Cold hardiness & minimum temperature

Is Small-flowered Cape Primrose (Streptocarpus micranthus)cold hardy? Hardiness zone & min temp

Also called Small-flowered Cape Primrose, Cape Primrose.

More about small-flowered cape primrose

About Small-flowered Cape Primrose

Streptocarpus micranthus · also called Small-flowered Cape Primrose, Cape Primrose · flowering

Streptocarpus micranthus is a caulescent (stemmed) Cape Primrose species from the Eastern Cape, South Africa, where it colonises damp, sheltered rock faces and forest understoreys. Unlike the stemless rosulate species, it develops a short upright stem bearing small, opposite leaves and produces numerous tiny pale lavender flowers over a long season. Its stemmed habit means it is more sensitive to overwatering at the stem base and requires particularly good drainage. The species is non-toxic to cats and dogs per the ASPCA.

Cold limit: USDA 10-12 (indoor in most climates) · RHS H1b (13-24°C)

What small-flowered cape primrose's hardiness rating actually means

Small-flowered Cape Primrose is not cold hardy. It is a tropical houseplant that dies if it is left out through frost — there is no zone where it overwinters outdoors in a UK or cold-US climate. Its RHS rating of H1b means: Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season. On the US scale that maps to USDA 10-12 (indoor in most climates) — 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 °C (sustained cold below this is damaging). Small-flowered Cape Primrose has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for small-flowered cape primrose as it gets too cold:

Below about about 10 °C, growth stalls and the leaves start to show cold stress — dark, water-soaked, or yellowing patches.
A single light frost blackens the foliage; a hard freeze kills the whole plant, roots included, and it does not recover.
Even a cold, draughty windowsill or an unheated porch in winter can be enough to damage it permanently.

Can small-flowered cape primrose go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above 10 °C, in shade or dappled light, hardened off gradually.
Bring it back indoors well before the first autumn frost — do not wait for a frost warning, move it when nights drop toward 10-12 °C.
It will never overwinter outside in a temperate climate; the indoors is its winter home, full stop.

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 small-flowered cape primrose can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1b figure above.

Small-flowered Cape Primrose hardiness — frequently asked questions

Is small-flowered cape primrose cold hardy?

Small-flowered Cape Primrose is not cold hardy. It is a tropical houseplant that dies if it is left out through frost — there is no zone where it overwinters outdoors in a UK or cold-US climate. Indoor-only in almost every home. Small-flowered Cape Primrose can only live outside year-round in genuinely frost-free climates (roughly USDA 10-12 (indoor in most climates)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature small-flowered cape primrose can survive?

What hardiness zone is small-flowered cape primrose?

Small-flowered Cape Primrose is rated USDA 10-12 (indoor in most climates) and RHS H1b — Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season.

Can small-flowered cape primrose survive winter outside?

It can holiday outdoors in summer once nights are reliably above 10 °C, in shade or dappled light, hardened off gradually. Bring it back indoors well before the first autumn frost — do not wait for a frost warning, move it when nights drop toward 10-12 °C. It will never overwinter outside in a temperate climate; the indoors is its winter home, full stop.

What happens to small-flowered cape primrose below its minimum temperature?

Below about about 10 °C, growth stalls and the leaves start to show cold stress — dark, water-soaked, or yellowing patches. A single light frost blackens the foliage; a hard freeze kills the whole plant, roots included, and it does not recover. Even a cold, draughty windowsill or an unheated porch in winter can be enough to damage it permanently.