Cold hardiness & minimum temperature

Is Forest Spurflower (Plectranthus fruticosus)cold hardy? Hardiness zone & min temp

Also called Forest Spurflower, Spur Flower, Blue Spurflower.

More about forest spurflower

About Forest Spurflower

Plectranthus fruticosus · also called Forest Spurflower, Spur Flower · houseplant

Plectranthus fruticosus is a fast-growing, erect evergreen shrub native to the forest margins and scrub of South Africa's eastern coast, where it can reach 2 m tall. It produces showy terminal spikes of soft blue to mauve flowers in late summer and autumn that are highly attractive to bees, and performs best in partial shade with humus-rich, well-drained soil. The most important care fact is to prune it back by up to one-third in late winter to keep growth compact and promote a flush of new flowering stems. This species is not individually listed on the ASPCA database; treat as mildly-toxic around pets.

Cold limit: USDA 9-11 · RHS H2 (5-28°C)

Watch for — Leggy, sparse growth: Without annual pruning, the shrub becomes woody and bare at the base. Cut back by one-third in late winter before new growth emerges to maintain a compact, bushy shape and maximise flowering.

What forest spurflower's hardiness rating actually means

Forest 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 — 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. Forest Spurflower shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for forest 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 forest spurflower 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 forest 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 forest spurflower

Forest 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.

Forest Spurflower hardiness — frequently asked questions

Is forest spurflower cold hardy?

Forest 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 (and sheltered UK gardens) forest 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 forest spurflower can survive?

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

What hardiness zone is forest spurflower?

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

Can forest spurflower 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 forest 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.