I can only assume the new growth was soft and tender, having been induced into premature growth, and was burnt by the sun in the window. I seem to recall you’re in a cold zone, but these are hardy plants so I’d start hardening them off if possible. Otherwise, have you got a cool bright place you could put them, like a glazed porch or something?

give it a week or two .. then tip it out of the pot and see if you have any new root growth ... if not.. i wouldnt keep it around long ... maybe a little bottom heat ... but the light stand probably took care of that...

what does the seller say about it ..

that said.. ive had stuff like this.. and just jammed it in the ground somewhere.. and was surprised to see it come fall .. lol .. ive also throw them on the compost pile.. and found them thriving months later.. lol ...

ken

“wasn't ready to plant outside...”

Do you just not have a place for them yet? Bare-root perennials, especially something perfectly hardy like D. exaltatum, can (and should) be planted outdoors any time the ground isn’t frozen solid. Especially if they’re waking up from dormancy as you describe with the new green growth, I’d be in a rush to install them in-ground ASAP.

Hmmm, maybe I should stick them in the ground this week. They've been under the lights for a couple weeks, so I'm a bit apprehensive to plop them into the cool ground. Should I just place the pots out on the deck until they start to grow? Or just go ahead and plant them?

Update: I planted them outside yesterday. When I took them out of their pots there were no growth buds. Sigh... Hopefully they'll recover eventually.

Note I said to harden them off first.