Cold hardiness & minimum temperature

Is Sea Knotgrass (Polygonum maritimum)cold hardy? Hardiness zone & min temp

Also called Sea Knotgrass, Coast Knotgrass, Sea Knotweed.

More about sea knotgrass

About Sea Knotgrass

Polygonum maritimum · also called Sea Knotgrass, Coast Knotgrass · flowering

Polygonum maritimum is a woody-based perennial herb in the Polygonaceae family, native to the sandy and shingle beaches of the Mediterranean, Atlantic Europe, and Black Sea coasts, with rare populations on a few south-west English beaches. It forms sprawling, grey-green mats of small elliptic leaves with distinctive silvery ochrea (papery sheaths at each leaf node) and produces tiny pinkish-white flowers from June to October. Its key requirement is non-compacted, freely draining coastal sand or fine shingle; it is a specialist of very open, disturbed beach environments. This species is not listed by the ASPCA and is classified mildly-toxic as a precaution.

Cold limit: USDA 7-10 · RHS H4 (-5–30°C)

What sea knotgrass's hardiness rating actually means

Yes — sea knotgrass is genuinely cold hardy. Rated RHS H4 and USDA 7-10, 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 7-10 — 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. Sea Knotgrass is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for sea knotgrass 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 sea knotgrass go outside or overwinter — and where?

Plant it out within USDA 7-10 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 sea knotgrass can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Frost protection for borderline sea knotgrass

Sea Knotgrass is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

At the cold edge of its range, mulch the root zone in late autumn to buffer the deepest freezes.
Protect container specimens — pots freeze through far faster than open ground, costing roughly a zone of hardiness.
Shelter new growth from late spring frosts with fleece if a hard night is forecast.

Sea Knotgrass hardiness — frequently asked questions

Is sea knotgrass cold hardy?

Yes — sea knotgrass is genuinely cold hardy. Rated RHS H4 and USDA 7-10, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Sea Knotgrass is hardy across USDA 7-10; 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 sea knotgrass can survive?

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

What hardiness zone is sea knotgrass?

Sea Knotgrass is rated USDA 7-10 and RHS H4 — Hardy in an average winter across much of the temperate world.

Can sea knotgrass survive winter outside?

Plant it out within USDA 7-10 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.

How do I protect sea knotgrass from frost?

At the cold edge of its range, mulch the root zone in late autumn to buffer the deepest freezes. Protect container specimens — pots freeze through far faster than open ground, costing roughly a zone of hardiness. Shelter new growth from late spring frosts with fleece if a hard night is forecast.