Cold hardiness & minimum temperature

Is Pygmy Rwandan Water Lily (Nymphaea thermarum)cold hardy? Hardiness zone & min temp

Also called Pygmy Rwandan Water Lily, Miniature Rwandan Water Lily.

More about pygmy rwandan water lily

About Pygmy Rwandan Water Lily

Nymphaea thermarum · also called Pygmy Rwandan Water Lily, Miniature Rwandan Water Lily · tropical

Nymphaea thermarum is the world's smallest water lily, native to a tiny area of damp mud formed by the overflow of a single freshwater hot spring in Mashyuza, southwest Rwanda; it was extinct in the wild by around 2008 when the spring was diverted, and its survival depends entirely on ex situ cultivation at Royal Botanic Gardens, Kew and partner institutions. Unlike other water lilies, it does not float its pads on open water but spreads across saturated mud margins, requiring very shallow, warm, calcium-rich water or consistently moist substrate. Pads can be as small as 1 cm in diameter and the white flowers barely reach 2 cm across. Toxicity has not been formally evaluated; treat as mildly-toxic as a precaution.

Cold limit: USDA 12 (indoor only outside tropical climates) · RHS H1a (18°C to 30°C)

What pygmy rwandan water lily's hardiness rating actually means

Pygmy Rwandan Water Lily 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 H1a means: Tropical — needs a heated room or greenhouse; no frost tolerance whatsoever. On the US scale that maps to USDA 12 (indoor only outside tropical climates) — 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 above about 15 °C (warm, never cold). Pygmy Rwandan Water Lily has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for pygmy rwandan water lily as it gets too cold:

Below about above about 15 °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 pygmy rwandan water lily go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above above 15 °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 pygmy rwandan water lily can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1a figure above.

Pygmy Rwandan Water Lily hardiness — frequently asked questions

Is pygmy rwandan water lily cold hardy?

Pygmy Rwandan Water Lily 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. Pygmy Rwandan Water Lily can only live outside year-round in genuinely frost-free climates (roughly USDA 12 (indoor only outside tropical climates)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature pygmy rwandan water lily can survive?

Minimum survivable temperature is roughly above about 15 °C (warm, never cold). Pygmy Rwandan Water Lily has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

What hardiness zone is pygmy rwandan water lily?

Pygmy Rwandan Water Lily is rated USDA 12 (indoor only outside tropical climates) and RHS H1a — Tropical — needs a heated room or greenhouse; no frost tolerance whatsoever.

Can pygmy rwandan water lily survive winter outside?

It can holiday outdoors in summer once nights are reliably above above 15 °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 pygmy rwandan water lily below its minimum temperature?

Below about above about 15 °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.