RHS Hardiness RatingsI've noticed something strange has happened to the RHS hardiness ratings - first
By Bamboo
London,
United Kingdom 
RHS Hardiness Ratings
I've noticed something strange has happened to the RHS hardiness ratings - first, Verbena bonariensis is now listed as fully hardy in the UK, when it didn't used to be. Today, I looked up Pennisetum setaceum on the RHS site. The description says its usually grown as an annual in the UK, and needs winter protection - yet the Hardiness rating is 6, which equates to 'hardy throughout all of Europe and the UK (down to -20deg C)', which it certainly isn't, so far as I know. Anyone else noticed these anomalies, or is it that they've changed their ratings (but not their descriptions) because of global warming? Or some other reason...or is it just plain wrong?
6 Sep, 2015
Answers
Well thanks for the link landgirl, but I know they've altered their hardiness ratings, making them narrower but with more categories - but it does not explain why a plant thats described as needing winter protection is rated as H6, hardy down to -20 degC. It would appear the rating of H6 has been misapplied, is my point....
I have actually emailed them regarding this, so I'll be interested to see how they explain it...
6 Sep, 2015
I'd be interested too Bamboo, I rely on such info as I need bone hardy plants here
7 Sep, 2015
Yea, Pam, I'll post whatever I find out here on this thread, so you'll see it anyway
7 Sep, 2015
Thanks ☺
7 Sep, 2015
Previous question
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Have a look here: https://www.rhs.org.uk/about-the-rhs/publications/magazines/the-garden/2013-issues/february/How-hardy-are-my-plants
It explains the hardiness ratings.
6 Sep, 2015