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Please may I ask what “H” number this clematis westerplatte has with regard to winter hardiness?

Hi Graham, I’ve never heard of ‘H numbers’
The hardiness of a plant is generally instilled into it durning the first year or 2 of its life and will depend on where it’s been grown and whether it’s been grown indoors or outside.
What I can tell you is that all our plants are 2 years old and have been grown outside here in Doncaster – they are all hardy down to about -15 to -20
However other people’s young and less mature plants will differ dramatically
Hope this helps

Chris and Suzy