Cold hardiness & minimum temperature

Is Wolffia arrhiza (Wolffia arrhiza)cold hardy? Hardiness zone & min temp

Also called Rootless Duckweed, Watermeal, Spotless Watermeal.

More about wolffia arrhiza

About Wolffia arrhiza

Wolffia arrhiza · also called Rootless Duckweed, Watermeal · houseplant

Watermeal is the world's smallest flowering plant — rootless green grains under 1 mm across that float like fine dust on still water. Each plant is a tiny ovoid frond with no root at all, multiplying by budding into a powdery surface film. A curiosity for nano-aquariums and ponds, it is even faster and finer than common duckweed and notoriously hard to remove.

Cold limit: USDA 6-10 (sinks and overwinters as turions in colder water) · RHS H4 (10 to 33°C)

What wolffia arrhiza's hardiness rating actually means

Yes — wolffia arrhiza is genuinely cold hardy. Rated RHS H4 and USDA 6-10 (sinks and overwinters as turions in colder water), it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H4 means: Hardy in an average winter across much of the temperate world. On the US scale that maps to USDA 6-10 (sinks and overwinters as turions in colder water) — 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 −10 to −5 °C. Wolffia arrhiza is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for wolffia arrhiza as it gets too cold:

It tolerates winter lows to about −10 to −5 °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 wolffia arrhiza go outside or overwinter — and where?

Plant it out within USDA 6-10 (sinks and overwinters as turions in colder water) 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 wolffia arrhiza can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Wolffia arrhiza hardiness — frequently asked questions

Is wolffia arrhiza cold hardy?

Yes — wolffia arrhiza is genuinely cold hardy. Rated RHS H4 and USDA 6-10 (sinks and overwinters as turions in colder water), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Wolffia arrhiza is hardy across USDA 6-10 (sinks and overwinters as turions in colder water); 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 wolffia arrhiza can survive?

Minimum survivable temperature is roughly about −10 to −5 °C. Wolffia arrhiza is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is wolffia arrhiza?

Wolffia arrhiza is rated USDA 6-10 (sinks and overwinters as turions in colder water) and RHS H4 — Hardy in an average winter across much of the temperate world.

Can wolffia arrhiza survive winter outside?

Plant it out within USDA 6-10 (sinks and overwinters as turions in colder water) 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 wolffia arrhiza below its minimum temperature?

It tolerates winter lows to about −10 to −5 °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.