Cold hardiness & minimum temperature

Is Silver Spurflower (Plectranthus argentatus)cold hardy? Hardiness zone & min temp

Also called Silver Spurflower, Silver Plectranthus, Silver Spur Flower.

More about silver spurflower

About Silver Spurflower

Plectranthus argentatus · also called Silver Spurflower, Silver Plectranthus · tropical

Plectranthus argentatus is a spreading, semi-shrubby perennial from eastern Australia, grown primarily for its large, striking leaves densely coated in silver-white hairs that give an almost metallic sheen. In late summer and autumn it produces tall spikes of small pale lilac to white flowers attractive to bees. It is vigorous, tolerates some shade, and works well as a bold textural foliage plant in containers or tropical-style beds. Toxicity data for this species is not confirmed by ASPCA; treat as mildly toxic and keep away from pets as a precaution.

Cold limit: USDA 9-11 (overwinter frost-free elsewhere) · RHS H2 (10–30°C)

What silver spurflower's hardiness rating actually means

Silver Spurflower 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 (overwinter frost-free elsewhere) — 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. Silver Spurflower shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for silver spurflower 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 silver spurflower go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 (overwinter frost-free elsewhere) 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 silver spurflower 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 silver spurflower

Silver Spurflower 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.

Silver Spurflower hardiness — frequently asked questions

Is silver spurflower cold hardy?

Silver Spurflower 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 (overwinter frost-free elsewhere) (and sheltered UK gardens) silver spurflower can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature silver spurflower can survive?

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

What hardiness zone is silver spurflower?

Silver Spurflower is rated USDA 9-11 (overwinter frost-free elsewhere) and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can silver spurflower survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 (overwinter frost-free elsewhere) 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 silver spurflower 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.