The hardiness ratings (H1 to H7) are part of the Royal Horticultural Society’s (RHS) system to indicate the minimum winter temperatures a plant can tolerate. Here are the temperature ranges associated with each hardiness rating:
- H1 – Heated Greenhouse:
- Plants that require a minimum temperature above 15°C (59°F).
- These are tropical plants that cannot tolerate cool or cold conditions.
- H2 – Tender:
- Plants that require a minimum temperature of 1 to 5°C (34 to 41°F).
- These plants are sensitive to frost and are typically grown as houseplants or treated as annuals in temperate climates.
- H3 – Half Hardy:
- Plants that can tolerate a minimum temperature of -5 to 1°C (23 to 34°F).
- These plants may survive light frosts but are generally not suitable for prolonged exposure to freezing temperatures.
- H4 – Hardy:
- Plants that can tolerate a minimum temperature of -10 to -5°C (14 to 23°F).
- These plants are considered hardy and can withstand average winter conditions in many temperate climates.
- H5 – Hardy in Most Places Throughout the UK even in Severe Winters:
- Plants that can tolerate a minimum temperature of -15 to -10°C (5 to 14°F).
- These plants are robust and can withstand colder temperatures, making them suitable for most regions in the United Kingdom.
- H6 – Hardy in All of UK and Northern Europe:
- Plants that can tolerate a minimum temperature of -20 to -15°C (-4 to 5°F).
- These plants are extremely hardy and can withstand even colder conditions, making them suitable for Northern European climates.
- H7 – Hardy in the Severest European Continental Climates:
- Plants that can tolerate a minimum temperature of below -20°C (-4°F).
- These plants are adapted to extremely cold climates and can survive severe winter conditions.
It’s important to note that while hardiness ratings provide valuable guidance, they are not the only factor influencing a plant’s success in a specific location. Other factors, such as soil type, moisture levels, and local microclimates, also play a role in determining a plant’s overall health and resilience. Gardeners should consider these factors and choose plants that align with the specific conditions of their garden or region. Additionally, the hardiness ratings are meant to provide a general guideline, and individual plant species may have specific variations within these categories.