Cold hardiness & minimum temperature

Is Hawaii Blue Flossflower (Ageratum houstonianum)cold hardy? Hardiness zone & min temp

Also called Flossflower, Bluemink, Blueweed, Pussy Foot.

More about hawaii blue flossflower

About Hawaii Blue Flossflower

Ageratum houstonianum · also called Flossflower, Bluemink · flowering

Hawaii Blue Flossflower is a compact, early-blooming annual producing a mass of fluffy, powder-blue flower clusters from late spring to autumn. One of the best edging plants for containers and borders, it thrives in full sun. Ageratum is listed by the ASPCA as toxic to horses and mildly toxic to dogs and cats.

Cold limit: USDA 2-11 (frost-tender annual) · RHS H1C (15-28°C)

What hawaii blue flossflower's hardiness rating actually means

Yes — hawaii blue flossflower is genuinely cold hardy. Rated RHS H7 and USDA 2-11 (frost-tender annual), 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 2-11 (frost-tender annual) — 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. Hawaii Blue Flossflower is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for hawaii blue flossflower 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 hawaii blue flossflower go outside or overwinter — and where?

Plant it out within USDA 2-11 (frost-tender annual) 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 hawaii blue flossflower can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Hawaii Blue Flossflower hardiness — frequently asked questions

Is hawaii blue flossflower cold hardy?

Yes — hawaii blue flossflower is genuinely cold hardy. Rated RHS H7 and USDA 2-11 (frost-tender annual), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Hawaii Blue Flossflower is hardy across USDA 2-11 (frost-tender annual); 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 hawaii blue flossflower can survive?

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

What hardiness zone is hawaii blue flossflower?

Hawaii Blue Flossflower is rated USDA 2-11 (frost-tender annual) and RHS H7 — Hardy in the severest European continental winters.

Can hawaii blue flossflower survive winter outside?

Plant it out within USDA 2-11 (frost-tender annual) 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 hawaii blue flossflower 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.