Cold hardiness & minimum temperature

Is Agastache 'Black Adder' (Agastache 'Black Adder')cold hardy? Hardiness zone & min temp

Also called Black Adder agastache.

More about agastache 'black adder'

About Agastache 'Black Adder'

Agastache 'Black Adder' · also called Black Adder agastache · flowering

Agastache 'Black Adder' is an aromatic hybrid hyssop with dense, smoky violet-blue flower spikes emerging from near-black buds from midsummer to autumn. Vigorous and long-blooming, it wants full sun and free-draining soil, tolerates heat and drought once established, and is a prolific nectar source for bees, butterflies and hummingbirds.

Cold limit: USDA 5-9 · RHS H5 (15-30°C)

Watch for — Winter wet rot: The main cause of loss; cold, wet soil rots the crown. Provide sharp drainage and avoid waterlogged or heavy ground.

What agastache 'black adder''s hardiness rating actually means

Yes — agastache 'black adder' 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. Agastache 'Black Adder' is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for agastache 'black adder' 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 agastache 'black adder' 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 agastache 'black adder' can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Agastache 'Black Adder' hardiness — frequently asked questions

Is agastache 'black adder' cold hardy?

Yes — agastache 'black adder' 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. Agastache 'Black Adder' 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 agastache 'black adder' can survive?

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

What hardiness zone is agastache 'black adder'?

Agastache 'Black Adder' is rated USDA 5-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can agastache 'black adder' 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 agastache 'black adder' 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.