Real Men Sow

What is Hardening Off?


This week, I tweeted about slipping my mangetout outside to acclimatise, or ‘harden off’, as it is commonly known.

‘What do you mean by hardening off?’ asked Ben, from the very likeable whoisben blog, and I thought to myself, ‘…there’s a useful blog post worth putting up on RMS’.

So, what is hardening off? Well, hardening off is something all of us are likely to need to do during the early growing season. If you raise veg plants indoors or in a greenhouse, they need to get used to different growing conditions outdoors.

Inside a greenhouse, the environment is very different – the temperature is warmer, the air more humid and there is little or no air circulation. If you move your plants out of this environment and straight into the new one on a permanent basis without gradual acclimatisation, then the plant can be damaged and die.

How to Harden Off
I’ve found that the best way to harden off plants is to let them adjust over a 2 week period. For the first week, bring the plants out first thing in the morning, and then place them back into the greenhouse for the night time.

For the second week, leave the plants outside permanently but cover with fleece if the temperature looks like it will be getting cold. Once into the third week, ditch the fleece and get ready to plant out onto your plot.

Hardening Off Seeds Grown In a Heated Environment
If you’ve started seeds off in a heated environment, such as a windowsill in your house, the seedlings will need to go into the greenhouse before being hardened off outside. Effectively you’re adding an extra stage and hardening them off twice, but it’s important to remember that the warmer the early growing conditions, the longer you’ll need to harden off.

Buying Seedlings from Nurseries and Garden Centres?
Watch out if you buy plants from nurseries or garden centres too. There is no way of telling how well these plants have been hardened off (if they have at all), so it is worth considering undertaking your own hardening off process before planting out.

Care and Organisation
When you’re hardening off, be savvy about how you look after the plants too. For example, don’t put them straight out into a gale. Try finding the plants a sheltered spot to begin with. Likewise, don’t make the first hardening day off an absolute scorcher if the weather has been dull for a period before.

Organisation is also key. It’s easy to forget to get out or put plants away (I’m useless at remembering before bed), so leave yourself a prominent note to remind you.

Get some trays to hold the plants as well. During peak sowing and growing times, there could be loads of plants being hardened off. Picking up a few trays to move outside is a lot easier than 50 odd individual pots every morning and night!


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  1. BenApril 3, 2015 at 8:31 amReply

    And now I understand!
    Nice one Jono!

  2. CatherineApril 8, 2015 at 10:12 pmReply

    It does get a bit overwhelming at times – I’m thinking that a greenhouse with retractable walls and ceiling might be useful if someone would invent it!

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About Real Men Sow

meIn 2007, I took on a redundant allotment plot with my gardening-mad mum Jan. As all good mums do, she went along with it, but I don’t think she held out much hope. However, over a decade later, and she now lets me do stuff without watching over my shoulder, so I must be doing something right. [ read more ]

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