The Royal Horticultural Society (RHS) has just announced the introduction of a new, more comprehensive rating for advising gardeners of a plant's hardiness.

The RHS has been working on the new rating for the past two years and the charity has consulted with garden owners, plant breeders and the horticultural industry throughout the UK. To date the RHS has rated a plant's hardiness on a simple four-step scale. The system was developed in the 1960s and was adapted from a system devised for rhododendrons.

"The original ratings have 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. This new system should satisfy this need."

Since the 1940s the USA has used a zone system for hardiness of ornamental plants, based on annual average extreme minimum temperatures. The latest version published last year divides the North American continent into 22 zones from the coldest (1a) to the warmest (11b). Plants are then rated for the coldest zone in which they survive. This approach has been generally followed by the nursery trade in the USA and increasingly in Europe and the UK.

"The US system really does not provide a reliable guide to plant performance in maritime areas such as the UK where temperatures are likely to fluctuate during the winter," says Jim. "We are also more prone to early and late frosts."

The previous RHS system composed four basic ratings, H1 to H4: H1 was for plants that have to be grown in greenhouse conditions and H4 was given to plants that can be grown outdoors in all parts of the UK. The new ratings (H1 to H7), are linked to winter minimum temperatures and a definition describing the performance of plants is outlined for each rating. The new ratings more clearly define what plants can tolerate.

The launch of the new hardiness ratings is being timed to coincide with the current review of the RHS Award of Garden Merit list, currently 7,400 plants. The new and revised RHS AGMs will be published with the new hardiness ratings in early 2013. This additional information and detail will ensure that UK gardeners can make an informed choice on the best plants for their gardens.

For more information about the RHS, please visit www.rhs.org.uk