How to Successfully Harden Off Seedlings

, written by Jeremy Dore gb flag

Cold frame

It has been said that being a good parent involves first learning to have your child around you 24/7 and then learning to let them go.  I think raising seedlings is very similar.  At first giving them all the attention they need to have the best possible start is the vital thing.  But soon it’s the letting them go that’s important, particularly when prize seedlings that have been nurtured for two months are about to face the outdoor world.  As the grand day approaches, the last thing you want is a late frost killing them off, so learning how to ‘let them go’ is crucial...

‘Hardening off’ is the vital process of getting seedlings ready for the big outdoors.  Without this vital step it is all too easy to lose precious plants or have them wilt from the sudden change in conditions between a warm windowsill and a draughty garden.  How best to harden plants off is the subject of much debate amongst experienced gardeners, so I thought I would round up the best advice available:

Hardening off seedlings

Seedlings need to be gradually introduced to the outdoors in stages over a period of at least a week and preferably two.  This generally involves taking them outside for just a couple of hours each day at first and then gradually lengthening that time until they are ready to spend a whole day and eventually nights out.  This time is required for them to adjust due to the:

  • Greater range of temperatures outside
  • Chill factor, moisture loss and buffeting of moving air
  • Greater variation in water supply due to the above factors which increase evaporation from plants outside
  • Direct strong sunlight which can scorch young tender leaves

To ease this transition, an intermediate stage can be introduced – either a greenhouse, cold frame or row cover.  Cold frames and greenhouses have the advantage that they can be opened up more and more over a few days, providing a further element of control.

Personally, I move my plants into my greenhouse for increasing amounts of time.  Over the last two weeks they have gone from a couple of hours in the weak morning sunshine to whole days out and now finally overnight when a frost doesn’t look likely.  As my greenhouse is unheated, the transition from there to outdoors will be easier and will probably just take a weekend.  I now have tomato plants, looking healthy and strong, surviving an overnight low of 40 degrees Fahrenheit (4 degrees Celsius) and ready for potting up or planting in the greenhouse bed.

Seedlings ready to plant out
Well-grown seedlings ready to plant out

Here are some tips I’ve found useful:

  1. Watch the weather closely – the best time to harden plants off is on wet or overcast days under cover of a greenhouse or cold frame. Overcast days don’t stress the plants with high afternoon temperatures and are unlikely to produce hard frosts at night.
  2. Once hardened off and planted out, still watch the weather – even if a late frost is forecast, plants can be covered with fleece or bubble-wrap and still survive
  3. When taking plants outside, don’t place them on the ground where slugs can access them or birds can knock them over looking for grubs.
  4. At first, limit the amount of direct sunlight they receive – choosing a place which is shaded in the afternoon.
  5. I always ‘hedge my bets’ and keep a few plants back inside as an insurance policy, just in case an unexpected frost kills the main batch off

One tip I haven’t tried is ‘tickling’ tomato plants.  It sounds crazy but some people swear by it – just brushing the tops of tomato seedlings with your hand or a piece of paper a couple of times each day is said to toughen them up a bit before hardening them off properly.  Apparently there is some research to support this method, though it could all be down to the extra vigilance of regularly checking up on the plants while doing so!

Whatever happens, it’s rapidly approaching that time to let them go and let the real growing begin.  Roll on the warmer weather!

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Show Comments


"Hi, I try to grow lots of veg from seed, I use a cold greenhouse, my problem is the plants get leggy very quickly before they appear strong enough to move out to a cold frame or plant out. what am I doing wrong? "
Mick Hallam on Friday 7 November 2008
"Mick, there are two causes of 'leggy' plant growth: insufficient light and a sheltered environment. To combat insufficient light you need to make sure that the glass is really clean, the greenhouse is not over-shadowed and you don't start your seedlings too early when light levels are poor. As for the sheltered environment, I've been told that fanning the plants with a large sheet of card a few times each day does the trick - forcing them to grow stronger stalks - although I have to say that sounds like excessive work to me. Trying to get a breeze going through the greenhouse on sufficiently warm afternoons is easier and is what I do."
Jeremy Dore on Saturday 8 November 2008
"Thanks for that Jeremy, I'm getting to like this Blog caper, I have just thought, could it be the compost I use, which is just general purpose bought stuff, I'm afraid I don't know the difference between compost types.where can I find out what's best?"
Mick Hallam on Saturday 8 November 2008
"Mick, I have had poor quality compost cause problems with seedlings - one year I bought an old batch of organic compost but the symptoms weren't that plants got leggy. Instead they germinated and grew well for the first few weeks and then just stopped growing. By the time I realised that they were staying small sized it was too late to re-sow the tomatoes and peppers. So it's certainly worth buying fresh good quality compost but I don't think it would cause this particular problem."
Jeremy Dore on Sunday 9 November 2008
"How do I harden off young plants a few hours a day when I am out at work? So far they have to go out all day or not at all this is where I think gardening gets hard. All gardeners talk like we're all home all day. - Not critism I think your sight is great - just frustrated."
Michele Key on Tuesday 17 March 2009
"Michele, It is possible... what I do is just start the hardening off process at the weekend. That way by the time Monday comes they are ready to be out for most of the day. In an ideal world they would probably get longer with just a few hours but in the real world this works OK."
Jeremy Dore on Tuesday 17 March 2009
"Talking about plants getting leggy, I prick off the top leaves and this helps to bush them out. I have done this this year and now all the 882 seedings I have in my greenhouse are coming along wonderfully, and bushy. Some of you could try this. "
Joan Angel on Wednesday 27 May 2009
"joan...are you pricking off the top leaves of tomatoe plants? herbs? thanks for your advise.........Dee"
dee on Thursday 8 April 2010
"In America we hear the term "pinch off" a lot. I've never understood that. Is that the same as "pricking off the top leaves?" And do we have to wait until there are two sets of leaves to take off the top one?"
Clarice McKenney on Friday 16 April 2010
"Hi Clarice, yes 'pinch off' is the same as 'pricking off the top leaves' but it isn't something you usually do until the plant is quite large and you want it to stop growing upwards and bush out to produce more edible crop."
Jeremy Dore on Saturday 17 April 2010
"Hi, I have little Echinacea seedlings started and they're not very tall yet, just about an inch over the "soil." The weather is getting increasingly wonderful and I was just unsure about how big my plants need to be before I harden them off? Like I said, they are about an inch above the "soil" and only have two "leaves." This is my first time doing something from seed and I need some help ^^;;; Much thanks! Peace and Love! "
Taiga on Saturday 10 March 2012
"I refuse to use chemical insecticides in my garden, even "safe" ones. Here in Texas, however, fire ants are a real menace. What organic measures can I take to rid my garden of these little terrorists?"
Todd Hiser on Tuesday 2 April 2013
"Greetings! I've been reading your site for some time now and finally got the bravery to go ahead and give you a shout out from Austin Texas! Just wanted to mention keep up the excellent work!"
Gunter on Thursday 15 August 2013
"Is there a recovery method for plants damaged by a hastened hardening off? My plants have lost there deep green and are now pale and a bit wilted. They are inside now until they look better, then I'll try again. This time I'll set a timer.."
Kevin Lingenfelter on Friday 18 April 2014
"Ants hate cucumber! Peel a cucumber and make a barrier. No ants will touch it. "
Danielle on Saturday 3 May 2014
"I'm new to gardening, and after a lifetime of 'duty' gardening, ie, keeping the grass cut and the weeds down, have recently developed an interest in the garden, and have a small poly tunnel. I have so much stuff growing in there that taking it all out to harden the plants off would take ages every day, so I have been opening the flap, which faces the prevailing wind, every day, also opening the 'Windows' to allow good air circulation. I'm also leaving the 'Windows' open at night, although I'm zipping up the door at about 6pm. I have lots of tomato plants, sweet peas, surfinia and geraniums which are very well grown, and wonder if what I'm doing will sufficiently harden them off to plant out soon. Please advise."
Rachel Forrest on Sunday 16 April 2017

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