RHS hardiness rating
H6 — Hardy (very cold winter)
minimum -20 to -15 °C · minimum -4 to 5 °F · 120-day growing season
What this rating means
The RHS rating system measures the lowest winter temperature a plant will reliably tolerate. H6 sits at minimum -20 to -15 °C. In practical UK terms, H6 suits upland scotland, the cairngorms foothills, the pennines, high ground in the north of england and north wales. For comparison with US references, H6 is roughly equivalent to USDA zone 6.
What grows at H6
Plants commonly recommended for H6 UK gardens:
- Picea, Pinus, Abies (conifers)
- Sorbus aucuparia (rowan)
- Birch (Betula pendula, B. utilis)
- Highland heathers (Calluna, Erica)
- Vaccinium (blueberries, bilberry)
- Rugosa roses
- Astrantia
- Lupins
- Cold-tolerant fruit — apple (Discovery, James Grieve), gooseberry, raspberry
- Vegetables — kale, cabbage, leeks, swede, parsnip, broad beans, early potatoes
- Rhubarb
- Native woodland species (bluebell, wood anemone, foxglove)
Climate notes for H6
In H6 country the growing season is short and the season-extension kit (cloches, polytunnels, fleece) earns its keep. Tomatoes and peppers want a polytunnel or sunny south-facing wall. Garlic and rhubarb stay reliable; tender perennials need lifting or heavy mulch.
Frost dates and growing season
| Typical last spring frost | late May to early June |
|---|---|
| Typical first autumn frost | early October |
| Growing season length | ~120 days |
| Temperature minimum (Celsius) | minimum -20 to -15 °C |
| Temperature minimum (Fahrenheit) | minimum -4 to 5 °F |
| Approx USDA equivalent | zone 6 |
Frost dates above are typical for the region the rating describes, not strict cutoffs. A south-facing walled garden in a H6 area can run a fortnight warmer; a frost pocket at the bottom of a valley can run a fortnight colder. Always cross-check against your local Met Office station before sowing tender crops.
When to plant — crop by crop in H6
Sowing, transplant, and harvest dates calibrated to a H6 winter:
- When to plant tomatoes in H6
- When to plant peppers in H6
- When to plant basil in H6
- When to plant garlic in H6
- When to plant lettuce in H6
- When to plant bush beans in H6
- When to plant cucumbers in H6
- When to plant summer squash in H6
- When to plant peas in H6
- When to plant carrots in H6
Source and methodology
Temperature thresholds from the RHS hardiness rating reference. Frost-date typicals are taken from Met Office regional climate summaries for the geographies each rating describes. Plant lists curated by the Growli editorial team from the RHS Find a Plant database and UK extension references. USDA-zone equivalents are approximate cross-references for readers comparing American catalogues.