An RHS hardiness ratingThe RHS's H1a–H7 scale for how much cold a plant can survive outdoors, H1a being tender indoor-only and H7 hardy anywhere in the UK. Full definition → is a band of temperature, not a single number: the lowest winter cold a plant is likely to survive outdoors, such as "−15 to −10°C". Read it as a worst case. The RHS built the H1a to H7 scale in 2012 around absolute minimum winter temperature, not the long-term average annual extreme minimum that the American USDA zones use. In Scotland, altitude, distance from the sea and frost pocketsA low-lying spot where cold air drains and pools on still nights, making it noticeably more frost-prone than the surrounding area. Full definition → decide which band a garden sits in, far more than the region it falls in.
The scale in full
| Rating | Temperature | RHS description | USDA zone |
|---|---|---|---|
| H1a | Warmer than 15°C | Heated glasshouse, tropical: grow as a house plant or under glass all year | 13 |
| H1b | 10 to 15°C | Heated glasshouse, subtropical: can go outdoors in summer in sunny, sheltered spots, but does best under glass | 12 |
| H1c | 5 to 10°C | Heated glasshouse, warm temperate: can go outdoors in summer throughout most of the UK | 11 |
| H2 | 1 to 5°C | Tender: tolerates low temperatures but not freezing, so needs a cool or frost-free glasshouse in winter | 10b |
| H3 | −5 to 1°C | Half-hardy: hardy in coastal or mild areas except in hard winters, at risk from sudden early frosts; may be hardy elsewhere with wall shelter | 9b/10a |
| H4 | −10 to −5°C | Hardy in an average winter, apart from inland valleys, altitude and central or northerly locations | 8b/9a |
| H5 | −15 to −10°C | Hardy through most of the UK even in severe winters; may not withstand open or exposed sites or central or northerly locations | 7b/8a |
| H6 | −20 to −15°C | Hardy throughout the UK and northern Europe | 6b/7a |
| H7 | Colder than −20°C | Very hardy: the severest European continental climates, including exposed upland UK | 6a to 1 |
The useful text is in H4 and H5, not the temperatures. H4 plants may suffer foliage damage and stem dieback in harsh winters in cold gardens, and many H5 evergreens suffer foliage damage. Neither line names Scotland, but inland valleys, altitude, and central and northerly locations describe a great deal of the country. H4 is the rating most often quoted on a generic plant label or seed packet, so it needs a harder look here than it gets in the mild south. Pots make it worse: H4 plants in pots are more vulnerable, H5 plants in pots are at increased risk, and many H6 plants in containers will be damaged unless given protection.
Testing the "Central Belt = H6, Highlands = H7" idea
It is a tidy shorthand and not baseless: much of Scotland falls outside what an H4 rating promises. But set against Met Office records, region-by-region shorthand collapses. The driver is altitude and distance from the sea, the same pattern behind spring frost timing.
These are extreme minimum records: the coldest single night at each station, the worst case a long-lived shrub or tree might eventually face in that spot.
| Station | Area | Recorded extreme minimum | Falls in |
|---|---|---|---|
| Glenlee | Dumfries & Galloway | −18.3°C (Jan 1940) | H6 |
| Glasgow Airport | Central Belt, lowland | −20°C (29 Dec 1995) | H6/H7 border |
| Carnwath | South Lanarkshire, inland | −24.8°C (11 Jan 1982) | H7 |
| Altnaharra | Sutherland, Highland interior | −27.2°C (30 Dec 1995) | H7 |
| Braemar | Aberdeenshire, Cairngorms fringe | −27.2°C (10 Jan 1982) | H7 |
Altnaharra and Braemar share the UK record low. Braemar sits in Aberdeenshire, in the north-east, not in the Highlands. Carnwath, inland in South Lanarkshire, has recorded −24.8°C, nearly 5°C colder than Glasgow Airport. The 1982 cold spell that set the Braemar and Carnwath records brought night minima below −15°C and day maxima below −5°C across Scotland, and on 29 December 1995 Glasgow Airport recorded −20°C at night and only −12°C by day.
Why the coast changes everything
The sea moderates temperature, and altitude and distance from the sea dominate everything else. Western Scotland is milder than Eastern Scotland because of the stronger maritime influence, and the Gulf Stream keeps annual mean temperatures at 9.5 to 9.9°C on the coasts of Ayrshire, Bute and Kintyre, against 8.0 to 9.4°C more typically inland. Northern Scotland shows the same split: mean daily minimum temperatures in January or February are around 2°C on west-facing coasts and in the Western and Northern Isles, but below −1°C over higher ground. In the east, January mean minima are about 2°C on the Firth of Forth and north-east Grampian coasts and below −2°C over higher ground. Air frost days show it too: fewer than 40 a year on the Fife coast, more than 90 on the Lammermuir Hills and Grampians.
Where the cold pockets are
The coldest records come from inland and upland stations such as Altnaharra, Braemar and Carnwath. Cold air also drains into valleys and pools there on still, clear nights, and the deep valley of the River Dee around Braemar is a named example. A garden in a frost pocket or well inland, wherever it sits on a regional map, is closer to H7 than H6, and plants labelled hardy on the strength of a generic "Scotland is H6" assumption are the ones most at risk there.
What this means for choosing plants
A garden's real exposure depends on three things that cut across every regional boundary: height above sea level, distance from open water, and whether cold air can pool overnight. Read an H4 rating with caution almost anywhere in Scotland, since the H4 definition already excludes inland valleys, altitude, and northerly or central locations. Treat H6 as a reasonable working assumption for a lowland or coastal garden, not a guarantee, and reserve H7 for anywhere that is elevated, well inland, or in a frost pocket. The real cold ground is the Cairngorms, the interior of Sutherland and the edge of the Southern Uplands, with Carnwath in the Central Belt a reminder that low ground inland can be just as cold. On Shetland and other exposed islands, wind rather than cold is usually the harder test. For plants in pots, allow roughly one band more protection, because roots lose heat from all sides, and lag the pot in a hard winter.
Common questions
Is my garden H6 or H7?
There's no answer by postcode. Glenlee in Dumfries and Galloway has recorded -18.3°C (H6), Glasgow Airport -20°C (the H6/H7 border), and inland stations in South Lanarkshire and Aberdeenshire have recorded -24.8°C and -27.2°C (both H7). Height above sea level, distance from the sea and whether cold air pools on your site tell you more than the region does.
Does an H4 plant have any place in a Scottish garden?
Yes, with care about where it goes: a sheltered wall, a courtyard or a spot against a house wall in a mild coastal area. H4 is defined as hardy through most of the UK apart from inland valleys, at altitude and central or northerly locations, which describes much of Scotland. An H4 plant in open, exposed ground may suffer foliage damage and stem dieback in a harsh winter.
Why are coasts and islands milder in winter than inland ground?
The sea. Mean daily minimum temperatures in January or February run around 2°C on west-facing coasts and in the Western and Northern Isles, against less than -1°C over higher ground in the same region. The sea moderates temperature, so maritime areas have a smaller range between the warmest and coldest months.
Do plants in pots need a hardier rating than plants in the ground?
Yes. RHS notes that H4 plants in pots are more vulnerable, H5 plants in pots are at increased risk, and many H6 plants in containers will be damaged unless given protection. Move pots somewhere sheltered or protect them in a hard winter.
Where can I check my own local frost risk more closely?
The Scottish frost dates page breaks the country down by named Met Office station, with a lowland and upland split, which is a closer match to a single garden than a postcode region.
Sources
Show the 4 sources
- RHS: RHS hardiness rating: Full H1a to H7 scale, temperature bands and descriptions, USDA equivalents, the absolute-minimum-winter-temperature basis, introduction in 2012, the H4/H5 wording on inland valleys, altitude and central or northerly locations, and the cautions for plants in pots
- Met Office: Eastern Scotland climate: Braemar's -27.2°C UK-record low (10 January 1982), night minima below -15°C and day maxima below -5°C in that cold spell, mean January minima of about 2°C on the Firth of Forth and north-east Grampian coasts against less than -2°C on higher ground, air frost days from under 40 on the Fife coast to over 90 on the Lammermuir Hills and Grampians, ground frost days, and cold air draining into the Dee valley around Braemar
- Met Office: Western Scotland climate: Glenlee's -18.3°C (January 1940), Carnwath's -24.8°C (11 January 1982), Glasgow Airport's -20°C minimum and -12°C maximum (29 December 1995), and the maritime and Gulf Stream explanation for coastal against inland annual mean temperatures
- Met Office: Northern Scotland climate: Altnaharra's -27.2°C (30 December 1995, UK record shared with Braemar) and mean daily minima of about 2°C on west-facing coasts and in the Western and Northern Isles against less than -1°C over higher ground