Maybe, but I would not count on it. Most windows nowadays screen out a significant amount of UV. There is also the difference in heat and wind to consider.

In your area of Texas, it may still be a bit cooler, but where I am at, the days have gotten fairly hot, for the most part.

Also, plants do also need to be hardened off and toughened to cooler night temps (and wind) too, so take that into account as well. IMO, gradual is best if you are in doubt.

I don't know why I was thinking you are in Texas, so never mind about my "in your area of Texas" comment...

Well, on a 60 degree day it can reach 90 degrees on the sill, and at night its in the low 60's or even upper 50' right up close to the window pane. But you are probably right about the UV part. I'll be gradual, but I won't take 2 weeks or anything like that. I don't mind if they get stunted a bit. They're too big right now to keep inside any longer.

Have to agree - windows aren't the same thing as direct sunlight. It isn't so much the temp of the light that is relevant, it is the light spectrum as suze said.

Even plants grown in greenhouses have to be hardened off. But you don't need 2 weeks - if done with care and gradually it can easily be done in 4-5 days.

Dave

I alreay put them out once today, but tomorrow it's either all day or not at all. Can I put them in a shady spot so that the direct sulight is minimal? Also, during this hardening off phase, should I be putting them back into the window where they were before, or should I keep them indoors away from windows after a period of being out?

Can I put them in a shady spot so that the direct sunlight is minimal?

Yes. that would be a good idea if you can't be around to watch them, especially on day two.

Also, during this hardening off phase, should I be putting them back into the window where they were before, or should I keep them indoors away from windows after a period of being out?

Once I start hardening off, I don't stop the process and bring them back inside -- full steam ahead. ;-)

They are either outdoors toughening up, or in the garage at night, but only if is simply too cold at night to have them out. I do go easy with cold nights the first couple of days, but they do need to get used to that, too.

I often stash my plants under a picnic table the first day or two out of the greenhouse. The partial shade and wind protection helps keep them from burning. After that they are ready to go on top of the table. By the way, I have a fan blowing on them for the last few weeks in the greenhouse
to strengthen up the stems before set out.

Last year we had 3 cloudy days for their first days out. No
protection needed and they got watered for free on the 3rd day! Wish I could arrange that with the Lord every year!

SO, lets say I change my mind about putting them out early, and they are fully hardened off... if I bring them back inside for two weeks does that undo the hardening?

That's an interesting question. I've never done that myself, but I think the basics can suggest an answer...

I think the existing leaves would retain their U/V tolerance, but new leaves would not have that. I don't really think that seedlings develop any temperature tolerance - it is either warm enough for growth or it isn't. One other benefit of outdoor exposure is stem thickness growth as a result of wind exposure. I would think the seedlings would keep that (in that I can't see them growing skinnier when kept out of wind).

Regarding the "wind" thing"; I have a cheap oscillating fan that I turn on for an hour a day when the seedlings are inside under artificial or windowsill light. I think it makes the stems stockier and the leaves thicker. While sudden all-day U/V exposure certainly causes leaf damage, I think many people under-estimate the dehydrating effects of wind.

Regarding the problem of U/V exposure; I know most people don't have the luxury of setting their seedlings outside for true gradual sunlight adaption an extra hour each day for a week (I'm retired so I can do that). But if you place them where they will be exposed to more direct sun each day (by placing them in locations that get an extra hour of sun per day), that will help.

What I mean is, place them where they will be shaded most of the whole day at first (by a wall or artificial barrier like a plywood or cardboard sheet), bring them in at evening, and move them to a slightly sunnier spot the next few days.

Yep. If they are outside and you bring them back in, it "undoes" your hardening off.

Hmm, I see conflicting ideas here. How exactly would hardening off get undone, besides from new leaves forming. Perhaps if an existing leaf continued to grow? Anyone care to explain?