Cold hardiness & minimum temperature

Is Compass Plant (Silphium laciniatum)cold hardy? Hardiness zone & min temp

Also called Compass plant, Pilot weed, Rosinweed compass plant.

More about compass plant

About Compass Plant

Silphium laciniatum · also called Compass plant, Pilot weed · flowering

Silphium laciniatum is a dramatic, deep-rooted native prairie perennial of the central and eastern US, famous for its deeply pinnately-lobed basal leaves that orient north–south along a compass axis (reducing midday sun exposure), and for towering spikes of yellow daisy flowers in midsummer. The plant develops a massive taproot that can reach 4.5 m (15 ft) deep, making it extremely drought-resistant but also meaning it strongly resents transplanting once established. The most critical care fact is to site it carefully in its permanent position before planting, as moving an established plant almost always kills it. Silphium is not listed on the ASPCA toxic plant database and is not considered toxic to pets.

Cold limit: USDA 3-9 · RHS H7 (-34 to 38°C)

What compass plant's hardiness rating actually means

Yes — compass plant is genuinely cold hardy. Rated RHS H7 and USDA 3-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H7 means: Hardy in the severest European continental winters. On the US scale that maps to USDA 3-9 — 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 below about −20 °C. Compass Plant is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for compass plant as it gets too cold:

It tolerates winter lows to about −20 °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 compass plant go outside or overwinter — and where?

Plant it out within USDA 3-9 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 compass plant can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Compass Plant hardiness — frequently asked questions

Is compass plant cold hardy?

Yes — compass plant is genuinely cold hardy. Rated RHS H7 and USDA 3-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Compass Plant is hardy across USDA 3-9; 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 compass plant can survive?

Minimum survivable temperature is roughly below about −20 °C. Compass Plant is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is compass plant?

Compass Plant is rated USDA 3-9 and RHS H7 — Hardy in the severest European continental winters.

Can compass plant survive winter outside?

Plant it out within USDA 3-9 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 compass plant below its minimum temperature?

It tolerates winter lows to about −20 °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.