What does Frost Hardy mean

Gardening books and magazines refer to plants as "Hardy" or "Frost hardy" what does it mean and why is it important?

 In basic terms its a system of classification which shows by symbols the level of cold and frost a plant can withstand. Plants sold in UK  are classified  for UK. Knowing the classification is more important as garden centres sell such a wide range of plants, and some are not hardy. A plant which is not frost hardy will not survive a UK winter without protection and it can be an expensive mistake.

The classification looks as follows:

* Half hardy will withstand temperatures down to 0 degree C.

** Frost hardy will withstand temperatures down to -5 degree C.

*** Fully hardy will withstand temperatures down to -15 degree C.

When picking plants for the garden, given recent winters its best to pick fully hardy and the plant label should display ***.

In addition some variation will occur by winters and plant situation and this means the classification is not as precise as is sometimes suggested. More exposed or wetter gardens may make the  plant less hardy than it's classification; equally all gardens have some more sheltered areas and micro climates.

For example a Pittosporum tenuifolium(link to read about Pittosporum) survived several years in the garden, in a sheltered spot, even though it is ** hardy; but perished in a bad winter. In a more protected spot in the garden it just may have survived.

Equally Lavender, more particularly the English Lavender Angustifolia, are fully hardy but really dislike the wet. English lavender may be fine in a really cold winter if it is in the right spot, dry and  well drained soil but it just loathes having it's roots in the wet which will kill it more quickly  than the cold. The French lavender, Stoechas is borderline and a cold winter will see it off.(link to lavender information)

The Hardy classification helps, but still right place right plant matters as well.