Cold hardiness & minimum temperature

Is Lobelia siphilitica (Lobelia siphilitica)cold hardy? Hardiness zone & min temp

Also called Blue Cardinal Flower, Great Blue Lobelia.

More about lobelia siphilitica

About Lobelia siphilitica

Lobelia siphilitica · also called Blue Cardinal Flower, Great Blue Lobelia · flowering

Lobelia siphilitica is a robust, moisture-loving perennial producing dense spikes of clear blue, tubular flowers above upright leafy stems in late summer and autumn. A native of wet meadows and streambanks, it is hardier and longer-lived than its scarlet cousin and a valuable late-season nectar source for bees and hummingbirds.

Cold limit: USDA 4-9 (hardy perennial) · RHS H5 (-30 to 30°C)

Watch for — Crown rot over winter: Basal rosettes can rot under heavy wet mulch or in waterlogged frozen ground. Keep the crown clear of smothering debris in winter.

What lobelia siphilitica's hardiness rating actually means

Yes — lobelia siphilitica is genuinely cold hardy. Rated RHS H5 and USDA 4-9 (hardy perennial), 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 (hardy perennial) — 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. Lobelia siphilitica is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for lobelia siphilitica 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 lobelia siphilitica go outside or overwinter — and where?

Plant it out within USDA 4-9 (hardy perennial) 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 lobelia siphilitica can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Lobelia siphilitica hardiness — frequently asked questions

Is lobelia siphilitica cold hardy?

Yes — lobelia siphilitica is genuinely cold hardy. Rated RHS H5 and USDA 4-9 (hardy perennial), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Lobelia siphilitica is hardy across USDA 4-9 (hardy perennial); 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 lobelia siphilitica can survive?

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

What hardiness zone is lobelia siphilitica?

Lobelia siphilitica is rated USDA 4-9 (hardy perennial) and RHS H5 — Hardy in most of the UK and in cold winters.

Can lobelia siphilitica survive winter outside?

Plant it out within USDA 4-9 (hardy perennial) 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 lobelia siphilitica 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.