TB: winter hardiness speculations

The climate in Moscow is temperate but risky for gardening. It is zone 4
here but my garden fall better to zone 3. Snow cover is usually deep and
stable. The ground under the snow is not deeply frozen, but sometimes there
are frosts down to -40 degrees F, and often frosts do occur without snow.
The summer is not very warm, and often is rather wet. Annual precipitations
are about 600 mm (25 in), mostly in summer: from June to August we have 228
mm (9 in). TB peak in my garden is about June 20. The climate is not like
central North America and Canada, is more like Northern NH I think. Summers
usually not warm enough for most TBs to properly mature, and as result their
terminal fans(buds) were damaged first of all during winter frosts. Usually
it was possible to make a range of TB varieties:
not damaged -
slightly damaged but terminal bud is alive and will blooming -
terminal bud is dead but rhizome is not damaged -
upper part of rhizome is damaged but increases are alive -
the rhizome is completely frozen.

The year of 2002 gave us a lot of surprises.
The summer of 2002 was exceptionally hot and dry until September. Then we
almost received our annual norm of water during October, and November ended
with good snow cover. I was not worried yet, but the troubles had only
started. By early December the snow cover was gone, and -22 degree F (-30oC)
frosts followed. Such hard frosts continued until Christmas so the ground
was frozen an unprecedented four to five feet deep. The snow cover returned
after January first and was reliable the rest of the winter. You can imagine
all those discussions we had during that winter!
When the snow has gone we found a lot of rhizomes completely dead, and not
simply frozen but sorta rotted (it was a gray wash in a shell of nut) -
after being 4 month as in freezer.
Another part of rhizomes looked well: leaves and terminal buds of matured
rhizomes were undamaged and sprouted, but they quickly scorched. Close
inspection showed that the roots and the lower parts of the rhizomes were
dead, and only several increases continued to grow.
Third part of rhizomes was only slightly damaged and bloomed. There was only
rare occasions when terminal buds was frozen and rhizomes was undamaged as
it usually happens in regular winters. This time, if rhizomes was not
completely dead and not lost its roots they usually bloomed.
This winter has turned over all my ideas about winter hardiness of irises.
Several variety I considered most winter hardy was dead completely this
time. It was for example Vanity, Silverado, Dusky Challenger, Pledge
Allegiance.
My impression it was dead not being frozen but being suffocated (or
retted)(?).
Such picture was more expressed in those areas where hard frosts followed by
thaws.
Those losses were almost only between TBs, IBs and SDBs was not damaged
except Jeweller's Art that died everywhere.

Juri Pirogov
in Moscow
zone 4-3

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