Cold hardiness & minimum temperature

Is Purple Giant Hyssop (Agastache scrophulariifolia)cold hardy? Hardiness zone & min temp

Also called Purple Giant Hyssop, Figwort-Leaved Giant Hyssop.

More about purple giant hyssop

About Purple Giant Hyssop

Agastache scrophulariifolia · also called Purple Giant Hyssop, Figwort-Leaved Giant Hyssop · flowering

A tall native North American perennial found in woodland edges, thickets, and moist roadsides from the eastern US through the Midwest. Bears dense spikes of purple to rose-purple flowers from midsummer into autumn, providing vital nectar for long-tongued bees and hummingbirds. More tolerant of partial shade and moist soils than western hyssops. Excellent for native and wildlife gardens.

Cold limit: USDA 4–8 · RHS H6 (−25°C to 35°C)

What purple giant hyssop's hardiness rating actually means

Yes — purple giant hyssop 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. Purple Giant Hyssop is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for purple giant hyssop 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 purple giant hyssop 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 purple giant hyssop can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Purple Giant Hyssop hardiness — frequently asked questions

Is purple giant hyssop cold hardy?

Yes — purple giant hyssop 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. Purple Giant Hyssop 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 purple giant hyssop can survive?

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

What hardiness zone is purple giant hyssop?

Purple Giant Hyssop is rated USDA 4–8 and RHS H6 — Hardy throughout the UK and northern Europe.

Can purple giant hyssop 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 purple giant hyssop 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.