What do plant hardiness ratings actually mean?

A hardiness rating is a useful shorthand and a poor prediction. It tells you about one variable — how cold the plant can get before its tissue is damaged — and almost every British winter fatality is caused by something the rating does not describe.

Step by step

1. Read the RHS scale as a ladder of situations rather than temperatures.H1 is a heated greenhouse or a house. H2 tolerates cold but no frost, so a frost-free glasshouse. H3 is half-hardy — survives outside in mild coastal or urban gardens, fails in a cold one. H4 is hardy through an average winter across most of the UK. H5 handles a cold winter. H6 handles a very cold one. H7 handles the coldest UK conditions.
2. Match the rating to your own site, not to the country.This is where your microclimate survey pays off. An H3 plant is a reasonable bet against a sheltered sunny wall in a city and a certain loss in an open rural frost pocket in the same town.
3. Treat American zone numbers with care.USDA zones are built from average annual minimum temperature and were designed for a continental climate. Most of lowland Britain maps to zones 8 and 9, which sounds generous — but our winters are long, dark and wet rather than short and dry, and plenty of things rated for zone 7 in the US rot here.
4. Add the three factors the rating omits.Drainage in winter, exposure to drying wind, and how ripened the growth was going into winter. A borderline plant on free-draining soil in a sheltered spot that had a good summer will take far more cold than the same plant wet, exposed and soft.
5. Consider the plant's age and size.Ratings assume an established plant. A first-year plant of the same variety, with a small root system and soft growth, is routinely a full grade less hardy and needs protecting for its first winter or two.
6. Use the rating for triage, then decide.H6 and H7 — plant and forget. H4 and H5 — fine in most gardens, worth siting thoughtfully. H3 — a decision: your best spot, protected, or in a pot you can move. H1 and H2 — indoors or under glass, full stop.

Tips

• A plant rated a grade hardier than you need is the correct choice for a frost pocket, a windy site or a first winter.
• The label in a garden centre reflects the species; the variety can differ substantially. Named cultivars selected for hardiness exist for many borderline genera and are worth seeking out.
• Growing something borderline in a pot means you can move it — into the lee of the house, under an overhang, or into a shed for the worst fortnight. That converts an H3 into a plant you can actually keep.
• If a plant has survived three winters in your garden including a bad one, you have better information about it than any rating.

Common mistakes

• Buying to the rating and ignoring the drainage — More borderline plants die of winter wet in Britain than of cold. An H4 plant on waterlogged clay is at more risk than an H3 plant on sharp-draining soil in the same garden.
• Assuming a rating covers a plant's whole life — Ratings describe an established specimen. A young plant, a recently moved one, or one that went into winter still growing softly after a late feed is meaningfully less hardy than its label.

Questions people ask