Gardening books, magazines and plant labels refer to plants as "Hardy" or "Frost hardy" with a rating, which until recently, was a fairly basic indication of the degree of cold and frost the plant or shrub would withstand during the winter. It is important to know the conditions a plant will survive, as a plant which is not frost hardy, or half hardy, will not survive a UK winter without protection. It is time consuming to have less hardy plants which need to be moved under glass, or sheltered and wrapped up for the winter and costly if they do not survive the winter. It is easy to spot a lovely looking plant in the garden centre only to find it is not fully hardy, which means the plant's ability to survive an English winter is limited.
Plants in the same group can have different hardy ratings. The most commonly grown Rosemary officinalis is described as H4 hardy, which is -5-10. Also widely sold is a form of trailing Rosemary, of the Prostratus Group, which is not as hardy and needs winter protection in some areas. It is easy to mix them up.
The original basic system of hardiness classification was to demonstrate the level of cold and frost the plant could withstand and was in widespread use until relatively recently. It was a simple *** rating. It is still relevant as some plants sold with labels which utilise this system.
The previous system was fine up to a point, but had several drawbacks. It was broadly based on the USA zones which didn't correspond with the UK climate. Also in practise, variation occurred in different areas and planting situations which suggested that the previous classification was not sufficiently detailed for the wide variety of growing conditions in the UK. The conditions experienced in more exposed Northerly, or wetter gardens, meant in effect the plant was in reality less hardy than its classification; equally most gardens have some more sheltered areas and micro climates.
With some plants their hardiness may be more obvious as in the images above. The Orchid (left) is known to most as an indoor plant, Sweet Peas (image center) are planted out in the Spring and are frost hardy to a point, and Heathers (image right) are very hardy, as tough as old boots growing as they do at high altitudes in the North and Scotland. Under the old system most plants in the UK were classified *** which didn't taken into account enough variations in conditions.
For example, a Pittosporum tenuifolium which initially survived for several years in my garden, in a sheltered spot, even though it is *** hardy under the old plant hardiness rating system, perished in a bad winter. Under the 2013 scheme, Pittosporum would be classified as H3 indicating a more tender nature. In a more protected spot or in a different garden the Pittosporum may have survived.
Equally Lavender, more particularly the English Lavender Angustifolia, is generally hardy but really dislikes the wet. English lavender may be fine in most winters if it is in the right spot, which is dry and well drained soil, but it loathes having its roots in the wet soil which will kill it more quickly than the cold. The French lavender, Stoechas is borderline, (for more about choosing and growing lavender.)The precise degree of hardiness isn't always indicative of how a plant will survive, and the current system was introduced in 2013 to take into account more variables.
The hardiness classification system is there to better inform the buyer. More detailed information has refined the classification. For example, on the question of lavender, Lavender Angustifolia 'Hidcote' has been moved to H5 so the buyer knows if choosing a Lavender for a more exposed site to pick H5 not H4. The same applies when choosing shrubs allowing a buyer to choose a shrub which has higher hardiness rating. All the RHS Garden merit plants are labelled under the new plant hardiness rating. This helps to avoid a situation where after a bad winter in the spring the border has gaps where plants and shrubs have failed to reappear in the following spring.