Cold hardiness & minimum temperature
Is Hygrophila polysperma (Hygrophila polysperma)cold hardy? Hardiness zone & min temp
Also called dwarf hygro, Indian swampweed.
More about hygrophila polysperma
About Hygrophila polysperma
Hygrophila polysperma · also called dwarf hygro, Indian swampweed · tropical
Hygrophila polysperma, dwarf hygro, is one of the fastest, hardiest stem plants in the aquarium hobby, with light-green oval leaves on upright stems. It tolerates low light, a wide temperature range and neglect, making it ideal for beginners. Note it is a federally listed noxious weed in the US and must never be released into waterways.
Cold limit: USDA Indoor/tropical aquarium plant; a federally listed noxious weed in the US — do not plant outdoors or release · RHS H1b (18-28°C)
What hygrophila polysperma's hardiness rating actually means
Hygrophila polysperma 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 Indoor/tropical aquarium plant; a federally listed noxious weed in the US — do not plant outdoors or release — 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). Hygrophila polysperma has no frost tolerance at all — it is an indoor plant in any climate with a real winter.
Concretely, for hygrophila polysperma 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 hygrophila polysperma 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 hygrophila polysperma can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1b figure above.
Hygrophila polysperma hardiness — frequently asked questions
Is hygrophila polysperma cold hardy?
Hygrophila polysperma 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. Hygrophila polysperma can only live outside year-round in genuinely frost-free climates (roughly USDA Indoor/tropical aquarium plant; a federally listed noxious weed in the US — do not plant outdoors or release); everywhere else it is a houseplant that summers out at most.
What is the minimum temperature hygrophila polysperma can survive?
Minimum survivable temperature is roughly about 10 °C (sustained cold below this is damaging). Hygrophila polysperma has no frost tolerance at all — it is an indoor plant in any climate with a real winter.
What hardiness zone is hygrophila polysperma?
Hygrophila polysperma is rated USDA Indoor/tropical aquarium plant; a federally listed noxious weed in the US — do not plant outdoors or release and RHS H1b — Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season.
Can hygrophila polysperma 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 hygrophila polysperma 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.
Keep reading
- Hygrophila polysperma care — the full brief (light, water, soil, problems, pet safety)
- USDA hardiness zones — find yours and what grows there
- Is hygrophila polysperma hardy in the UK? — the RHS-rating version
- RHS hardiness ratings — the UK system explained
- Frost-date calculator — your real outdoor window
- The USDA hardiness zone map, explained
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- All 5561plant hardiness & min-temp guides