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hardening off - wilting plants

peter pepper

peter pepper

Hey guys -

my plants are doing pretty good considering I had no grow lights and this is my first year. but I have a question about hardening off. i set my plants outside for the first time this past weekend when it was sunny, breezy and about 80. they were placed in a box on my table on the deck. they were in direct sunlight minus whatever cover the clouds gave. i could see from the breeze they were moving too. i only left them out there for about 1.5 - 2 hours and brought them back in. later that night i see they all looked wilted and bad, but then in the morning they were all perked back up. i take it this is usual? why do they do this and what is it called?
 
Plants have structures on the bottom of the leaves called stomata that regulate gas exchange. They're essentially portholes through which Oxygen and water vapor escape, and Carbon dioxide can enter. The stomata open when conditions are favorable for photosynthesis; when photosynthesis is "induced", and close when conditions are less favorable. When closed, the plant can maintain turgor pressure in the cells by limiting water loss, and thus remain nice and stout.

However, when your plants are inside, they aren't exposed to much wind, high temperatures, or extreme light conditions, all of which can increase transpiration (read gas-exchange). Thus, they're stomata have developed in an environment in which they do not need to close quickly with changing conditions. So when you stick them out under full sun, high winds, and high heat, they can't close the portholes fast enough and H2O vapor leaks out so fast your plants wilt. Gradually acclimating your plants to these conditions gives them a chance to adapt to these conditions over time.
 
Very well said 5star! I've seen this in action many times, but never had a clue as to why. Great info.
 
alright thought of another question: as i harden off how much do i increase the time outside? i was thinking of 1/2 to 1 hour each time. is this too fast or good?
 
I usually play it by ear but try to push them at least 15-20 minutes extra each day....but it all depends on daytime temps, what they look like, etc etc.
 
I don't know why but mine never wilted at all this year. I moved them outside Sunday evening and Monday was bright, very sunny and temps in the low 80s. They were in the sun for about six-seven hours, maybe a little more. I had planned to move them to the shade after a couple of hours but did not see a reason to.

Mike
 
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