How much does temperature matter when hardening off plants?

bart1(6/7 Northern VA)March 20, 2014

I know you're supposed to gradually introduce new seedlings/plants to more and more direct sunlight to get them used to the powerful rays of the sun compared to grow lights.

What about temperatures? Do the plants care if it's say 45 degrees when they're set out in the early morning?

Being a working stiff, when I leave in the morning it's often much cooler than it will be by the afternoon but by the time I get home, there's very little sunlight left for hardening off.

So will I stunt the plants by putting them out on cool mornings?

I'm asking specifically about broccoli now but the question goes for all plants. Even when I'm ready to harden off tomatoes and peppers we still have some cool mornings.


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Temperature plays a smaller role than many think but keep in mind that when you set plants out, unprotected, the wind can be a problem, especially with low temperatures.

Some means of protection is usually best but I like to be there to monitor the response of translocated plants. You can tell alot in the first 20-30 minutes when plants are placed into a new environment.

    Bookmark   March 20, 2014 at 10:05AM
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With cool-season plants like broccoli, 45 is fine. I like to harden off other cool-season crops (but not broccoli due to the buttoning risk) with temps. in the low to mid 30s to encourage frost tolerance.

With warm season crops, especially peppers, try not to expose them to anything under 50 degrees. Soil temperatures under 55 degrees drastically decrease their ability to uptake water.

    Bookmark   March 20, 2014 at 11:46AM
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wayne_5 zone 6a Central Indiana

As already said, 45ð and up is ok for cool weather plants. Things like melons need to be 50ð or more.

I understand your problem of setting out plants when it is too cold but gets warmer shortly. I am retired and can care for the plants perfectly most days. I start taking them outside from the get go. Also unattended plants may get hit by a storm...not good at all. I have a south and a north place to protect from strong winds

    Bookmark   March 20, 2014 at 12:40PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Hope you don't mind if I just link you to a recent discussion of this on the Tomatoes forum. Saves typing it all again.


Here is a link that might be useful: Hardening off while working

    Bookmark   March 20, 2014 at 2:25PM
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I've seen accounts that say prolonged exposure to temps between 35 - 50 will cause bolting and buttoning in brassicas. From which I assume that even lower temps would be worse, which is what we're getting next week.

My first batch of seedlings are already getting oversized. I've put them out to harden, but I doubt if I'll be able to plant them within a month.

    Bookmark   March 20, 2014 at 2:56PM
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bart1(6/7 Northern VA)

Thanks all!

Dave that link is great. I had no idea that the first day or two of hardening off required NO direct sunlight. I aways figured an hour or two of direct sun was fine (and then build up time from there)

    Bookmark   March 20, 2014 at 6:22PM
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wayne_5 zone 6a Central Indiana

bart, If the plants are of some size, then they need to be broken in more slowly. With my setup they can be out all day from the get-go when the weather is don't shade newly germinated seedlings that are garden sown!

This post was edited by wayne_5 on Thu, Mar 20, 14 at 21:42

    Bookmark   March 20, 2014 at 9:41PM
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I'm a working stiff too. When I start hardening off my plants, I set them outside on partly sunny or cloudy days and have them right next to the house. That way they only get a few hours of direct sunlight. I bring them in at night for the first week or two. I gradually (every couple of days) move them further away from the house so they get more sunlight.

    Bookmark   March 21, 2014 at 8:53AM
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