Cold hardiness & minimum temperature

Is Riccia fluitans (Riccia fluitans)cold hardy? Hardiness zone & min temp

Also called crystalwort, floating liverwort.

More about riccia fluitans

About Riccia fluitans

Riccia fluitans · also called crystalwort, floating liverwort · tropical

Riccia fluitans, crystalwort, is a rootless aquatic liverwort that naturally floats as a bright green tangled mat. Famously pinned down with mesh in aquascaping, it forms a dazzling pearling carpet under strong light and CO2. Vivid but demanding, it sheds oxygen bubbles when thriving and needs high light, injected CO2 and frequent trimming to stay anchored.

Cold limit: USDA Not applicable (indoor tropical aquarium plant) (18-28°C)

What riccia fluitans's hardiness rating actually means

Riccia fluitans is not cold hardy. It is a tropical houseplant that dies if it is left out through frost — there is no zone where it overwinters outdoors in a UK or cold-US climate. Its RHS rating of H1b means: Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season. On the US scale that maps to USDA Not applicable (indoor tropical aquarium plant) — 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 °C (sustained cold below this is damaging). Riccia fluitans has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for riccia fluitans as it gets too cold:

Below about about 10 °C, growth stalls and the leaves start to show cold stress — dark, water-soaked, or yellowing patches.
A single light frost blackens the foliage; a hard freeze kills the whole plant, roots included, and it does not recover.
Even a cold, draughty windowsill or an unheated porch in winter can be enough to damage it permanently.

Can riccia fluitans go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above 10 °C, in shade or dappled light, hardened off gradually.
Bring it back indoors well before the first autumn frost — do not wait for a frost warning, move it when nights drop toward 10-12 °C.
It will never overwinter outside in a temperate climate; the indoors is its winter home, full stop.

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 riccia fluitans can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1b figure above.

Riccia fluitans hardiness — frequently asked questions

Is riccia fluitans cold hardy?

Riccia fluitans is not cold hardy. It is a tropical houseplant that dies if it is left out through frost — there is no zone where it overwinters outdoors in a UK or cold-US climate. Indoor-only in almost every home. Riccia fluitans can only live outside year-round in genuinely frost-free climates (roughly USDA Not applicable (indoor tropical aquarium plant)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature riccia fluitans can survive?

What hardiness zone is riccia fluitans?

Riccia fluitans is rated USDA Not applicable (indoor tropical aquarium plant) and RHS H1b — Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season.

Can riccia fluitans survive winter outside?

It can holiday outdoors in summer once nights are reliably above 10 °C, in shade or dappled light, hardened off gradually. Bring it back indoors well before the first autumn frost — do not wait for a frost warning, move it when nights drop toward 10-12 °C. It will never overwinter outside in a temperate climate; the indoors is its winter home, full stop.

What happens to riccia fluitans below its minimum temperature?

Below about about 10 °C, growth stalls and the leaves start to show cold stress — dark, water-soaked, or yellowing patches. A single light frost blackens the foliage; a hard freeze kills the whole plant, roots included, and it does not recover. Even a cold, draughty windowsill or an unheated porch in winter can be enough to damage it permanently.