Fruit & Vegetable Gardening - Gardening Forums

Hardening off

Interested in growing a vegetable garden or a fruit garden? Whether you’re gardening indoors or outdoors, planting an edible garden can be easy and fun

Hardening off

Postby Dumbo » Mar 20, 2012 7:28 am

So my head isn't wrapping around some things to do with hardening off.

1: In most of the articles I read they state to put veggies or flowers out for only a couple of hours a day and gradually increase that time while at the same time lowering the amount of water they get over the course of 7 to 10 days.

Other sites/articles state to do this over the course of 3 weeks.

Which is it?

2 (two part question): Most all sites say instead of doing the process in #1, put your plants in a cold frame or green house thing.

So being an impulse buyer I bought one of those cheap 4.5-feet high mini-greenhouses. Then I realized how ultra-thin the plastic is. It's tough and sturdy, but ultra-thin.

So, confusion A:
If they say expose your plants to the sun for less than 2-hrs a day and gradually increasing that for 7 to 21 days, why do they in the same breath tell you to toss it in the sun in a cold frame or greenhouse 24/7?

Isn't this kind of contradicting?

Confusion B:
If I do toss my seedlings in that mini-greenhouse I bought, and if the plastic is too thin for nights that get to 10C or so, can I just toss a tarp over it at night and take it off in the morning? Will that work?

3. Since the weather here is kind of kooky this year (25C for the next few days, then it's supposed to snow) is there any issue with putting them out then bringing them in again for a couple of weeks, then putting them out again? Does hardening off only need to be done once? Or is it the type of thing that needs to be repeated for them to re-acclimatise?
User avatar
Posts: 2070
Joined: Feb 06, 2012 11:20 pm
Location: Zone 5

Re: Hardening off

Postby CdnChelsea » Mar 20, 2012 8:28 am

Confusion A
Exposing the seedlings for only 2 hours a day will cause the seedlings to grow tall and spindley because they are reaching for the sun; then they will flop over. Your plants should have a minimum of 8 hours of light a day.

Leaving the plastic cover on over your seedlings and setting them outside for the day will fry them. The sun will bake your plants unless you lift the corner of the plastic lid to let air in. Seedlings grown in a proper greenhouse will be ok as long as the vents are open to allow the greenhouse to cool down a bit and will create air circulation.
If I left my greenhouse closed up, the temperature in there would be around 100 F. Too hot for anything to grow.

Confusion B
If the nights are cold, I bring my seedlings inside every evening and if the weather is nice during the day, I put them back outside in the greenhouse. It is a lot of work to bring them back and forth every day, but I find it is worth it.

When I first started gardening, I too thought I could just throw a blanket over the seedings that were outside and they would be ok. I was wrong. Everything died. Lesson learned the hard way and I had to start over.
"Those who dwell among the beauties and mysteries of the earth
are never alone or weary of life" ~ Rachel Carson
User avatar
Posts: 1106
Joined: Nov 06, 2010 11:26 pm

Re: Hardening off

Postby kelly_m » Mar 20, 2012 9:14 am

Oh my...My post just disappeared!

A good suggestion would be to put your seedlings out in the shade or filtered light to start. Although they are not getting direct sunlight, they are getting undiffused light, better than under plastic or through a window.

Then gradually increase the amount of direct sun they get.

Keeping them under the plastic cover, in full sun in these temps will fry them. And putting them outm, even for 2 hours, with no cover risks sunburn!

And CC's suggestion to bring them in when the nighttime temps get to low is spot on...a PITA but well worth it if you don't want to start over!!
Zone 5a/b

User avatar
Posts: 5905
Joined: Nov 14, 2006 12:14 pm
Location: Smaller town Ontario, Zone 5a/5b

Re: Hardening off

Postby Smitty » Mar 21, 2012 9:29 am

I put mine out in cold door during day and close it up at night
""Life isn’t about waiting for the storm to pass. It’s about learning to dance in the rain."
Smitty BBS :-)
User avatar
Posts: 5299
Joined: Jun 11, 2008 2:07 pm
Location: manitoba zone3

Re: Hardening off

Postby OGrubber » Mar 21, 2012 9:36 am

Sometimes research results in information overload.

Hardening off is simply acclimatizing pampered seedlings to real life outdoor conditions of intense sun and wind.... and there are many, many ways of doing it.

As Kelly suggested, a sheltered spot with shade or dappled light is probably the easiest way to go, and requires the least amount of fuss by you. It should only take about 7 to 10 days if you increase exposure daily, longer if exposure is by a less frequent regime. Also it's important to note that the time frame or frequency of increased exposure is dependant on the strength/health of your seedlings when you bring them out. Seedlings that are spindly would need more care and time, for instance.

The references to cold frames and greenhouse thingy's would be about a secondary/alternative use for these tools - and it would be to create a "sheltered" spot, as in a wind buffer, if you didn't already have one. However, as CdnChelsea pointed out, using the greenhouse thingy, you run the very real risk of frying your seedlings. You'd have much better control putting your seedlings into an open cardboard box where the sides were as tall as the height of the seedlings - and that is what the cold frame would be used for as well. Also, a cold frame would allow you to trap some heat at the end of the day, giving seedlings a couple of degrees advantage overnight if you were leaving them out.

Temps - this one is tougher. It's easy to say 10 C is the cut-off point, but the truth is that some veggies can tolerate lower temps and some are ultra temp sensitive. Basil comes to mind for not tolerating cold. ....
Market Gardening - Just another day at the plant.
User avatar
Posts: 3026
Joined: Nov 14, 2006 5:17 pm
Location: Ontario, 6a

Re: Hardening off

Postby Dumbo » Mar 21, 2012 3:54 pm

OGrubber wrote:information overload

Likely it.

I was going to toss them under the plastic. Good thing I asked first.
User avatar
Posts: 2070
Joined: Feb 06, 2012 11:20 pm
Location: Zone 5

Re: Hardening off

Postby angelinaottawa » Mar 21, 2012 9:33 pm

in ottawa, it has been nice and i am leaving my coleus outdoors. 100 of them germinated for me.
now, it is hardening phase, right?
i just thought, i have to bring them in by sunday, when it is suppose to snow?


"A sustainable, eco-friendly community starts with you and me!"
My blog is at
User avatar
Posts: 56
Joined: Sep 28, 2011 10:40 am

Re: Hardening off

Postby Eeyore » Mar 21, 2012 10:48 pm

Coleus is very frost sensitive so definately bring it in if it is going to snow.

Angelina check your PM's.
AB, Zone 3A
“Those who say it can't be done are usually interrupted by others doing it.” ` James Arthur Baldwin"
User avatar
Posts: 11182
Joined: Nov 14, 2006 10:47 pm
Location: AB, Zone 3A

Return to Fruit & Vegetable Gardening

Who is online

Users browsing this forum: No registered users and 7 guests

Follow Style At Home Online



Latest Contests

more contests