Half your houseplants could survive a UK winter outdoors

Key findings

1. 49.3% of catalogued species (4,122 of 8,359) are rated cold-hardy enough to survive a UK winter outdoors. On minimum-temperature alone, nearly one in two houseplants clears the RHS H4 threshold — hardy through most of the UK. That is a larger share than most gardeners would expect from plants sold as indoor specimens.
2. 8,359 of 8,452 catalogued species carry a hardiness rating; the remaining 93 (1.1%) were excluded. Coverage is high enough that the 49.3% figure is representative of the full catalogue rather than a selected subset.
3. Most of England and Wales sits in USDA zone 9 — milder than the zone-8 threshold used here. A zone-8-rated plant is over-qualified for a typical English winter minimum. The binding limit for many of these species in the UK is not extreme cold but persistent wet-cold and lack of summer heat — factors that hardiness ratings do not capture.
4. RHS H4 is defined as -10°C to -5°C, described as 'Hardy through most of the UK apart from inland valleys, at altitude and central/northerly locations'. The RHS's own language supports the headline framing: H4 is the baseline for mainstream UK outdoor hardiness, and our zone-8 definition aligns with it.
5. Winter wet, not frost, is the primary killer in the UK maritime climate for many zone-8-rated species. Mediterranean and drought-adapted plants rated to zone 8 in continental climates can fail at -2°C in saturated UK soil. Free-draining conditions are as important as the hardiness rating itself.

Here is the outdoor-hardiness breakdown across the catalogue.

Methodology

Dataset: 8,452 plant species catalogued in the Growli plant-care database, of which 8,359 carry a populated USDA hardiness-zone rating. The remaining 93 species (1.1%) were excluded from percentage calculations. Definition: "UK-hardy" = USDA zone 8 or colder (zone 1–8), which maps approximately to RHS hardiness H4–H7 (H4 = -10°C to -5°C, described by the RHS as "Hardy through most of the UK"). Because most of England and Wales sits in USDA zone 9 (milder than zone 8), a zone-8-rated plant clears a typical UK winter minimum comfortably in most of the country. The USDA→RHS mapping follows the official RHS hardiness-rating PDF (published 2012, confirmed current as of June 2026); it is an approximation — the RHS rates plants, not places, precisely because UK maritime climate is not fully captured by minimum-temperature zones. The 49.3% figure was computed as 4,122 ÷ 8,359. All percentages are of the tagged (8,359-species) denominator, not the full catalogue. Caveats: cold-hardiness is minimum-temperature only; RHS ratings do not encode winter wet, drainage, wind exposure, or summer heat. A plant rated to -10°C in dry continental conditions can still rot at -2°C in three weeks of waterlogged UK soil. "Hardy enough" is a necessary condition, not a guarantee of outdoor success.