The term 'frost
hardy' is often misleading because of the degrees of
light frost vs.
hard killing frost) in fact there is not an universal standard
to define frost
resistance of plants according to their ability to
degrees of frost. Frost hardines is very much relative to
Hardy plants can tolerate frost but vary in the severity and
duration they can survive.
Here a common classification of plant hardiness:
Frost hardy plants can stay
alive even when temperatures go down to -5°C
Fully hardy plants can cope
with short spells as low as -15°C.
Half-hardy plants cannot
tolerate frost can survive when temperatures go down to 0C.
Tender plants maybe damaged
by low temperatures below 5°C.
These are general guidelines
because if a plant is in a protected or sheltered spot it might
well survive lower temperatures.