Key takeaways
- UK labels use the RHS H1a to H7 scale, where each rating means a set temperature range
- H4 (down to -10C) suits most lowland gardens; choose H5 or above for cold inland or high gardens
- The UK roughly matches USDA zones 8 and 9, but our cool summers make that comparison misleading
- Winter wet rots more "hardy" plants than frost does, especially on heavy clay
- Frost pockets, walls and shelter can make one part of a garden much colder or warmer than another
UK gardens use the RHS hardiness ratings, not USDA zones. The ratings run from H1a, for tropical plants that need a heated greenhouse, to H7, for plants that shrug off the coldest British winter. Each one tells you roughly how cold a plant can take.
This guide explains what each rating means, how it compares with USDA zones and why the zone number can mislead in a British garden. It also covers the thing labels don’t mention: wet soil kills more plants here than cold does.
What do the RHS hardiness ratings mean?
The RHS hardiness ratings replaced the old labels “fully hardy”, “half hardy” and “tender” in 2012. Those old terms had no agreed temperatures behind them. Each H rating gives a range instead.
The rating tells you the lowest air temperature an established plant should survive in the open. It doesn’t take account of your soil, drainage, wind or how long the cold lasts. Those things matter just as much in a real garden.
Weather temperatures are taken in a screen like this, above the grass. Your plants live at ground level, where a clear night can be colder.
RHS ratings and USDA zones compared
The USDA conversions below are rough, based on winter lows only.
| RHS rating | Lowest temperature | Rough USDA zone | Example plants | Where it works in the UK |
|---|---|---|---|---|
| H7 | Below -20C | 6 and colder | Scots pine, birch, most native trees | Anywhere |
| H6 | -20C to -15C | 6b to 7a | Yew, hornbeam, many hardy shrubs | Almost anywhere |
| H5 | -15C to -10C | 7b to 8a | Many shrubs and perennials | Most gardens, given good drainage |
| H4 | -10C to -5C | 8b to 9a | Hebe, Phormium, many roses | Most lowland gardens in an average winter |
| H3 | -5C to 1C | 9b to 10a | Agapanthus, some salvias, cordyline | Mild coastal and city gardens, or with winter protection |
| H2 | 1C to 5C | 10b | Pelargoniums, many fuchsias | Frost-free greenhouse in winter |
| H1c | 5C to 10C | 11 | Many houseplants | Warm greenhouse or indoors; outside in summer |
| H1b | 10C to 15C | 12 | Many tropical foliage plants | Heated greenhouse or a warm room |
| H1a | Above 15C | 13 | Tropical plants | Heated greenhouse all year |
Plants in the same group can vary, so check the rating of the exact variety you’re buying.
Why USDA zones mislead UK gardeners
The USDA hardiness zone map was made for the United States. It sorts places by just one thing: the average coldest winter night.
That works fairly well in places with hot summers and cold winters. Britain is different. The Atlantic keeps our winters mild for how far north we are, so much of the UK lands in USDA zones 8 and 9. That sounds generous, but our summers are cool too.
Many shrubs need summer warmth to ripen their stems and set flower buds. A plant listed as “zone 8 hardy” in America may grow here but never flower well, or go into winter with soft growth that frost then damages. Treat USDA zones as a rough guide to winter cold only. Our winters are also changing, and the guide to adapting your garden to climate change looks at what that means.
The mildest coastal gardens of the far south west can grow echiums and tree ferns outdoors. Go a few miles inland and they struggle.
Winter wet kills more plants than cold
Waterlogged soil is the main reason “hardy” plants die in British winters. This is especially true of Mediterranean plants such as lavender, rosemary and many salvias.
Wet soil holds little air, and roots need air to live. In waterlogged clay they suffocate, then rot sets in. By spring the plant looks frost-killed, but the damage was done underground during the wet months.
To fix it, improve the drainage of the whole bed rather than the planting hole. Dig grit into the top of the soil, or build a raised bed. Don’t just fill a planting hole in heavy clay with grit, because it collects water like a sump. The guide to improving drainage in clay soil covers the work, and the list of plants for clay soil is often the easier answer.
This lavender didn’t die of cold. Its roots sat in wet clay all winter and rotted.
Gardener’s tip: To check drainage, dig a hole about 30cm deep, fill it with water and let it drain. Fill it again. If water is still standing hours later, treat the spot as wet and choose tougher plants or improve the soil.
Why spring frosts do so much damage
Plants harden up in autumn as nights get colder. A mild spell in late winter or spring makes them start growing again and lose that toughness quickly.
That is why a late frost in April or May can blacken new shoots on a plant that sailed through a much colder January. New spring growth is soft and easily damaged. Keep fleece handy for borderline plants until the risk of frost has passed. The guide to protecting plants from frost explains how.
Frost damage usually starts at the soft shoot tips, which turn brown and crisp. Wait until spring growth starts before you cut it back.
How cold it gets across the UK
The UK is small, but winters vary a lot. The far south west and the Isles of Scilly rarely get hard frosts. Inland, upland and northern gardens are much colder, and parts of the Scottish Highlands are among the coldest places in Britain.
As a rough guide:
- Mild coastal and big-city gardens: H4 plants are usually safe, and some H3 plants survive with a little protection.
- Most lowland gardens in England and Wales: stick mainly to H4 and hardier.
- Cold inland spots, high ground, northern England and much of Scotland: choose H5 or hardier.
Tender plants in pots can still be grown anywhere if you bring them under cover. The guide to overwintering plants explains your options.
In exposed upland gardens, very hardy plants such as heathers and native shrubs are the safe choice.
Microclimates inside one garden
Every garden has warmer and colder spots. Knowing yours makes a big difference.
A frost pocket forms where cold air flows downhill and gets trapped. Cold air sinks like water and pools in hollows, or behind a solid fence or dense hedge at the bottom of a slope. Leaving a gap at the base of the fence lets the cold drain away.
A south-facing wall soaks up warmth by day and gives it back at night. It is the best spot for borderline plants such as figs. The soil at the foot of a wall is often dry, though, because the wall and roof keep rain off. That suits Mediterranean plants, but new plants there need watering.
Cold air has drained downhill and pooled against the fence. Plant only the hardiest things in a spot like this.
A sunny brick wall stays warmer at night. It’s the best home in the garden for a fig or other borderline plants.
How to read a plant label
Look for the H number on the label, often on the back. If there isn’t one, look up the plant’s full name before you buy.
Be careful with tender-looking plants sold early in spring. Many are raised under glass and haven’t been hardened off. Keep them somewhere sheltered and harden them off before planting out. The guide to hardening off plants shows how.
Month-by-month hardiness calendar
| Month | What to do |
|---|---|
| January | Note your coldest spots on frosty mornings |
| February | Check borderline plants for rot at the base |
| March | Keep fleece handy for late frosts |
| April | Plant borderline shrubs now so they root in before winter |
| May | Plant out tender plants once frosts are over, usually late May in the south and later further north |
| June | Test drainage in any bed where plants died over winter |
| July | Water new plants against warm walls |
| August | Take cuttings of tender and borderline plants as a backup |
| September | Stop feeding shrubs, so soft new growth doesn’t go into winter |
| October | Dig grit into wet beds. Lift or mulch tender bulbs and tubers before the first frost |
| November | Fleece borderline evergreens. Raise pots onto feet so they drain |
| December | Check stakes and ties, as wind rock lets water pool round the stem |
Common mistakes with hardiness ratings
- Trusting a USDA zone from an American website. It only tells you about winter cold, not our cool summers or wet winters.
- Ignoring drainage. An H5 plant in waterlogged clay can die in a mild winter.
- Planting borderline plants in autumn. They go into winter with few roots. Plant them in spring instead.
- Putting tender plants in a frost pocket. Use the warmest, best-drained spot you have.
- Cutting back frost damage too early. The damaged growth shields what’s below. Wait until spring growth starts.
For tough plants that cope with cold, wet and dry spells, the guide to climate-resilient plants is the next step. Browse more plants by condition on the plants hub.
Frequently asked questions
What hardiness zone is the UK in?
Most of the UK falls roughly in USDA zones 8 and 9. The mildest coasts of the far south west are milder still, and the coldest Highland glens are colder. UK plant labels don’t use these zones, though. They use the RHS H1a to H7 ratings instead.
Why do UK plant labels not use USDA zones?
The RHS brought in its own H1a to H7 ratings in 2012. Each rating gives a temperature range a plant should survive. USDA zones were made for the United States and only measure winter cold, which doesn’t suit a mild, wet island.
Does “fully hardy” on a plant label mean anything?
Not much, as it has no set temperature behind it. It’s an older term that some growers still use. Look for the H number on the label instead, or look the plant up before you buy.
Is the UK really the same zone as Georgia and Texas?
For winter cold, parts of it can be, but the summers are very different. USDA zones only measure winter lows. Hot American summers ripen plants in a way cool British summers can’t, so the same plant can behave very differently here.
Why do hardy plants die in my clay soil?
Winter waterlogging usually kills them, not frost. Wet clay holds little air, so roots suffocate and rot. The plant often looks frost-killed in spring, but the damage started underground in the wet months.
How much colder is a frost pocket?
A frost pocket can be several degrees colder than higher ground nearby. Cold air sinks and collects in hollows or behind solid fences and hedges. A gap at the bottom of the barrier lets it drain away.
What RHS rating do I need for my garden?
H4 suits most lowland gardens in England and Wales. Choose H5 or hardier for cold inland spots, frost pockets, high ground and much of Scotland. Mild coastal and city gardens can often risk some H3 plants.
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
