Last winter was a very bad winter. I guess you could call it a test winter for a lot of trees. We were in the negatives lots of days opposed to a few days last winter. Some I wonder if they were dead because of decline earlier last year and the cold winter just ended them though. My spitzenburg, standard Comice and sweetheart apricot were not looking the best before winter. All of those revealed with a scratch test not to be surviving. I also broke branches to see if there was living tissue in the branches and there was no living tissue. My Comice on OHxF 87 survived so I am wondering if it was just in decline with that pear. The spitzenburg and sweetheart are supposed to be rated to my area but I wonder if a few harsh hits can do them in. I had heard spitzenburg was a challenging apple so whatever it has the previous season and the Comice may have done them in but the apricot is a shame. It shows if you are zone 5 maybe not grow those ones. My Pink Lady and Mountain Rose apple are still to come out but are showing green. I am just not sure what is going on with those apples.
I’m sorry to hear, are your trees in pots or in ground?
They are in pots. 100 gallon pots though. The mountain rose apple and pink lady are in 25 inch regular pots though. I am wondering about the trilite peach plum though. That being said I am not sure if it was the cold that killed them or the fact many of these were struggling and the cold was just the final straw. Like I mentioned I have one Comice that is alive but the one on standard rootstock died. The sweetheart apricot did not appear to be in the best shape when Stark Bros sent them to me. The Spitzenberg has something on the leaves before last summer. Something I noticed too was all the trees that died had little root growth. The weird part is those died while my spice zee nectaplum seemed to have lived. I think this shows that maybe it is not just a factor of how cold it is but if the tree is already in decline the cold can just end them.
I find weak trees will die easier in winter. I found my trees that have struggled with deer browsing in summer have died off in winter way more than ones that were unharassed.
I’m betting the rootstocks are what mattered more in your case since putting them in pots and leaving them outside may be something similar to dropping a USDA hardiness zone (just a hunch).
Like I said it is weird because zone 6 plants survived and so did many zone 5 plants. It was those specific plants that died. In fact I still have a Comice living. It is possible it was different rootstocks since the rootstocks changed. The Comice was on a wild callery rootstock and the rootstock seemed to live but not the scion. The Spitzenberg was on M111 and the sweetheart apricot was on unknown rootstock. All my Fuji types and Zestar! seemed to live and are actually thriving more than last year but are on unknown rootstock as I got them from Stark Bros. I know Pakistan which did not make it is not hardy in general so it likely just got too cold even in my garage and Rossyanka persimmon is likely just not that hardy even if on American rootstock as that was as dead as can be. I thought Comice was zone 4 so I would not think that even lowering a zone could kill the Comice.
It is probably a combination of the harsh winter, the rootstock, the vigor of the plant and the fact that they were in pots which caused the particular plants to die. Potted plant roots will be a lot colder than the ground, 20 degrees or more, which is two full zones.
It is possible you are correct on that. I seem to be losing my other Comice now and the other pears doing well are hardy to zone 4. The other pears are Warren, Seckle and Magness which are all more cold hardy. That being said my Fuji are zone 5 so I am not sure why my Comice pears are being affected so much while only one of my apples have been.
Were the pots outside all winter? Plants in pots have many issues that can affect hardiness. You’re reaching to infer much from those results.
It is hard on a tree to overwinter on a pot, the soil offers a lot of insulation. If in addition they were not looking happy after a full season of growth most likely than not they were not in the best of shapes to taken on winter. I would dial in soil and irrigation in order to buy them the best chance. You may want to pull them out of their pots to do some root pruning.
Often what kills the tree is not the lowest temperature but the temperature swing. If it warms up for a week in the later winter they start shedding cold hardiness. If a sharp cold snap follows they may not be prepared for it.