H6 - Hardy (very cold winter)

Very cold-hardy plants for severe winter conditions, useful in northern, inland and upland gardens where lower-rated plants face greater risk.

What it means

H6 plants tolerate around -20 to -15 degrees C and are hardy through very cold winters. They are valuable in northern UK, exposed inland areas, upland gardens and colder parts of Scotland, where H5 plants may be tested in severe spells. In milder lowland gardens they are generally very reliable, assuming suitable soil and moisture. The RHS scale rates the hardiness of each plant, rather than assigning a rating to place.

Example plants

Examples include many birches, rowans, hardy maples, sorbus, hardy conifers, pulmonarias, bergenias and robust alpine perennials. Numerous woodland plants and northern-origin shrubs also have H6 tolerance. These are often chosen for exposed gardens, though a plant's resilience to cold does not automatically mean it enjoys coastal salt, shade, drought or heavy clay, so match it to the site.

Protecting these plants

H6 plants need little frost protection once established, but site preparation still matters. Ensure winter drainage, especially for alpines and plants with crowns vulnerable to wet. Mulch young roots, secure newly planted trees against wind and protect pots from repeated freezing. Avoid forcing early soft growth with excessive feeding, as late spring frosts can still mark otherwise very hardy plants.

RHS hardiness ratings describe a plant's cold tolerance, not a geographic zone. Local conditions vary; the RHS or a local garden centre can advise. Page updated 18 Sep 2026.