How to harden off plants (and tickle tomatoes)

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Hardening off plants - feel free to use photo as long as you link back to
Hardening off plants (

One of the joys of sunny days is that they call you out into the garden even when you’ve got proper grown-up things to do (which is why gardening and freelance writing work so well together, because you can garden by day and write stuff to pay the bills by night).  I decided to start hardening my seedlings off today because there’s still a reasonable amount of cloud cover (baby seedlings freak out in direct sunlight if they’re used to being indoors). Basically, this means getting plants used to being outside after growing up n a windowsill or indoors. (Should you be interested, the above picture shows peas, strawberries, garlic, tomatoes, pumpkin, mint and cucumber, all of which will be moving outside at various stages over the next fortnight or so.)

Generally speaking, plants don’t like being shocked (though there are some that respond best to harsh treatment, which I’ll write about another day.) As such, you have to ease them into outdoor living gently, over the course of a week or two, so that they get used to sun, wind, rain and all the other things they’ll have to deal with outdoors. It’s incredib ly simple, if a tad inconvenient should you have a day job.

  • Day one: Put your plants outside for two hours.
  • Day two: Put your plants outside for four hours.
  • Day three: Put your plants out for six hours.
  • Day four: Put your plants outside for eight hours.
  • Day five: Put your plants outside for ten hours.
  • Day six: Put your plants outside for 12 hours.
  • Day seven: Put your plants outside for 14 hours.
  • Day eight: Put your plants outside for 16 hours.
  • Day nine: Put your plants outside for 18 hours.
  • Day ten: Put your plants outside for 20 hours.
  • Day 11: Put your plants outside for 22 hours.
  • Day 12: Introduce your plants full time to the joys of outdoor living.

It doesn’t have to be this prescriptive – judge how long to leave your plants outside depending on how well they’re thriving, and look at the weather forecast first – if it predicts frost or scorching sun, keep your plants indoors. If you put them outside and the weather becomes cold, scorching hot or frosty, bring your plants back in again even if it’s before the allotted time as it may be too much for them to cope with otherwise. The further along the process you are, the more they should be able to deal with – after two or three days they should be able to cope with a sunny afternoon (they’re only this high maintenance to start with – the older a plant gets, the less demanding it tends to be, so have patience.)

If you start the process but the plants don’t respond well, bring them back in and leave it for a week or so before trying again: some seed packets include details of when to harden off the seedlings, so read what the packet says if you’ve had the foresight to keep it (I keep all my seed packets in a folder so they’re easy to refer to).

Keep your seedlings well-watered (but not overwatered) when you start the hardening off process to help give them as much strength as possible to deal with the experience of being moved outside, but gradually reduce the amount of water you give them with every passing day and don’t fertilise them until hardening off is complete to make them hardy – this is tough love. Make sure you keep your seedlings out of reach of slugs and snails so that they don’t think outdoors is full of predators (they’ll learn, given time, but why take their innocence so early?) and cover them with a net if you have seedling-hungry birds on the prowl.

Overcast days are best to start hardening your plants off as they don’t put as much pressure on the seedlings who are only just getting used to this thing called weather, after a protected upbringing indoors. If you have a cold frame or greenhouse, so much the better as you can start them off in there and move them outside (in a similarly staged way) once they look strong enough. It’s as much about instinct as anything else – if a plant is unhappy it’ll show you by wilting or otherwise looking disgruntled.

If you want your seedlings to grow strong, it’s a good idea to tickle them. This trick is commonly used on tomatoes but I apply it to all my seedlings. It simply entails brushing your hand lightly over the seedling tops every day, to emulate the movement they’d naturally experience outdoors through wind blowing them around. It apparently makes them stronger, and I reckon it works, so for the sake of a brief stroke every day, it’s worth a go.

Plant your seedlings out on an overcast day that’s not too cold, so that they have a gentle first night outside and with any luck they’ll be happily growing in your garden in no time. If you’re worried about the frost getting your seedlings, once you’ve planted them outside, cover them with a cloche (cut a soft drink bottle in half and use the bottom half to cover the seedlings), fleece or bubble wrap at night to help keep them warm. Hardening off may sound like a faff but if you skip this step, you’ll probably lose all your seedlings and have to start again, meaning your produce will come later, so it’s essential that you put in the work. Sorry.


  1. Hard herbs tend to like rough treatment and will often sulk if treated too well in my experience. Think I may have written about it a while ago – will have a dig through the archives

  2. Did you ever write about the plants that LIKE a good shock? I’d love to know which ones they are. 😉

  3. VERY helpful- thanks! my tomato seedlings are still too little to cope, but will be putting all this into practice in a week or so…

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