Cold hardiness & minimum temperature

Is Thai Basil (Ocimum basilicum var. thyrsiflora 'Siam Queen')cold hardy? Hardiness zone & min temp

Also called Siam Queen Basil.

More about thai basil

About Thai Basil

Ocimum basilicum var. thyrsiflora 'Siam Queen' · also called Siam Queen Basil · herb

Thai basil 'Siam Queen' is a sturdy, heat-loving basil with narrow green leaves, striking purple stems and flower spikes, and a distinctive sweet anise-liquorice flavour central to South-East Asian cooking. More bolt- and heat-tolerant than sweet basil, this tender annual thrives in full sun and warmth, cropping prolifically through summer when pinched regularly to stay bushy.

Cold limit: USDA Tender annual; grown outdoors in zones 4-11 after frost, year-round indoors · RHS H1c (tender; killed by frost, protect below ~10°C) (18-32°C)

Watch for — Root rot in cold, wet soil: Overwatering or cold conditions rot the roots; use free-draining mix and ease off watering in cool spells, as this tropical herb hates soggy feet.

What thai basil's hardiness rating actually means

Thai 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 Tender annual; grown outdoors in zones 4-11 after frost, year-round indoors — 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). Thai Basil has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

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

Thai Basil hardiness — frequently asked questions

Is thai basil cold hardy?

Thai 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. Thai Basil can only live outside year-round in genuinely frost-free climates (roughly USDA Tender annual; grown outdoors in zones 4-11 after frost, year-round indoors); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature thai basil can survive?

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

What hardiness zone is thai basil?

Thai Basil is rated USDA Tender annual; grown outdoors in zones 4-11 after frost, year-round indoors and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can thai 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 thai 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.