Cold hardiness & minimum temperature

Is Swamp Sunflower (Helianthus angustifolius)cold hardy? Hardiness zone & min temp

Also called Swamp Sunflower, Narrowleaf Sunflower.

More about swamp sunflower

About Swamp Sunflower

Helianthus angustifolius · also called Swamp Sunflower, Narrowleaf Sunflower · flowering

Swamp Sunflower is a late-blooming eastern US native perennial that erupts in masses of golden-yellow flowers in autumn, often coinciding with fall foliage. Unlike most sunflowers, it tolerates consistently moist or even seasonally wet soils, making it ideal for rain gardens, pond margins, and boggy borders. A magnet for bees and butterflies in late season.

Cold limit: USDA 6-9 · RHS H5 (−20°C to 38°C)

What swamp sunflower's hardiness rating actually means

Yes — swamp sunflower is genuinely cold hardy. Rated RHS H5 and USDA 6-9, 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 6-9 — 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. Swamp Sunflower is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for swamp sunflower 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 swamp sunflower go outside or overwinter — and where?

Plant it out within USDA 6-9 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 swamp sunflower can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Swamp Sunflower hardiness — frequently asked questions

Is swamp sunflower cold hardy?

Yes — swamp sunflower is genuinely cold hardy. Rated RHS H5 and USDA 6-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Swamp Sunflower is hardy across USDA 6-9; 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 swamp sunflower can survive?

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

What hardiness zone is swamp sunflower?

Swamp Sunflower is rated USDA 6-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can swamp sunflower survive winter outside?

Plant it out within USDA 6-9 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 swamp sunflower 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.