Which plants will survive our winters
28 June 2012
The RHS is introducing a new, improved hardiness rating system to give gardeners a much clearer idea of the temperatures specific plants can tolerate.
The seven-rating system will replace the current four-step scale, which was developed in the 1960s, and will be available on thousands of plants early next year.
The Society has been working on the new system for two years, in consultation with the UK’s garden owners, plant breeders and horticultural industry.
Recent climate changes and fluctuating temperatures have made it even more important for gardeners to choose the right plants for their conditions.
'The original rating system has served us well,' says Jim Gardiner, RHS Director of Horticulture. 'But we have felt for some time that it doesn’t provide enough detail on degrees of hardiness. Today’s gardeners are much more aware of changes in the climate and are looking for more information.'
He says that the zone system, used in the USA and based on annual average extreme minimum temperatures, does not really work well in maritime areas such as the UK, where winter temperatures are likely to fluctuate and we are prone to early and late frosts.
The new ratings (H1 to H7, where H7 is the most hardy) are linked to winter minimum temperatures. Each rating has a definition of the conditions in which plants with that rating wil perform best. Under the previous system H1 was for plants requiring greenhouse conditions and H4 was for plants that can be grown outdoors anywhere in the UK.
The launch of the new system coincides with the current review of the RHS Award of Garden Merit list, featuring 7,400 plants. When the new and revised AGMs are published in early 2013 they will have their new hardiness rating.
See the new, improved RHS hardiness ratings systems (508kB pdf)