Cold hardiness & minimum temperature

Is Amphibious Bistort (Persicaria amphibia)cold hardy? Hardiness zone & min temp

Also called Amphibious Bistort, Water Knotweed, Longroot Smartweed, Water Smartweed.

More about amphibious bistort

About Amphibious Bistort

Persicaria amphibia · also called Amphibious Bistort, Water Knotweed · flowering

Persicaria amphibia is a vigorous amphibious perennial native to ponds, lakes, ditches, and wet meadows across Europe, Asia, and North America, growing in two distinct forms: a submerged aquatic form with floating leaves, and a terrestrial form growing in moist soil on land. It produces attractive upright spikes of bright pink flowers in summer that are highly attractive to bees and other pollinators. The most important care point is managing its vigorous spreading habit — it can colonise large areas of pond surface or wet ground rapidly. Not confirmed toxic by the ASPCA; treat as mildly toxic as a precaution.

Cold limit: USDA 3-10 · RHS H7 (-30°C to 30°C)

What amphibious bistort's hardiness rating actually means

Yes — amphibious bistort is genuinely cold hardy. Rated RHS H7 and USDA 3-10, 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 3-10 — 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. Amphibious Bistort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for amphibious bistort 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 amphibious bistort go outside or overwinter — and where?

Plant it out within USDA 3-10 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 amphibious bistort can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Amphibious Bistort hardiness — frequently asked questions

Is amphibious bistort cold hardy?

Yes — amphibious bistort is genuinely cold hardy. Rated RHS H7 and USDA 3-10, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Amphibious Bistort is hardy across USDA 3-10; 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 amphibious bistort can survive?

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

What hardiness zone is amphibious bistort?

Amphibious Bistort is rated USDA 3-10 and RHS H7 — Hardy in the severest European continental winters.

Can amphibious bistort survive winter outside?

Plant it out within USDA 3-10 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 amphibious bistort 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.