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The ability of a plant to survive certain temperatures, especially low temperatures over the winter period, or as an indication of when to plant out as bedding plants.
• Hardy
This describes a plant that can withstand cold freezing temperatures.
• Half-hardy
These plants can be grown outdoors once the risk of frost has passed. They can survive in low temperatures. Examples of half-hardy annuals are the African Marigold; Lobelia; Zinnia. Examples of half-hardy perennials are Impatiens, Petunia (however, these plants are often grown as annuals).
• Tender
These plants must not be exposed to freezing conditions otherwise they will die; they should also be protected from cold conditions, especially from cold winds. Examples of tender plants are Dahlia, Begonia (tuberous), Gladiolus (corms); Cannas.

Hardiness classifications
Geographical regions can be categorised into hardiness zones which provide an indication of the likely survivability of plants in different zones.

The United States Department of Agriculture (USDA) has devised a scheme which has 13 zones relating to the average minimum temperature in that zone.

The minimum requirements for plants in the UK cover ranges from zone 6 to 10 (each zone being divided into a and b), which spans the temperature range -10°F to 40°F.
• Zone 6: - 10°F to 0°F (-17.8 to -23.3°C).
• Zone 7: 0°F to 10°F (-12.2 to -17.8°C).
• Zone 8: 10°F to 20°F (-12.2 to -6.7°C).
• Zone 9: 20°F to 30°F (-6.7 to -1.1°C).
• Zone 10: 30°F to 40°F (-1.1 to 4.4°C).
(Source: https://planthardiness.ars.usda.gov, accessed 2nd March 2022)

The Royal Horticultural Society (RHS) also uses a hardiness rating from H1 to H7 indicting minimum temperatures for plant survival.
• H1a to H3 require greenhouse protection for at least some of the year, to all of the year for tropical plants - or used as house plants.
• H4 plants are hardy plants for an average winter (0.5°C to -10°C)
• H5 - H7 are hardier plants surviving colder conditions
(Source: https://www.rhs.org.uk/plants/pdfs/rhs-hardiness-rating.pdf, accessed 2nd March 2022)