the end they are ready to thrive outdoors instead of struggling.
Why Hardening Off Matters in the UK
It matters more in the UK than in most other places because our weather is so changeable. A warm sunny morning can become a cold windy afternoon within hours. A mild week can be followed by a sudden cold snap. Plants that have not been hardened off cannot cope with this.
You can spot a plant that has been put out too soon almost straight away. The leaves often look bleached or pale, growth stops, and the plant just sits there for weeks. Tomatoes are particularly vulnerable. So are courgettes, cucumbers, French beans and most flowering bedding plants.
The other reason it matters in the UK is timing. Most tender plants need to go outside in May, which is exactly when the weather is at its most unreliable. If you want to know when frost is finally past in your area, UK Last Frost Dates by Postcode is worth checking.
Which Plants Need Hardening Off?
Any plant that has been grown indoors or in a greenhouse needs hardening off before going outside. The most common ones in UK gardens are:
Tomatoes, courgettes, cucumbers, squash, pumpkins, French beans, runner beans, sweetcorn, and peppers all need it. So do tender bedding plants like cosmos, marigolds, petunias, busy lizzies and dahlias.
Plants you do not need to harden off include hardy seeds you have sown directly outside in the ground, plants bought already growing outside at a garden centre, and tough plants like brassicas which are usually fine to plant out without much fuss.
If you are not sure which crops are tough and which are tender, Vegetables That Survive Late Frost is a useful guide.
How to Harden Off Plants Step by Step
This is the bit most people overcomplicate. It is genuinely simple. Here is exactly what to do.
Day 1 to 3: A Couple of Hours Outside
Put your plants outside in a sheltered spot for two or three hours during the warmest part of the day. Somewhere out of direct wind is ideal, especially on the first day. A patio against a south-facing wall works well. Take them back inside before the temperature starts to drop in the afternoon.
Day 4 to 6: Half a Day Outside
Now give them four or five hours outside. They can be in slightly more direct sun by now and they will start to look stronger and stockier. Keep bringing them in at night.
Day 7 to 9: A Full Day Outside
Leave them out for the whole day. They should be coping well at this stage, and direct sun and wind exposure are now part of normal life. Bring them inside in the evening before night temperatures drop.
Day 10 to 14: Overnight
If overnight temperatures are above 10°C and no frost is forecast, leave them outside overnight. Start with a sheltered spot, ideally with some kind of cover or against a wall. After two or three nights they should be tough enough to plant out properly.
Day 14 Onwards: Plant Out
By now your plants are properly hardened off. They will go into the ground and keep growing rather than stalling for weeks. Water them in well after planting and they will be away.
How to Harden Off Tomato Plants
Tomatoes are the plant most people ask about when it comes to hardening off, so they deserve their own section.
Tomato plants grown on a windowsill from March or April are some of the most coddled plants in any UK garden. They have never felt wind. They have never had bright direct sunlight on their leaves. The leaves can actually scorch if they go into full sun without being eased into it. This is why so many people put their tomatoes outside in May and watch the top leaves go white or crispy within a couple of days.
The fix is the same gradual process described above, but with one extra rule. Watch out for sun scorch in the first few days. If the leaves look bleached or have white patches, move them into shade for the next session and try again the day after with a few minutes less direct sun.
Most UK tomato growers start hardening off in the last week of April or first week of May, with the aim of planting out from mid to late May. When to Plant Tomatoes in the UK covers the full timing.
How to Harden Off Courgettes and Other Tender Crops
Courgettes, cucumbers, squash and pumpkins all hate cold and wind even more than tomatoes do. They need a slightly slower hardening off process and absolutely should not go outside until overnight temperatures are reliably above 10°C.
Start them off later than tomatoes. Mid May is a good time to begin hardening them off in most parts of the UK, with planting out from the end of May or early June. Even one cold night can set them back badly. If your courgette seedlings have been struggling already, Why Courgette Seeds or Seedlings Fail in the UK is worth a read.
What If You Forget to Bring Plants In?
It happens. You put plants out for the day, get distracted, and remember at midnight when it is 4°C outside. What now?
Honestly, just leave them. Going out at midnight to bring them in does not help much because the damage if any is already done. Check them in the morning. If the leaves look fine, you got lucky and you can carry on as if nothing happened. If they look bleached, drooping or damaged, bring them in to recover and start the hardening off process again from a few days earlier.
Tomatoes and courgettes can usually bounce back from one cold night. Damaged growing tips are slower to recover than damaged lower leaves, so check those particularly.
Common Hardening Off Mistakes
The mistakes are nearly always the same.
Skipping it altogether. The most common mistake by far. Plants moved straight from indoors to outdoors almost always struggle, even in mild weather. The two weeks really do make a difference.
Rushing through it. Doing the whole process in three or four days is not enough. Plants need time to physically toughen up. Two weeks is the minimum and even that is sometimes not enough for very pampered plants.
Putting them out in full sun on day one. Strong direct sunlight is one of the harshest things plants experience after being indoors. Always start in shade or filtered light and work up.
Leaving them out overnight too early. A clear, still night in early May can drop to 2 or 3°C even when the days have been warm. Overnight temperatures matter more than daytime ones for tender plants. If you are caught out by an unexpected frost, How to Protect Vegetables from Frost in the UK covers what to do quickly.
Forgetting about wind. Wind dries plants out and physically stresses stems. A sheltered spot for the first few days makes a real difference. Direct exposure to a stiff breeze on day one is too much for plants that have only ever known still indoor air.
FAQs
How long does it take to harden off plants in the UK?
About two weeks for most tender plants. You can do it in 10 days if conditions are mild, but rushing it usually causes more problems than it solves. Two weeks is the safe answer.
Can you skip hardening off?
You can but you almost certainly should not. Plants moved straight from indoors to outdoors stall, suffer leaf damage, and often take longer to recover than the two weeks hardening off would have taken in the first place.
What temperature should plants be hardened off at?
Above 10°C overnight is the safe rule for most tender plants. Daytime temperatures between 12 and 20°C are ideal for the daily sessions outside.
Can I harden off plants in a greenhouse?
A greenhouse helps because it gives plants more light and a more variable temperature than indoors, but it is not a complete substitute for hardening off. Plants from a greenhouse still need a few days of being outside before being planted out, particularly to get used to wind.
What happens if I harden off plants too early?
Cold damage. Tomatoes, courgettes and cucumbers in particular can be set back badly by overnight temperatures below 10°C. Wait until at least mid-April for most tender crops and look at the forecast carefully.
Do I need to harden off plants from a garden centre?
Usually not, if they have been kept outside at the garden centre. Ask if you are not sure. Some garden centres sell plants that have been kept under cover, in which case yes, harden them off for at least a week before planting out.
A Sensible Place to Start
Hardening off is one of those gardening jobs that sounds fiddly but really is not. A few hours outside in a sheltered spot, building up gradually over two weeks, and that is it. The plants do all the actual work themselves.
If you only ever do one new thing in your garden this May, make it this. The difference between hardened off plants and plants that went straight outside is huge. Once you have done it once and seen the results, you will never skip it again.
For everything else worth doing in the garden this month, May Gardening Jobs in the UK covers the full list.