Know your climate

Plant Hardiness Ratings (RHS)

In the UK, plant hardiness is described by the RHS ratings H1a to H7, which rate how much cold a plant can take, from tender (H1a) to very hardy (H7). This is different from the USDA zones used in the US, which map geography. Most of lowland Britain sits around H4.

Rating	Minimum temperature	Meaning
H1a - Under glass (heated)	above 15 degrees C	Plants needing reliably warm, heated indoor conditions all year, with no tolerance of cool winter rooms, frost, or outdoor British weather.
H1b - Tender, warm	10 to 15 degrees C	Warm-climate plants requiring winter protection under glass, but often happy outside for the British summer once nights are reliably mild.
H1c - Tender, cool	5 to 10 degrees C	Tender plants that enjoy summer outdoors but need cool, frost-free cover through winter because ordinary British frosts can seriously damage or kill them.
H2 - Tender	1 to 5 degrees C	Frost-sensitive plants able to tolerate a cool, frost-free winter, usually in an unheated greenhouse or a protected indoor place.
H3 - Half-hardy	-5 to 1 degrees C	Plants that can survive light frost and sheltered winters, yet remain vulnerable during prolonged freezing weather or in exposed, colder British gardens.
H4 - Hardy (average winter)	-10 to -5 degrees C	Plants hardy through an average British winter, making them dependable outdoors in most lowland gardens when properly planted and established.
H5 - Hardy (cold winter)	-15 to -10 degrees C	Plants hardy in cold British winters, well suited to most gardens including many inland locations where H4 plants can be less dependable.
H6 - Hardy (very cold winter)	-20 to -15 degrees C	Very cold-hardy plants for severe winter conditions, useful in northern, inland and upland gardens where lower-rated plants face greater risk.
H7 - Very hardy	below -20 degrees C	The toughest plants, able to endure exceptionally severe cold and suited to demanding British gardens, particularly where winters are intensely frosty or exposed.