Cold hardiness & minimum temperature

Is Climbing Onion (Bowiea volubilis)cold hardy? Hardiness zone & min temp

Also called Climbing Onion, Sea Onion, Zulu Potato.

More about climbing onion

About Climbing Onion

Bowiea volubilis · also called Climbing Onion, Sea Onion · houseplant

Climbing Onion is a fascinating South African geophyte with a large, green, above-ground bulb that sends up slender twining stems armed with thread-like leaves. It prefers bright indirect light, infrequent watering, and a dry rest period after the vine dies back. An unusual and easy-to-grow collectors' succulent bulb that tolerates neglect well.

Cold limit: USDA 9–11 · RHS H2 (10–28 °C)

Watch for — Vine fails to emerge in season: Usually caused by the bulb not receiving a dry rest period. Ensure the bulb is kept dry for at least 2 months in summer, then resume watering in autumn–winter to trigger vine growth. Very old or damaged bulbs may be slow to respond.

What climbing onion's hardiness rating actually means

Climbing Onion is half-hardy (RHS H2). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Its RHS rating of H2 means: Tender — survives a frost-free greenhouse or a very mild, sheltered spot. On the US scale that maps to USDA 9–11 — 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 1 to 5 °C — tolerates cold but no real frost. Climbing Onion shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for climbing onion as it gets too cold:

Down to roughly about 1 to 5 °C it copes, especially if dry and sheltered.
A sustained hard frost collapses the top growth; whether it returns depends on whether the roots, crown or tubers froze.
Wet cold is far more lethal than dry cold for this plant — soggy, frozen soil is the usual killer.

Can climbing onion go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 9–11 or a frost-free UK microclimate.
In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter.
A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

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 climbing onion can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H2 figure above.

Frost protection for borderline climbing onion

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

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost.
Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse.
Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones.
Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.

Climbing Onion hardiness — frequently asked questions

Is climbing onion cold hardy?

Climbing Onion is half-hardy (RHS H2). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Borderline outdoors. In its mild end of USDA 9–11 (and sheltered UK gardens) climbing onion can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature climbing onion can survive?

Minimum survivable temperature is roughly about 1 to 5 °C — tolerates cold but no real frost. Climbing Onion shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is climbing onion?

Climbing Onion is rated USDA 9–11 and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can climbing onion survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 9–11 or a frost-free UK microclimate. In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter. A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

How do I protect climbing onion from frost?

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost. Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse. Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones. Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.