Cold hardiness & minimum temperature

Is Stomandra Cape Primrose (Streptocarpus stomandrus)cold hardy? Hardiness zone & min temp

Also called Stomandra Cape Primrose, Cape Primrose.

More about stomandra cape primrose

About Stomandra Cape Primrose

Streptocarpus stomandrus · also called Stomandra Cape Primrose, Cape Primrose · houseplant

Streptocarpus stomandrus is a caulescent (stemmed) species native to the Nguru Mountains of the Morogoro District in Tanzania, where it grows in shaded, moist highland conditions. Unlike the familiar rosulate Cape Primroses, it forms a branching, semi-erect herb to about 25 cm tall with opposite, ovate-elliptic, softly hairy leaves and small clusters of tubular flowers on axillary peduncles. It requires consistently warm, humid conditions and bright filtered light, and is more sensitive to cold and drought than most cultivated Streptocarpus. According to the ASPCA, the Streptocarpus genus is non-toxic to cats, dogs, and horses.

Cold limit: USDA 10-12 (indoor in most climates) · RHS H1c (16–24°C)

Watch for — Stem rot: The succulent young stems are susceptible to rot at the base if grown in waterlogged compost or cold, damp conditions; ensure free drainage and keep temperatures above 15°C.

What stomandra cape primrose's hardiness rating actually means

Stomandra 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 H1c means: Warm-temperate — can summer outdoors but must come in well before the first frost. 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 5 °C (and never frost). Stomandra Cape Primrose has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for stomandra cape primrose as it gets too cold:

Below about about 5 °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 stomandra cape primrose go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above 5 °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 stomandra cape primrose can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1c figure above.

Stomandra Cape Primrose hardiness — frequently asked questions

Is stomandra cape primrose cold hardy?

Stomandra 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. Stomandra 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 stomandra cape primrose can survive?

Minimum survivable temperature is roughly about 5 °C (and never frost). Stomandra Cape Primrose has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

What hardiness zone is stomandra cape primrose?

Stomandra Cape Primrose is rated USDA 10-12 (indoor in most climates) and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can stomandra cape primrose survive winter outside?

It can holiday outdoors in summer once nights are reliably above 5 °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 stomandra cape primrose below its minimum temperature?

Below about about 5 °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.