Cold hardiness & minimum temperature

Is Scarlet Giant Hyssop (Agastache coccinea)cold hardy? Hardiness zone & min temp

Also called Scarlet Giant Hyssop, Red Giant Hyssop.

More about scarlet giant hyssop

About Scarlet Giant Hyssop

Agastache coccinea · also called Scarlet Giant Hyssop, Red Giant Hyssop · flowering

Scarlet Giant Hyssop is a striking North American perennial bearing vivid scarlet to orange tubular flower spikes from midsummer into autumn, irresistible to hummingbirds and long-tongued pollinators. More heat- and drought-tolerant than many Agastache species, it suits sunny, well-drained borders and xeriscape gardens. Often treated as a short-lived perennial or annual in cooler climates.

Cold limit: USDA 5–9 · RHS H5 (-10–35°C)

Watch for — Short lifespan in cool, wet climates: In UK and northern European conditions, plants may behave as short-lived perennials or annuals. Overwinter cuttings or collect seed in autumn to ensure continuity. Mulch crowns in late autumn for added cold protection.

What scarlet giant hyssop's hardiness rating actually means

Yes — scarlet giant hyssop is genuinely cold hardy. Rated RHS H5 and USDA 5–9, 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 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 −15 to −10 °C. Scarlet Giant Hyssop is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for scarlet giant hyssop 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 scarlet giant hyssop 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 scarlet giant hyssop can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Scarlet Giant Hyssop hardiness — frequently asked questions

Is scarlet giant hyssop cold hardy?

Yes — scarlet giant hyssop is genuinely cold hardy. Rated RHS H5 and USDA 5–9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Scarlet Giant Hyssop 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 scarlet giant hyssop can survive?

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

What hardiness zone is scarlet giant hyssop?

Scarlet Giant Hyssop is rated USDA 5–9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can scarlet giant hyssop 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 scarlet giant hyssop 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.