Cold hardiness & minimum temperature

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

Also called Mexican sunflower, Tithonia.

More about mexican sunflower

About Mexican sunflower

Tithonia rotundifolia · also called Mexican sunflower, Tithonia · flowering

Mexican sunflower is a vigorous, heat-loving half-hardy annual from Mexico and Central America producing vivid orange-red, daisy-like blooms up to 8 cm across on tall, branching plants from midsummer to first frost. An exceptional pollinator plant and cut flower. Thrives in poor, dry soil in full sun; struggles with rich soil, excess nitrogen, and root disturbance.

Cold limit: USDA 2-11 · RHS H1c (18–35°C)

Watch for — Root disturbance at transplanting: Mexican sunflower has a sensitive taproot and transplant shock is common if roots are disturbed. Sow into biodegradable pots and transplant without breaking the root ball. Direct sowing after frost avoids this issue entirely.

What mexican sunflower's hardiness rating actually means

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). Mexican sunflower has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for 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 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 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.

Mexican sunflower hardiness — frequently asked questions

Is mexican sunflower cold hardy?

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. 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 mexican sunflower can survive?

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

What hardiness zone is mexican sunflower?

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 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 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.