It's a horse chestnut (Aesculus) and not a chestnut (Castanea). These are prone to physiological leaf damage during warm conditions.

horse chestnut physiological leaf burn - Bing images

when you were hardening it off outside.. did you keep it out of sun for the first few weeks?

it was stressed from repotting .. and stressed more from hardening off .. so my guess is that the leaves are stressed ..

it should have been planted outside when it was dormant.. no leaves to get stressed ..

why did you have it inside for winter.. why didnt you plant it in fall or winter

and btw.. where are you.. big city name ... if its fully hardy to plant in the ground.. why did you keep it potted for so long ..

as long as damaged leaves have any green.. they are doing their work ... most trees can releaf if severely damaged ... if all other things are ok ... your repotting make that hard to figure out ...

wait a minute.. isnt this a deciduous tree .. but you kept it under lights all winter.. how long have those leaves been on it???

how old is it??

ken

There appears to be green algae on the soil surface indicating that it’s too wet. I’d harden it off ASAP and plant it outside.

Thanks everyone! I am near Boston and was advised to sprout it with peat moss in the refrigerator and plant it in the spring. It was planted about 1.5 months ago. I did put it in full sun when hardening it which seems to be the issue. I am wondering how best to harden it before planting it outside ASAP?

I'd assume instead the warmth of the propagation environment caused the damage.

In your area you could simply have planted it in a pot in autumn and left it outside all winter. No need to refrigerate if you have a natural fridge outside the window. Now I’d put it outside in a sheltered spot and leave it. It might lose its leaves but that won’t kill it.

That’s certainly the recommended procedure and I expect I’m used to my own climate of weak sun and moderate temps where a seedling would be fine put outside immediately at this time of year. It might stall a bit but I’ve often done it, being lazy.

you have a tree trunk that thick in 1.5 months???

full bright shade for any pot is the way to go .... if a FREEZE is forecasted put it the the garage ...

first part is to get it out of the house for good ...

then in a few weeks it could get some morning or evening sun ... which will be many times effective to anything you had indoors ...

tree time is counted in decades .. so you may as well count hardening off in at least weeks ...

and you MIGHT be best off.. keeping it the pot until fall for planting .. since you just up-potted anyway ...

ken

Aesculus come out of their plump seeds with already stout shoots.

That's awfully small for a 1 and a half month old horse chestnut you sure it's a horse chestnut

It is 100% a horse chestnut and not particularly small for one grown indoors in sub optimal conditions

That's mine it was done indoors put outside yesterday it's 4weeks and 40cm tall already

And when mine started to look ill I gave it a bit of plant food and it just shot up hope you find this useful

Don't fertilize a stressed plant!

That's what we were told to do and it worked so

Often times the cause of a stressed plant is lack if fertilization. Has this plant been fertilized any? Some of the leaf damage could be from a deficiency in one or more nutrients. Leaves that are deficient in nutrients will burn much easier when they are taken outside for the first time. Are there any other plants near this one that look sick? You certainly want to be careful with fertilizing a stressed plant. If it has adequate fertility and you load it up with another dose, bye bye! But if your plant is stressed because of a lack of fertilization, then you need to fertilize immediately! Just don’t overdo it. Start out with a light dose of fertilizer and if you get a positive response (you can usually see that within a few days to a week, depending on plant type), up the fertilizer rate to the recommended level. If you don’t fertilize a plant that’s stressed from lack of fertilizer, the effect is the same as over fertilizing. It just takes the under fertilized plant longer to die. Any plant, in potting soil especially, relies on you providing 100% of required nutrients for it to grow and stay healthy. Natural soil contains a lot of the necessary nutrients, especially the micronutrients, although that can vary substantially from area to area.