Gardening books, magazines and plant labels refer to plants as "Hardy" or "Frost hardy" with a "H" 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 because if the plant is not sufficiently hardy for UK winters, it will die over winter, or will need winter protection. It is time consuming to grow less hardy plants, which need to be moved under glass, or sheltered and/or wrapped up for the winter. It is also 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.
Confusingly similar plants and plants in the same group can have different hardy ratings. For example, some varieties of Lavender are more hardy than others, as are different types of Salvia. This means when buying a plant you need to check the label for information not just about the group, or genus, but also the variety.
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.
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, 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.
Illustrated above are the three extremes of plant hardiness: the Orchid (left) originally from tropical areas is very tender and really only an indoor plant, Sweet Peas (image center) are hardy annuals and will take a degree of frost, and Heathers (image right) are very hardy, as tough as old boots growing as they do at high altitudes in the North and Scotland.
It isn't just the cold which takes its toll on plants, other factors such as rainfall, chilly winds, high levels of water. Pittosporum tenuifolium which initially survived for several years in my garden, in a sheltered spot, was classified as *** hardy under the old plant hardiness rating system, perished in a bad winter, because my garden was no just cold but wet as well. Under the 2013 scheme, Pittosporum would be classified as H3 showing a more tender nature. In a more protected spot or in a different garden the Pittosporum may have survived.
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 hardy. The the current system introduced in 2013 takes into account more variables.
The hardiness classification system is there to 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 make it through the winter.