Rachel Sullivan wrote:

In article , Roger Van Loon
writes

Do others have experienced problems with the hardiness of C. montana,
anywhere in the UK?

As young plants I think they can be susceptible to hard frosts (but they
have to be hard), but as mature & young plants they can get a thing
called 'slime flux'. Some years ago we had a mild spell in March/April
and then some quite severe frosts in May. Lots of people lost their big
montana's - the RHS at Wisley was inundated with Wails! It seems the
sap had started to rise, then the frost had burst the stems - bit like a
water pipe I suppose. A creamy yellow slime oozes out of the stems at
the bottom and the plant dies, though if you prune it back below the
damage they often grow again. A small comfort though when your plant is
a beautiful 10 year old specimen.
This slime flux phenomenon is, I think, fairly unusual though, and
generally speaking I find montana's to be very amiable and perfectly
hardy, especially here in the UK. I have heard from Canadian gardeners
who have problems with it - but they have winters of horrid and lengthy
severity.
Mary Toomey & Everett Leeds, in their excellent book on clematis reckon
montana's are hardy from zones 7 to 9 - that is not as tough as the
large flowered hybrids and viticellas that are hardy from zones 4
upwards. But it's still minus 17.7 to minus 12.3 C - which is still
pretty darned cold.

Thanks, Rachel. I had read about 'slime flux' (Mary Toomey's book, p.
89) but had not thought too much about it. But I now realise - that's
no too much different from Nick Mclaren's idea either, I think.
Anyhow, I did take a closer look at my plants (as I said, two of them
in different locations in my garden, many years old, one with a stem
of about one inch). The stems are now totally dead, down to the
ground. I don't see any sign of "creamy slime oozing from the stems" -
but that could have happened a few weeks ago, without my noticing.
Anyhow, this winter we had several cold spells, and the last one may
have been the cause, coming when the sap was already rising.
The thing that still puzzles me - it happened to my plants in the
past. And, thinking about it, I realise a few montanas in gardens
nearby have disappeared over the years. I know of others that were
growing nearby, and I will take a look how they have done this winter.
I know my garden is not that very much colder than Nick's (in
Cambridge), and I'm surprised that this plant can behave so
differently.
Regards,
Roger.