Cold hardiness & minimum temperature

Is King George aster (Aster amellus 'King George')cold hardy? Hardiness zone & min temp

Also called King George aster, King George Italian aster.

More about king george aster

About King George aster

Aster amellus 'King George' · also called King George aster, King George Italian aster · flowering

'King George' is a long-established cultivar of Italian aster, prized for its large, rich violet-blue daisy flowers with deep golden-yellow disc centres produced freely from late August through September. It is compact, reliably mildew-resistant, and thrives in alkaline, well-drained soils. An AGM-awarded plant from the Royal Horticultural Society, it is one of the finest selections of A. amellus for late-season colour.

Cold limit: USDA 4–8 · RHS H6 (-20 to 32°C)

Watch for — Root rot in wet or clay soils: The main risk for this cultivar is sitting in waterlogged soil over winter. Excellent drainage is non-negotiable. Plant on a slight slope or raise the bed if the native soil holds moisture.

What king george aster's hardiness rating actually means

Yes — king george aster 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. King George aster is built for winter — once established it takes hard frost and snow in its stride.

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

King George aster hardiness — frequently asked questions

Is king george aster cold hardy?

Yes — king george aster 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. King George aster 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 king george aster can survive?

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

What hardiness zone is king george aster?

King George aster is rated USDA 4–8 and RHS H6 — Hardy throughout the UK and northern Europe.

Can king george aster 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 king george aster 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.