Cold hardiness & minimum temperature

Is Ostbo Red Mountain Laurel (Kalmia latifolia 'Ostbo Red')cold hardy? Hardiness zone & min temp

Also called Ostbo Red mountain laurel, Mountain laurel.

More about ostbo red mountain laurel

About Ostbo Red Mountain Laurel

Kalmia latifolia 'Ostbo Red' · also called Ostbo Red mountain laurel, Mountain laurel · flowering

A classic American garden cultivar of mountain laurel bearing distinctive deep red buds that open to soft pink flowers in May–June. Broader and more ornamental than the straight species, it forms a handsome, dense evergreen mound. Thrives in cool, acidic, humusy woodland soils in partial shade. All parts are highly toxic; grayanotoxins affect humans, dogs, and cats.

Cold limit: USDA 4-9 · RHS H5 (-26°C to 32°C)

What ostbo red mountain laurel's hardiness rating actually means

Yes — ostbo red mountain laurel is genuinely cold hardy. Rated RHS H5 and USDA 4-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H5 means: Hardy in most of the UK and in cold winters. On the US scale that maps to USDA 4-9 — the zones where it can be left outdoors year-round.

New to these scales? The USDA hardiness zone map explained covers how the zone numbers work, and you can find your own zone with the zone finder.

Minimum temperature — and what happens below it

Minimum survivable temperature is roughly about −15 to −10 °C. Ostbo Red Mountain Laurel is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for ostbo red mountain laurel as it gets too cold:

It tolerates winter lows to about −15 to −10 °C once established.
Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root.
First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.

Can ostbo red mountain laurel go outside or overwinter — and where?

Plant it out within USDA 4-9 and it overwinters with little or no help.
It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy.
The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

Work back from your local frost dates with the frost-date calculator: the last spring frost and first autumn frost are what really decide when ostbo red mountain laurel can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Ostbo Red Mountain Laurel hardiness — frequently asked questions

Is ostbo red mountain laurel cold hardy?

Yes — ostbo red mountain laurel is genuinely cold hardy. Rated RHS H5 and USDA 4-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Ostbo Red Mountain Laurel is hardy across USDA 4-9; it belongs in the ground or a frost-proof container, not on a windowsill, and many types actively need a cold winter to perform.

What is the minimum temperature ostbo red mountain laurel can survive?

Minimum survivable temperature is roughly about −15 to −10 °C. Ostbo Red Mountain Laurel is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is ostbo red mountain laurel?

Ostbo Red Mountain Laurel is rated USDA 4-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can ostbo red mountain laurel survive winter outside?

Plant it out within USDA 4-9 and it overwinters with little or no help. It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy. The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

What happens to ostbo red mountain laurel below its minimum temperature?

It tolerates winter lows to about −15 to −10 °C once established. Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root. First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.