How to Harden Off Indoor-Sown Plants

, written by Benedict Vanheems gb flag

Hardening off seedlings

It’s a proud moment: planting out all those seedlings and plants you’ve so carefully raised indoors over the past few months. It’s important to take steps to acclimatise them to their new outdoor home however, or you risk losing your plants and wasting all that hard work. This is a process known to gardeners as ‘hardening off’.

Hardening Off

Hardening off should take a minimum of a week and may take up to two. Suddenly moving plants from a stable environment to one with wide variations in temperature, light and wind can seriously weaken plants.

For most plants, start hardening off about a week before the final frost date for your area. Our Garden Planner uses data from your nearest weather station to give an indication of when it’s safe to plant outside, providing a helpful guide to work back from.

Hardening off young plants in the greenhouse

An intermediate home, such as an unheated greenhouse or cold frame, is a great tool for hardening off. Place seedlings and plants into the structure for a couple of hours on the first day, then gradually increase the length of time they are in place by two or more hours per day. After a week they can then be left there overnight, so long as there’s no danger of frost. These structures have the added advantage of windows or doors that can be opened increasingly wider with time to gradually make under cover conditions more similar to those outside.

Harden Off Plants in a Sheltered Spot

Wind speeds up evaporation, which can cause plants to wilt surprisingly quickly. Choose a sheltered position to harden off your plants, and water them before they go outside so there’s less risk of them drying out.

Pots can be clustered into crates, tubs or buckets. This not only stops them from blowing over, but acts as a windbreak around the foliage.

Sheltering seedlings in a bucket to aid hardening off

Avoid Bright Sunshine

Sudden bright sunshine on delicate plants can cause leaves to scald – the plant equivalent of sunburn – so begin hardening off on a still, cloudy day when temperatures are fairly steady. Set plants out for two hours in a shady part of the garden. The next day, leave them out for two more hours, with perhaps an hour’s direct sunshine in the morning. Gradually increase the length of outdoor time and direct sunshine over the course of one to two weeks.

Remember that areas of shade will move over the course of the day. If you can’t be at home all of the time, pay close attention to where and when the shade and sun fall. You could also use fleece to shield delicate seedlings from strong light – simply drape it over the top and tuck it in at the sides so it can’t blow off.

Introduce Plants to Cool Nights Gradually

In regions with cold winters, plants will need to be prepared for the cooler nights experienced earlier on in the growing season. This is especially important for tender plants such as tomatoes and peppers, which are easily damaged by low temperatures.

If you don’t have a greenhouse or cold frame, just set your plants outside for increasingly longer periods of time. Towards the end of the hardening off period you can use fleece or row covers to protect foliage against the chill of night. Even after crops have been planted into their final positions, be on hand with crop protection to guard against any unexpected late cold snaps.

Hardening seedlings off in a cold frame

Tips for Hardening Off Success

As well as these guidelines, it’s worth considering some further points. First, avoid placing plants on the ground where they can easily be knocked over by birds or nibbled by slugs.

If you have the space, grow a few more plants than you need so you can hold some back, just in case. If you don’t need them you can always give them away.

Remember that bought-in plants may also need hardening off, particularly if they have been kept in sheltered conditions.

And finally, don’t hurry the process. Hardening off takes time but will give you stronger, more resilient plants that will ultimately be more productive.

If you’ve got a tip on hardening off plants that you’d like to share, please leave a comment below – we’d love to hear it.

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Show Comments


"I harden-off my plants using home-made hoop-house covered in window screen material. "
Brian on Saturday 25 March 2017
"Hi Brian. That's a great idea - it will shield the plants from strong sun and wind, giving them a chance to settle in."
Ben Vanheems on Tuesday 28 March 2017
"It's important to get your plants used to wind throughout the hardening off timeframe as well. It not only prepares them for wind, but also it will strengthen the stems if done periodically when still young. I've used an oscillating fan on low when seedlings are a little bigger."
Gail Epping Overholt on Saturday 9 March 2019
"Thanks Gail. I've heard that lightly brushing over the top of your plants a couple of time a day has a similar hardening off effect to a gentle breeze."
Ben Vanheems on Monday 11 March 2019
"The past few years I've been using Floating Row Cover, rather than moving the plants as much. I start with 3 layers, then take off a layer every couple days. So far, no damage to the plants and they've done well. I watch for cloudy days to start this process."
Jeff on Friday 10 May 2019
"Thanks for that Jeff. I guess it's the same with us humans: wear lots of layers and you can remove/add them according to the temperature!"
Ben Vanheems on Monday 13 May 2019
"How would I calculate the daily water needs of plants in hardenening off areas?"
Stacey Spasojevic on Tuesday 10 September 2019
"It all depends on the weather, temperature, size of pots/plug trays and a number of other factors. Generally the best way to tell whether something needs watering is to lift it up and judge the weight of it. If a pot is very light then it probably needs watering. Over time you get a feel for this and it becomes more intuitive."
Ben Vanheems on Tuesday 10 September 2019
"Are my tomato seedlings beginning to yellow because Im moving too fast with the hardening off process? ( Leave them outside all day but more often thatn not its been overcast) "
Gillian Jones on Monday 14 October 2019
"It could be that, but also it could be due to the nutrients contained within the potting mix being exhausted. Continue to harden off, being careful not to go too fast, too soon. Once they are planted the roots will have more resources to draw on and should soon settle in, helping plants to regain their verdant green."
Ben Vanheems on Monday 14 October 2019
"Could you give me some information on the cold frame/small greenhouse shown in this video? "
Jennifer West on Monday 13 January 2020
"Hi Jennifer. The greenhouse in the video is a small Gardman wooden plant house. You can find it on Amazon by searching for 'wooden polycarbonate growhouse'. I use it mainly to harden plants off in spring, but also to start some seedlings off. To vent it you simply open up the doors and lid. I also overwinter a few plants in there, removing the shelving if necessary, which just slot in and out."
Ben Vanheems on Monday 13 January 2020
"Can tomatoes and peppers go from my basement to a greenhouse that is covered with a shade cloth to harden off? "
Brian on Thursday 20 February 2020
"Hi Brian. Quite possibly, yes. Just make sure the greenhouse doesn't get close to freezing - be on hand to cover plants with a layer of row cover/horticultural fleece or similar if it looks to get too uncomfortably close to freezing point. Shade cloth may not be necessary if the sun isn't very strong where you are - at this time of year plants welcome all the sunshine they can get. But that is assuming you don't live in a very hot climate!"
Ben Vanheems on Friday 21 February 2020
"I started all my seedlings in an unheated sunroom. It has tons of light. Temps have been staying in upper 30 to 40's at night for a month and won't hit 50's until almost mid June. My issue is the beans and peas are getting leggy and really need a trellis. If our last frost date is last week of May(25th), but night temps will still be in 40's, which they have had in the sunroom for a month, can I start hardening now and put out end of May? They are getting too tall!"
Julianne Raines on Monday 10 May 2021
"Yes, I'd get on and harden them off immediately! At this stage they'll be craving better light and time outside will do them a lot of good."
Ben Vanheems on Wednesday 12 May 2021
"I've transplanted my tomato, pepper and eggplant seedlings from their seed tray to 4x4 cups already. But they are now outgrowing those, and need supports to keep them from falling over. Will transplanting them to bigger parts AND hardening at the same time cause too much stress on the plants. I'm on Day 3 of Hardening off. Any suggestions??? ~Signed "The Panicked Gardner" "
Chanda on Sunday 16 May 2021
"Hi Chanda. You're probably almost there with hardening off. I would just stick with it and maybe get them out in the next two to three days if there's no risk of frost. But transplanting them to bigger pots certainly wouldn't do them any harm - so you could do that as well if you'd prefer to harden off for a bit longer."
Ben Vanheems on Monday 17 May 2021
"Very helpful. I only wish I had discovered all of this earlier, before half- killing my latest batch of greenhouse seedlings. A thought: cloches seem to offer a possible halfway house."
Eric on Tuesday 25 May 2021
"That's a good thought Eric - cloches are a great way to ease plants in to their new outdoor home."
Ben Vanheems on Tuesday 25 May 2021
"Hi, I've got loads of tomatoes, pumpkins, beans etc in pots in my unheated greenhouse. Can I harden them off by just opening the greenhouse door for a couple of hours a day? "
fay Foster on Saturday 7 May 2022
"Hi Fay. That would be a great way to start hardening them off. To be honest, that's all I have done, but best practise would be to then leave them outside - probably in a sheltered spot - for increasingly longer spells. But if the weather is mild (and set to be mild) when you plant out, I think leaving the doors and windows open would be just fine as a way to harden them off."
Ben Vanheems on Tuesday 10 May 2022
"Great article for a first-time veg grower; thanks! Hubby bought the planner subscription, and that has been really helpful, too. I was wondering if there is anything different you need to do with respect to hardening off if growing seedlings in soil blocks. Of course, I can't put them out if it's raining, because the soil blocks would dissolve, but are there any other considerations?"
Chelle on Friday 20 May 2022
"Hi Chelle. I would think the hardening off considerations would be exactly the same as any other plant. Just be sure to keep them well hydrated so they don't struggle, particularly if it's warm and windy."
Ben Vanheems on Monday 23 May 2022
"Good Articles"
indoorplants on Monday 20 March 2023
"Hi, can you leave plants all day in unheated grrejouse for the day?"
Melissa St-Onge on Friday 12 May 2023
"Hi Melissa. Yes you can. Greenhouses are a great first-step in hardening off plants that have been indoors, and you can then go from greenhouse to outdoors gradually. Please be aware, though, that frost-sensitive plants will still need protecting from frost - so just be careful if it's a particularly cold night as plants inside a greenhouse could still be vulnerable."
Ben Vanheems on Monday 15 May 2023
"Thanks for all the helpful information. This is my first time hardening off seedlings. If I harden them off gradually, perhaps over 2 weeks, but then have a delay in planting (3-4 days,) should I leave them outside 24 hours a day before planting or bring them back inside for periods?"
Mary Arnould on Saturday 4 May 2024
"I would keep them outside, once hardened off, till you're ready to plant."
Ben Vanheems on Tuesday 7 May 2024

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