Hardiness

How much cold a plant can take. Every care sheet has a code from H1a to H7: the Royal Horticultural Society scale, with the lowest temperature the plant survives in the ground.

H1aheated glasshouseTrue houseplant, needs above 15°C. Not really an outdoor plant in the UK climate at all, even in summer, except perhaps a very brief spell in a heatwave in full sun.African violet

H1babove 10 °CTender, needs above 10°C. Only suited to a sheltered spot outdoors in the warmest part of summer; otherwise wants a warm greenhouse or conservatory.Begonia

H1cabove 5 °CTender, needs above 5°C. Can spend summer outside in a sheltered spot, but must come in well before the first frost and stay above 5°C over winter.Aeonium

H2down to 1 °CTender. Fine outdoors for summer, but needs to be moved somewhere frost-free for winter (roughly 1°C to 5°C).Echeveria

H3down to −5 °CHalf hardy. Survives outdoors in a mild winter or a sheltered, coastal or urban garden, but can be damaged or killed in a harder one (roughly -5°C to 1°C).Dahlia

H4down to −10 °CHardy through an average UK winter. The baseline 'fully hardy' rating for most garden plants, reliable down to about -10°C to -5°C.Penstemon

H5down to −15 °CHardy in most of the UK, but can struggle in the coldest inland or upland spots. Comfortable down to roughly -15°C to -10°C — covers most of what a South East England garden will ever see.Cherry laurel

H6down to −20 °CHardy in the vast majority of UK gardens. Copes with a typical harsh winter down to around -20°C to -15°C.Box

H7below −20 °CFully hardy anywhere in the UK. Survives even a severe inland or upland winter, well below -20°C.Hardy geranium

The bar is cold tolerance: longer means hardier. On the right, an example plant.

In a pot, one step down

A plant's roots in a container experience a noticeably colder microclimate than the same plant's roots in open ground. Soil in a border is insulated by the sheer mass of earth around it; compost in a pot has only a thin wall of ceramic, plastic or terracotta between the roots and the outside air, so it chills — and freezes — much faster and more deeply.

A plant's effective hardiness drops by roughly one whole H-band when grown in a pot, and potentially more if the pot is small, thin-walled, or standing somewhere exposed to wind.

In the ground

Oleander · H3

In a pot

counts as H2

An H5 shrub in open ground might need treating as an H4 (or worse) in a container, benefiting from extra winter protection it wouldn't need in a border.

Surviving isn't thriving

A hardiness rating says what a plant survives, not what keeps it happy. The two diverge most in succulents: a care guide will say "keep above 10°C" while the same plant is rated H2, tolerant down to 1°C. Both are true — the first is the temperature at which it grows well, the second the temperature it comes through. And the rating assumes the plant is dry: the same succulent kept wet dies well above its rating. Read the number as the floor it survives on a dry root, not as the thermostat setting.