Final clarity requested on the issue of Hardening Off...

[huntsman]

I got to thinking this through today (always dangerous!) and I don't understand the idea of introducing your plants to sunlight gradually.

They spend 24 hours a day under lights, which at this time of year make the plant hotter than the sun does, so why take them outdoors for an only hour or two at a time?? Is it perhaps the colder temps outside compared to your grow room, rather than the Sun's rays that are an issue?

I had mine outside yesterday for two hours and today for four hours and there is no sign of stress, so if someone can clarify this point I'd be most grateful.

As long as they are hydrated, what danger is there to them spending every day, all day outside, even at 10cm?

 

[POTAWIE]

The sun is more intense than your indoor lights and can quickly fry your young plants, and you need to harden off for wind as much as for sun

 

[MrArboc]

It's the combination of the intense light of the sun, the wind and usually lower humidity that gets them.

Some people who never even heard of hardening off can have success bringing their plants straight to full days in the sun, but the next year their plants are wilting after 30 minutes.

Watch your plants, and read this:

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.