hardiness definition

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).

hardiness scale

Classification of hardiness in perennial plants:

USDA Classification 1-14

RHS Classification H1a to H7

  1. H1a Tropical plants >15C all year round (sometimes = ‘tender’ e.g. Phalaenopsis orchids and Fuchsia ’Firecracker’)

  2. H1b Subtropical (Monstera delicious (Swiss cheese plant) sheltered outside during warm spells)

  3. H1c Warm temperate (tomatoes and Pelargonium outside in summer)

  4. 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

  5. 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

  6. 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)