Cold hardy Paw Paw

I've planted a few Paw Paw trees, and some from seed, but they don't overwinter here well.  A couple died back to the ground and came back a year or two, but then didn't make it.  If anyone has a source for Paw Paw trees or seeds that survive Zone4, I would love to have some.

Pretty sure Pawpaws are only hardy up to zone 5 (a winter temp drop to -20F),  They can grow in Southern Ontario, I know that much.
There aren't many specific cultivars of pawpaw to begin with - most are focused on increased fruit production, not additional cold-hardiness.

They tend to be quite tender as new saplings as well, so maybe keeping them in large pots so you can move them... or covering them during the winter may help keep them alive long enough to grow thicker bark.  Gotta mulch the FUCK outta their roots, since root death from frost will kill them faster than anything.

You could also check around your yard, or build an earthworks to create a warmer microclimate that doesn't freeze as hard in the winter.
Something that helped me grow zone 7 subtropical plants in zone 6 was to find a warm microclimate. For me, the spot was right up next to the eastern side of my house. It's full-sun, my house blocks any cold air that would flow down the hill (cold air sinks, and will pool in lower areas, making harsher freezes - walls and structures that touch the ground can divert it.) and the house's brick warms up in the sun during winter, so the snow near that side of the house is never quite as deep. Plus it's shielded from winter winds. A nice little spot.

I've also had good luck piling hay up when the temps get low, dousing it with compost tea, and throwing a tent over the plants this past spring, when temps started to drop below-freezing after a long warm spell. The hay both insulated the plants from cold drafts, and the decomposition generated a bit of heat. Idk if it'd work through a whole winter, but it definitely saved my sprouts from a couple frosts - maybe it could help you get through the deepest freezes.

So yeah, I'd try to find or build a microclimate for your pawpaws if you're growing in zone 4.  

Good luck!

I bought 100 pawpaw seeds from Sean at Edible Acres in New York about 18 months ago. I first kept them cool, then warmed them, then planted them to air-prune beds. Something like two dozen of them were two inches high by the time the summer was closing down, so I built a shield-wall of 3-high hay-bales around them to help insulate and block wind. They appear to be entirely gone. Not a one survived the winter. I'm giving them time to see if anything other than weeds pops up in those bed-boxes, but not holding my breath. But hey, I knew it was a longshot.

If you find a source, I'm interested too!

There are some facebook pawpaw groups that have members that are in zone 4.  Pawpaw Fanatics is one that has members from Europe and Asia also.  They may know of a few cultivars that may work if you are close to Zone 5.

Maybe try to get some one- or two-year seedlings from a nursery close to your zone?

I am in zone 5 upstate NY. I got lots of seeds a few years ago and over 2 summers tried planting them all out. They usually made it till following spring. Bareroot plants never took. I read rodents eat the young and young plants burn easy. I bought two older potted plants, huge size as my last ditch effort before giving up. I planted them behind my grape vines for shade, inside the apple orchard fencing, and caged them.  So they are double caged essentially. They actually survived. I bought fresh fruit off of Etsy and saved the seeds and am growing them in pots. This time half I will pot grow till much larger and half I intend to shade and protect from mice or rabbits again. Quarter inch hardware cloth dug a bit into the ground so they can't pry under would protect the young plants from vermin. I suspect that killed most as things loved to pull them up and sonetimes they just vanished over might. I had covered all my tenders with chicken wire but something ate my pencil sized apple whips when caged this past Spring. Quarter inch keeps mice out. I read Pawpaw don't grow usually up north because the winter kills the seeds. I heard of others around me who do grow pawpaw. When I took a small farmers course some guy even said they grew cold hardy banana in zone 5 which was edible. That was all he shared. In my research, I think he meant the pink velvet banana. But who needs a banana when you can grow pawpaw? If you can't grow the pawpaw, try the banana. It appears just as seedy.