Cold hardiness & minimum temperature

Is African Blue Basil (Ocimum kilimandscharicum × basilicum 'Dark Opal')cold hardy? Hardiness zone & min temp

Also called Perennial Basil.

More about african blue basil

About African Blue Basil

Ocimum kilimandscharicum × basilicum 'Dark Opal' · also called Perennial Basil · herb

African Blue basil is a sterile interspecific hybrid that is unusually long-lived and vigorous, with camphor-scented purple-veined leaves and near-constant lavender flower spikes that are magnets for bees. Because it sets no seed it never truly bolts, so it stays productive for months. Tender to frost, it is often overwintered indoors as a perennial.

Cold limit: USDA 10-11 (overwinter indoors or as an annual in cooler zones) · RHS H1c (18-30°C)

Watch for — Frost kill: Has no cold hardiness and dies at frost. Take cuttings or pot it up and bring indoors before the first frost to keep the perennial going.

What african blue basil's hardiness rating actually means

African Blue 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 (overwinter indoors or as an annual in cooler zones) — 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). African Blue Basil has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

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

African Blue Basil hardiness — frequently asked questions

Is african blue basil cold hardy?

African Blue 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. African Blue Basil can only live outside year-round in genuinely frost-free climates (roughly USDA 10-11 (overwinter indoors or as an annual in cooler zones)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature african blue basil can survive?

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

What hardiness zone is african blue basil?

African Blue Basil is rated USDA 10-11 (overwinter indoors or as an annual in cooler zones) and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can african blue 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 african blue 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.