I have 4 properties 15 miles south of the 45th parallel, northern Michigan, zone 5. My neighbor has chestnuts that are 35 to 40 years old with unknown origin, but I am guessing they are Chinese. I have grown some of them and also American Hybrid, Dunstan, and Italian chestnuts.
Neighbor's Chestnuts are winter hardy.
American hybrid from Oikos nursery in Michigan are winter hardy - one got blight
Dunstans from Chestnut Ridge, have some winter die back so far. This is my 3rd year of growing them from seed. Year one (2017) I planted six month old trees out in the field and they had winter die back and resprouted. So did some of the oaks I planted. 2 year old trees from 2017 hardened off much better this year and were planted out this fall.
Italian chestnuts are definitely southern climate trees and have had almost total winter die back and then resprouted and survive.
Last year I had a timbur harvest and removed a lot of thermal cover causing die back. Part of the harvest was to create a south facing ridge with 85% full sun and that is where I planted 10 or so, 2 year old Dunstans and an Italian avoiding north winds.
3 years ago, I began growing seedlings in my backyard in metro Detroit area - 200 miles south, and still had some winter die back on Dunstans there along with some oaks.
I stopped fertilizing after July as trees will harden off better that way.
I used to plant in the spring, but now I plant primarily in fall as the rainy season approaches in September. This avoids Spring frosts in Northern climate. We also have lots of ticks in Spring now. That was not the case 8 years ago and prior.