What does "hardy" actually mean on a plant label?

It describes a plant's ability to survive cold, and the labels are more specific than most people realise — but they are also a simplification of something with several variables.

The RHS hardiness ratings, used across UK horticulture, run from H1a to H7:

H1a–H1c — heated glasshouse to warm summer outdoors only. Houseplants and tender exotics.

H2 (tender) — tolerates down to 1°C. Needs frost-free protection. Most bedding plants.

H3 (half hardy) — tolerates −5 to 1°C. Survives coastal and mild areas; killed inland in a hard winter.

H4 (hardy) — −10 to −5°C. Survives an average UK winter in most of the country. The largest category for garden plants.

H5 (hardy, cold winter) — −15 to −10°C.

H6 (hardy, very cold winter) — −20 to −15°C.

H7 (very hardy) — below −20°C.

Why the number alone is not enough. Several factors determine survival beyond the minimum temperature:

Winter wet is frequently the actual killer. Many Mediterranean and alpine plants survive far lower temperatures than their rating suggests if kept dry, and rot in a mild wet winter. Drainage matters as much as hardiness.

Duration. Surviving −8°C overnight is different from a week of it.

Wind chill and exposure.

Microclimate. A south-facing wall, an urban location or a sheltered corner can be a full rating warmer than open ground nearby.

Establishment. A young plant in its first winter is far more vulnerable than an established one.

Container plants are much less hardy than the same plant in the ground, because roots have no soil mass insulating them and can freeze solid. Assume roughly one rating less.

American labels use USDA zones, a different system based on average annual minimum temperature, and the two do not map neatly.