Hardiness is a horticultural measure of the tolerance of plants for cold, particularly frost.
For annual plants, hardiness determines when and where plants should be sown or transplanted outdoors.
Hardy annuals: outdoors in spring; half-hardy and tender annuals under glass and outdoors in late spring/early summer respectively (after frost risk passed).
Classification of hardiness in perennial plants:
USDA Classification 1-14
RHS Classification H1a to H7
H1a Tropical plants >15C all year round (sometimes = ‘tender’ e.g. Phalaenopsis orchids and Fuchsia ’Firecracker’)
H1b Subtropical (Monstera delicious (Swiss cheese plant) sheltered outside during warm spells)
H1c Warm temperate (tomatoes and Pelargonium outside in summer)
H2 Tender (or frost-tender) cannot survive outside where min. less than +1 to +5C. Okay in greenhouse (frost-free) in winter, outside in summer
H3 Half-Hardy cannot survive outside where min. less than -5 to +1C. Unheated greenhouse ok, or protected outside/mild winter. Seeds need warmer conditions to germinate than hardy perennials (Note perennials treated as half-hardy annuals e.g. Salvia splendend ‘Blaze of Fire’, Salvia patens
H4 - H7 Hardy (frost-hardy) cannot survive where min. less than -5 to -10C (or -20C for H7) e.g. Fagus sylvatica (Beech), Iris pseudacorus. Some annuals survive winter as seed e.g. Love-in-a-mist (Nigella damascena), sweet pea (Lathyrus odoratus)