Cold hardiness & minimum temperature

Is Torch Mexican sunflower (Tithonia rotundifolia 'Torch')cold hardy? Hardiness zone & min temp

Also called Torch Mexican sunflower, Torch tithonia, Mexican sunflower Torch.

More about torch mexican sunflower

About Torch Mexican sunflower

Tithonia rotundifolia 'Torch' · also called Torch Mexican sunflower, Torch tithonia · flowering

Torch is the classic, AAS Award-winning 1951 cultivar of Tithonia rotundifolia, producing intensely vivid, single orange-red blooms up to 8 cm across on robust plants reaching 120–180 cm. One of the most reliably dramatic annuals for the back of a border, wildlife garden, or cutting plot. Thrives in full sun and poor to average soil; may need staking in exposed sites.

Cold limit: USDA 2-11 · RHS H1c (18–38°C)

Watch for — Transplant shock: The taproot is easily damaged at transplanting. Always use biodegradable pots and transplant the intact root ball after last frost. Do not leave in modules too long — pot-bound plants suffer worse transplant shock. Direct sowing avoids this risk entirely.

What torch mexican sunflower's hardiness rating actually means

Torch Mexican sunflower 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 2-11 — 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 °C (and never frost). Torch Mexican sunflower has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

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

Torch Mexican sunflower hardiness — frequently asked questions

Is torch mexican sunflower cold hardy?

Torch Mexican sunflower 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. Torch Mexican sunflower can only live outside year-round in genuinely frost-free climates (roughly USDA 2-11); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature torch mexican sunflower can survive?

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

What hardiness zone is torch mexican sunflower?

Torch Mexican sunflower is rated USDA 2-11 and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can torch mexican sunflower 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 torch mexican sunflower 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.