Cold hardiness & minimum temperature

Is Starry Rosinweed (Silphium asteriscus)cold hardy? Hardiness zone & min temp

Also called Starry rosinweed, Starry silphium.

More about starry rosinweed

About Starry Rosinweed

Silphium asteriscus · also called Starry rosinweed, Starry silphium · flowering

Starry rosinweed is a native prairie perennial from the southeastern and central United States, thriving in open woodlands, roadsides, and dry to moderately moist meadows. It produces cheerful, daisy-like yellow flowers with a prominent central disk throughout summer, attracting bees, butterflies, and goldfinches to the seed heads. The single most important care fact is excellent drainage — like all silphiums, it will rot in soggy soil but is highly drought-tolerant once established. Toxicity to cats and dogs is not documented in the ASPCA database; classify with caution as mildly-toxic until confirmed.

Cold limit: USDA 5-9 · RHS H6 (-20 to 35°C)

Watch for — Root rot in poorly drained soil: The most common cause of plant loss; avoid clay-heavy sites or low spots where water pools, especially in winter.

What starry rosinweed's hardiness rating actually means

Yes — starry rosinweed is genuinely cold hardy. Rated RHS H6 and USDA 5-9, 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 5-9 — 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 −20 to −15 °C. Starry Rosinweed is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for starry 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 starry rosinweed go outside or overwinter — and where?

Plant it out within USDA 5-9 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 starry rosinweed can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Starry Rosinweed hardiness — frequently asked questions

Is starry rosinweed cold hardy?

Yes — starry rosinweed is genuinely cold hardy. Rated RHS H6 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Starry Rosinweed is hardy across USDA 5-9; 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 starry rosinweed can survive?

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

What hardiness zone is starry rosinweed?

Starry Rosinweed is rated USDA 5-9 and RHS H6 — Hardy throughout the UK and northern Europe.

Can starry rosinweed survive winter outside?

Plant it out within USDA 5-9 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 starry 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.