Cold hardiness & minimum temperature

Is Ruellia Brittoniana (Ruellia simplex)cold hardy? Hardiness zone & min temp

Also called Mexican petunia, Britton's wild petunia, Ruellia.

More about ruellia brittoniana

About Ruellia Brittoniana

Ruellia simplex · also called Mexican petunia, Britton's wild petunia · flowering

Ruellia simplex is a tough, fast-growing perennial grown for its slim upright stems and near-continuous flush of petunia-like purple-blue flowers through the warm months. Despite the common name it is not a true petunia. Heat- and drought-tolerant once established, it thrives in sun and damp soils alike, but is invasive in warm regions and should be sited with care.

Cold limit: USDA 8-11 (root-hardy and can be invasive in zones 9-11) · RHS H3 (15-32°C)

Watch for — Frost dieback: Top growth is killed by frost, though established roots resprout in mild zones. Cut back dead stems in spring; in colder areas treat it as a tender plant or overwinter the rootstock frost-free.

What ruellia brittoniana's hardiness rating actually means

Ruellia Brittoniana is half-hardy (RHS H3). 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 H3 means: Half-hardy — comes through mild UK winters outside but is killed by a hard freeze. On the US scale that maps to USDA 8-11 (root-hardy and can be invasive in zones 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 −5 to 1 °C — a light, short frost only. Ruellia Brittoniana shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for ruellia brittoniana as it gets too cold:

Down to roughly about −5 to 1 °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 ruellia brittoniana go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 8-11 (root-hardy and can be invasive in zones 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 ruellia brittoniana can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H3 figure above.

Frost protection for borderline ruellia brittoniana

Ruellia Brittoniana 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.

Ruellia Brittoniana hardiness — frequently asked questions

Is ruellia brittoniana cold hardy?

Ruellia Brittoniana is half-hardy (RHS H3). 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 8-11 (root-hardy and can be invasive in zones 9-11) (and sheltered UK gardens) ruellia brittoniana can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature ruellia brittoniana can survive?

Minimum survivable temperature is roughly about −5 to 1 °C — a light, short frost only. Ruellia Brittoniana shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is ruellia brittoniana?

Ruellia Brittoniana is rated USDA 8-11 (root-hardy and can be invasive in zones 9-11) and RHS H3 — Half-hardy — comes through mild UK winters outside but is killed by a hard freeze.

Can ruellia brittoniana survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 8-11 (root-hardy and can be invasive in zones 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 ruellia brittoniana 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.