Plant Hardiness Zones were devised in the United States of America by the Department of Agriculture (USDA)to give a geographically-defined zone with respect to the minimum temperature where particular plants or groups of plants would survive. The zones were initially defined using the average minimum temperatures over a five year period. A more recent upgrade of the zones in USA uses averages taken over a 30 year period from 1976 to 2005.
Similar hardiness zones have been drawn up in the United Kingdom by the Royal Horticultural Society (RHS) and in the European Garden Flora (EGF). The RHS has four zones ranging from Fully Hardy (H) to Frost Tender (FT). Following the severe winters of 2009/10 2010/11 the RHS is in the process of producing a revised rating system. (see below)
The EGF has seven from hardy (H1) to requiring glasshouse protection (G2). Most plant guides tend to use the US zones as they have been more widely researched and are aiming at the largest market for their publication.
In plant guides and some labels the RHS zones can be indicated with snowflake symbols. Fully hardy plants have three |
Most areas of the British Isles are in zone 8 of the US system with some western and coastal regions in zone 9. Central, mountain areas of Scotland are in zone 7. The south-westerly tips of Ireland, Cornwall and the Isles of Scily are the mildest areas and can grow some plants designated for zone 10. Due to the length of winter with variable periods of cold and short days, not all plants in zone 9 or 10 are suitable.
The unique effects of the Gulf Stream mean that the British Isles are the furthest north of any place on earth to experience these conditions and has allowed the development of a wide range of gardening styles. Also after centuries of plant hunters bringing plants from all parts, the British Isles now has the widest range of any place in the world which have been able to adapt and thrive.
As mentioned above a new hardiness rating system is to be introduced by the RHS. It will expand the existing zones, but confusingly, for those familiar with the USDA or EGF ratings, the numbers are in reverse. So H1a is the most tender in this new system whereas H1 is the most hardy in the USDA and EGF zones.
The new ratings are due for introduction in spring 2013 when a revised list of plants given the Award of Garden Merit (AGM) is issued. Endorsements on plant labels may include red, amber and green 'traffic light' warnings.
| Rating | Temperature ranges (°C) | Category | Definition | USDA Zones |
| H1a | over 15 | Heated greenhouse – tropical | Under glass all year. | 13 |
| H1b | 10 to 15 | Heated greenhouse – subtropical | Can be grown outside in the summer in hotter, sunny and sheltered locations (such as city centre areas), but generally perform better under glass all year round. | 12 |
| H1c | 5 to 10 | Heated greenhouse – warm temperate | Can be grown outside in the summer throughout most of the UK while day-time temperatures are high enough to promote growth. (Most bedding plants, tomatoes and cucumbers). | 11 |
| H2 | 1 to 5 | Tender – cool or frost-free greenhouse | Tolerant of low temperatures, but not surviving being frozen. Except in frost-free inner-city areas or coastal extremities requires glasshouse conditions. Can be grown outside once risk of frost is over. (Most succulents, many subtropical plants, annual bedding plants, many spring-sown vegetables). | 10b |
| H3 | 1 to -5 | Half hardy – unheated greenhouse/mild winter | Hardy in coastal and relatively mild parts of the UK except in severe winters and at risk from sudden (early) frosts. May be hardy elsewhere with wall shelter or good microclimate. Likely to be damaged or killed in cold winters, particularly with no snow cover or if pot grown. Can often survive with some artificial protection in winter. (Many Mediterranean-climate plants, spring sown vegetables for later harvesting). | 9b/10a |
| H4 | -10 to -5 | Hardy – average winter | Hardy though most of the UK apart from inland valleys, at altitude and central/northerly locations. May suffer foliage damage and stem dieback in harsh winters in cold gardens. Some normally hardy plants may not survive long wet winters in heavy or poorly drained soil. Plants in pots are more vulnerable to harsh winters, particularly evergreens and many bulbs. (Many herbaceous and woody plants, winter brassicas, leeks). | 8b/9a |
| H5 | -15 to -10 | Hardy - cold winter | Hardy in most places throughout the UK even in severe winters. May not withstand open/exposed sites or central/northern locations. Many evergreens will suffer foliage damage, and plants in pots will be at increased risk. (Many herbaceous and woody plants, some brassicas, leeks). | 7b/8a |
| H6 | -20 to -15 | Hardy – very cold winter | Hardy in all of UK and northern Europe. Many plants grown in containers will be damaged unless given protection. (Herbaceous and woody plants from continental climates). | 6b/7a |
| H7 | below -20 | Very hardy | Hardy in the severest European continental climates including exposed upland locations in the UK. (Herbaceous and woody plants from continental climates). | 6a to 1 |
The increased variability of the weather lately has been causing the loss of many more plants so the sort of guarantees that used to be given by plant sellers regarding replacements will have to be revised or may not be given in the future. Improved labelling and advise should help gardeners to utilize plants better.
As the temperature is a mean value it is not the lowest reached and local conditions can affect what will happen in any site. Frost pockets can occur in low spots where cold air becomes trapped as it flows downwards, eg. at the base of a wall or hedge. Soil type will influence the penetration of the cold so waterlogged soil will freeze solid and damage the crown or roots of a plant. The aspect of the site and any shelter given by walls, hedges or overhanging trees and shrubs will improve the chances of survival. In large urban areas the temperature can be a degree or two higher due the shelter or radiant heat from buildings, either artificial or stored in solid materials from daytime sunshine. A covering of snow will insulate the soil, but after a thaw a hard frost can penetrate deeper with the wetter conditions. It is usually cold wet soil that kills plants in winter, even those that are native to freezing temperatures in mountainous areas where it is normally relatively dry - so some alpine plants can survive with a transperent cover in the winter to shield them from the wet.
Growing in raised beds or planting on mounds will improve the drainage, as will incorporating grit and organic matter and applying protective mulches of bark chips or dry plant material can allow plants rated for a cooler zone to be used. Keeping more tender plants in containers which can be moved under cover in winter is another way to increase the range that can be grown.
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