Cold hardiness & minimum temperature

Is Purple Glasswort (Salicornia ramosissima)cold hardy? Hardiness zone & min temp

Also called Purple Glasswort, Branched Glasswort.

More about purple glasswort

About Purple Glasswort

Salicornia ramosissima · also called Purple Glasswort, Branched Glasswort · edible

Salicornia ramosissima is a highly branched annual halophyte native to saltmarshes of western Europe, including the British Isles, where it is the most common Salicornia species. It forms bushy, succulent jointed stems that mature from bright green to vivid purple or red in late summer, making it both an ecologically important saltmarsh plant and a gourmet edible. Full sun and permanently saline growing conditions are essential; it cannot survive in non-saline soil. As with other Salicornia species, it is not confirmed safe for pets by the ASPCA and its high salt content poses a risk of salt toxicity if eaten by cats or dogs.

Cold limit: USDA 5-9 (grown as annual) · RHS H5 (5-28°C)

What purple glasswort's hardiness rating actually means

Hardiness works differently for purple glasswort: it is grown as a seasonal crop, not overwintered. The question is not "what zone" but "how long is your frost-free growing window". Its RHS rating of H5 means: Hardy in most of the UK and in cold winters. On the US scale that maps to USDA 5-9 (grown as annual) — 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

As an annual crop, its "minimum temperature" is the first hard frost — that is the end of the plant's life, not a survivable low. Many types are also damaged by light frost (around 0 °C).

Concretely, for purple glasswort as it gets too cold:

Light frost (around 0 to −2 °C) damages or kills tender summer crops outright; cold-hardy types take a few degrees of frost.
The plant does not "survive winter" — its life cycle simply ends, by design, when frost arrives or it finishes cropping.
A surprise late spring frost can also kill young transplants set out too early, before the season even starts.

Can purple glasswort go outside or overwinter — and where?

Time it to your frost dates: sow or plant out after the last spring frost, and aim to harvest before the first autumn frost.
In short-season zones, start it indoors or under cover to stretch the effective growing window.
Hardier crops in this group can be sown for an autumn or overwintered harvest in mild zones — check the specific crop.

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 purple glasswort can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Frost protection for borderline purple glasswort

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

Use fleece, cloches or a cold frame at each end of the season to dodge a borderline frost and add growing weeks.
Have row cover ready for an unexpected late spring or early autumn frost.
Know your local last- and first-frost dates and count back the crop’s days-to-maturity to schedule the sowing.

Purple Glasswort hardiness — frequently asked questions

Is purple glasswort cold hardy?

Hardiness works differently for purple glasswort: it is grown as a seasonal crop, not overwintered. The question is not "what zone" but "how long is your frost-free growing window". A seasonal crop, not a perennial. Purple Glasswort is grown 5-9 (grown as annual); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature purple glasswort can survive?

As an annual crop, its "minimum temperature" is the first hard frost — that is the end of the plant's life, not a survivable low. Many types are also damaged by light frost (around 0 °C).

What hardiness zone is purple glasswort?

Purple Glasswort is rated USDA 5-9 (grown as annual) and RHS H5 — Hardy in most of the UK and in cold winters.

Can purple glasswort survive winter outside?

Time it to your frost dates: sow or plant out after the last spring frost, and aim to harvest before the first autumn frost. In short-season zones, start it indoors or under cover to stretch the effective growing window. Hardier crops in this group can be sown for an autumn or overwintered harvest in mild zones — check the specific crop.

How do I protect purple glasswort from frost?

Use fleece, cloches or a cold frame at each end of the season to dodge a borderline frost and add growing weeks. Have row cover ready for an unexpected late spring or early autumn frost. Know your local last- and first-frost dates and count back the crop’s days-to-maturity to schedule the sowing.