Cold hardiness & minimum temperature

Is Pillwort (Pilularia globulifera)cold hardy? Hardiness zone & min temp

Also called Pillwort, Pepper Grass.

More about pillwort

About Pillwort

Pilularia globulifera · also called Pillwort, Pepper Grass · houseplant

Pilularia globulifera is a diminutive, native British aquatic fern in the family Marsileaceae, found at the margins of seasonally fluctuating ponds, ditches, and lakes on acidic clay or sandy substrates across western Europe. Unlike most ferns, it produces slender, grass-like, rush-like fronds up to 8 cm tall rather than flat leaves, and bears distinctive round spore-bearing structures (sporocarps) resembling tiny peppercorns at the base. The key care requirement is a seasonal fluctuation in water level — it thrives where the pond dries to mud in summer. Extremely frost-hardy to below -20 °C and not regarded as toxic to pets.

Cold limit: USDA 4-8 · RHS H7 (-20 to 25°C)

What pillwort's hardiness rating actually means

Yes — pillwort is genuinely cold hardy. Rated RHS H7 and USDA 4-8, 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 4-8 — 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. Pillwort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for pillwort 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 pillwort go outside or overwinter — and where?

Plant it out within USDA 4-8 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 pillwort can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Pillwort hardiness — frequently asked questions

Is pillwort cold hardy?

Yes — pillwort is genuinely cold hardy. Rated RHS H7 and USDA 4-8, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Pillwort is hardy across USDA 4-8; 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 pillwort can survive?

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

What hardiness zone is pillwort?

Pillwort is rated USDA 4-8 and RHS H7 — Hardy in the severest European continental winters.

Can pillwort survive winter outside?

Plant it out within USDA 4-8 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 pillwort 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.