Scott,

Several years ago a smaller Poncirus was growing at the Arboretum in another location that was more exposed. Perhaps that tree is still there, but I tried looking for it a year later and couldn't find it. The welcome center can show you where all the trees are.

In any case, the trees you photographed might be two different accessions from the Boyce Thompson Institute-which explains the difference in leaf retention.

I found that small Poncirus trees which tend to retain their leaves dry out in the winter and die from that cause rather than the cold, so a breeding program for a hardier citrus in zone 5 or 6 would need to include deciduousness.

Alan

That's an interesting observation regarding leaf retention and dessication of the trees. My FD was evergreen the first winter I had it in the ground, but since then it has only held a small fraction of its leaves in winter, and these have generally been on lower branches which were shielded from direct sun in the fall.

The two PTs in my pictures are both seedlings, so they certainly could be genetically different. Since the Arnold Arboretum is not too far from me, I'm going to try to stop by a couple times this year and see if I can spot any differences between the two trees.

I'm not sure there are any other Poncirus in the Arboretum, at least there are no other listings in their online plant inventory, it is possible that it perished at some point.

PT's are pretty tough though, I know of a FD that is ~8Ft tall and is several years old growing in zone 5B central Massachusetts.