Cold hardiness & minimum temperature

Is Purple Basil (Ocimum basilicum 'Purpurascens')cold hardy? Hardiness zone & min temp

Also called purple basil, dark opal basil.

More about purple basil

About Purple Basil

Ocimum basilicum 'Purpurascens' · also called purple basil, dark opal basil · herb

Purple basil is a striking deep-burgundy-leaved selection of sweet basil grown for both kitchen use and ornament, with a slightly spicier, clove-tinged flavour. A tender warm-season annual, it loves heat, sun and rich, moist soil, and rewards frequent pinching with bushy growth. Cold, wet conditions and the first frost will finish it off.

Cold limit: USDA 10-11 as a tender perennial; grown as a warm-season annual in most regions · RHS H1c (18-27°C)

Watch for — Sudden collapse in cold: Basil is frost-tender and chills below about 10°C, blackening and rotting. Keep it warm and bring container plants indoors before nights turn cold.

What purple basil's hardiness rating actually means

Purple Basil 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 10-11 as a tender perennial; grown as a warm-season annual in most regions — 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). Purple Basil has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for purple basil 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 purple basil 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 purple basil can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1c figure above.

Purple Basil hardiness — frequently asked questions

Is purple basil cold hardy?

Purple Basil 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. Purple Basil can only live outside year-round in genuinely frost-free climates (roughly USDA 10-11 as a tender perennial; grown as a warm-season annual in most regions); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature purple basil can survive?

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

What hardiness zone is purple basil?

Purple Basil is rated USDA 10-11 as a tender perennial; grown as a warm-season annual in most regions and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can purple basil 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 purple basil 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.