No, you can't see by looking, unless they visibly start wilting. But you can watch the weather to tell if you can hurry them up a bit. If you are hardening to heat and sun and you get cool overcast days they can stay out longer sooner. I have to harden to cool, wet windy weather rather than sun so I can push them if the weather is still and mild.

This only issue I ever had I did see wilting, if you watch closely you will not loose anything, I missed it by an hour and many were wilted to the ground, 90% came back fine. I pretty much double the average hours per day with no issues. Second day was hard direct sun for 3 hours, no issues.

Are we talking about temperature hardening, or sunlight hardening? For sudden exposure to lots of new sunlight, sure, wilting will happen if left out in the sun too much. If you take a seedling that has been growing at 80F, and suddenly stick it outside at 50F, no, it won't wilt, but it may get stunted a bit if left there too long. In the latter case, of course, you can't see it happening.

Depends on the weather, the plants, and where the plants are going for me. I start night hardening seedlings pretty early with matching the older seedling lights with the actual day and night hours. I half harden a lot of older seedlings in an enlosed porch till they start to get their full hardening in the outdoor mini greenhouses and caps. The porch is a step cooler than indoors, and a step warmer than outdoors in the mini greenhouses and under caps. Full hardening can take a week or two depending. I do it in the greenhouses and caps because of the wind around here. They are already night hardened, and getting day hardened. But they need extra protection while wind hardening. I have them set up where some natural shade at times of day helps temper the day hardening, and I'm using the heavy green caps that cut the sun a bit too.

I rarely get wilting unless I accidently miss a cell during watering in the morning. I don't really watch them. The trays that need to move go out in the morning and pull in early evening. Since I've been repeating this process for many years with success, I know what works for the plants I grow.

I am talking all hardening, good point about stunting growth. I do have a fan on inside whenever grow lights are on. I am doing 2, 4, 6, 8 hours this year then outside but in a cold frame - to "moderate" sunlight, wind and cold nights. Thoughts?

For a start placing the young plants in Dappled light such as under a shade tree works well for me. Wind burn is another consideration so best to keep them inside on windy days.

Sunscald damage doesn't always show up right away. So if you leave the plants out in direct sun for too long in the early stages of hardening off, even if they look fine initially there may already be damage done that shows up later. As long as the plants don't die they will grow out of it but it is annoying, sets the plants back, and should be prevented when possible. Short answer: No, you can't just keep an eye on the plants.

Rodney

Very good info guys, thanks! How long does it take for sunscald to show up?

I think you can assume that if sun exposure leads to seriously wilted leaves, it will end up as sunscald (otherwise known as leaf scorch). In this instance, large volumes of water are being removed from the plant through the leaves and may not be replaced in time to prevent cellular damage.

Sun issues can show up pretty quick, and sometimes it don't really show till later. Like a human can get too much sun and sometimes you feel like you are getting pinked while you are out in the sun, and sometimes you really don't notice as much till you have been inside for a couple hours and still feel the heat. After the fact you realize you got too much sun. Sometimes you don't realize it was way too much till the cancer shows up much later. Sometimes you think you have more sunburn, when you have a combo of sun and windburn that hurt your skin. And then think of how much you would expose a baby or small child skin to sun vs adult skin. Little hardening off plants are like toddler skin and they haven't gotten their tough adult skin yet.

Plants can't talk, but their skin gets the sun like ours does. How much sun is too much depends on the plant, their skin, their tolerance, how much sun exposure they have had. It also depends on the temps and wind exposure in combination with the sun. Many gardeners choose not to push their plants to hurt, burnt, or potental death to see how much it soo much, or how long is too long. That's why many recommendations are to harden off slowly instead of fast and hard to push it. Reccomendations of utilizing shade or dappled sunlight instead of direct sunlight.