Cold hardiness & minimum temperature

Is Whorled Rosinweed (Silphium trifoliatum)cold hardy? Hardiness zone & min temp

Also called Whorled rosinweed, Three-leaved rosinweed, Starry rosinweed.

More about whorled rosinweed

About Whorled Rosinweed

Silphium trifoliatum · also called Whorled rosinweed, Three-leaved rosinweed · flowering

Silphium trifoliatum is a distinctive native prairie and open-woodland perennial of the eastern and central US, recognisable by its leaves arranged in whorls of three or four around smooth or slightly rough stems — unusual in a genus where most species have opposite leaves. It produces cheerful yellow ray flowers with a yellow disc from midsummer to early autumn and is one of the more shade-adaptable Silphium species, tolerating the partial shade of woodland edges. The most important care fact is adequate soil drainage — root rot in waterlogged soils remains the main cultural challenge. Silphium trifoliatum is not listed by the ASPCA as toxic to cats or dogs.

Cold limit: USDA 4-8 · RHS H6 (-28 to 35°C)

What whorled rosinweed's hardiness rating actually means

Yes — whorled rosinweed is genuinely cold hardy. Rated RHS H6 and USDA 4-8, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H6 means: Hardy throughout the UK and northern Europe. On the US scale that maps to USDA 4-8 — 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 −20 to −15 °C. Whorled Rosinweed is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for whorled rosinweed as it gets too cold:

It tolerates winter lows to about −20 to −15 °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 whorled rosinweed go outside or overwinter — and where?

Plant it out within USDA 4-8 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 whorled rosinweed can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Whorled Rosinweed hardiness — frequently asked questions

Is whorled rosinweed cold hardy?

Yes — whorled rosinweed is genuinely cold hardy. Rated RHS H6 and USDA 4-8, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Whorled Rosinweed is hardy across USDA 4-8; 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 whorled rosinweed can survive?

Minimum survivable temperature is roughly about −20 to −15 °C. Whorled Rosinweed is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is whorled rosinweed?

Whorled Rosinweed is rated USDA 4-8 and RHS H6 — Hardy throughout the UK and northern Europe.

Can whorled rosinweed survive winter outside?

Plant it out within USDA 4-8 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 whorled rosinweed below its minimum temperature?

It tolerates winter lows to about −20 to −15 °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.