I agree that proper hardening off can be very difficult for those who can't be there to monitor the plants. The solution IMO is to create ahead of time an outdoor transition "room".

For example, our greenhouses have screened lean-to annexes for hardening off plants where they can be put and left and still be shielded from direct sun and protected from wind. That way all we have to worry about is temps.

But you don't need to build a screed annex. Depending on the number of plants you can still make a protected enclosure where it will be safe to leave them all day on day 3 and from then on - no in and out and in and out.

Trudi has suggested in the past an upside down plastic laundry basket with a rock on top to hold it in place. Put in a shaded spot - one that will be shady all day - and the plants well watered before leaving them it is good for a flat of plants.

Got a sheltered side of the house with an overhang? Hanf a piece of shade cloth from it. A large cardboard box with slats cut out of it on 3 sides and a piece of screen or shade cloth on top works. A low tunnel or tent made out of Reemay or any row cover or some shade cloth and open on the non-windy ends works great. Shoot if you have 4 stakes and 50' of row cover or similar fabric you can make a 4 sided room with roof big enough for 10 flats of plants. You just have to think outside the box so to speak.

The mistake I think too many make is expecting direct sun tolerance - believing that sun tolerance is the goal - far too quickly. Instead cloudy overcast and shade are more than sufficient. Get them use to shaded UV light and a breeze over several days and the rest takes care of itself.

Hope this helps.

Dave

PS: Google Images shade enclosures and see if any of the images trigger an idea

I heard plants in the shade receive more light than bright bulbs inches over them indoors. I've heard this info from a few well known gardeners on youtube.

Thank you, I am going to have to think about how to do this. I didn't realize a completely shaded enclosure would still help hardening them off. I don't have overhangs on the house. I usually wait for a little bit of sun at least, then then start with an hour or two and increase from there. Can you do more than one trip outside per day at beginning? Like one in the morning and another in the late afternoon or evening? I am not sure how long it takes them to recover from the first trip outside. If they are in a shaded enclosure, they can right from that to full sun in garden? Or maybe one extra day of half sun then full sun? How long does it take when removing them from the enclosure?