Cold hardiness & minimum temperature

Is Mexican zinnia (Zinnia haageana)cold hardy? Hardiness zone & min temp

Also called Mexican zinnia, Haage zinnia, Persian carpet zinnia.

More about mexican zinnia

About Mexican zinnia

Zinnia haageana · also called Mexican zinnia, Haage zinnia · flowering

A compact, mounding annual native to Mexico bearing masses of small, jewel-toned bicolour and tricolour flowers in shades of orange, red, gold, and mahogany from early summer to frost. More heat- and drought-tolerant than common zinnia and notably more resistant to powdery mildew, it is an excellent choice for hot, dry gardens and long-blooming summer containers.

Cold limit: USDA 2-11 · RHS H1c (16–35°C)

Watch for — Poor germination in cold soil: Seeds fail to germinate or rot when sown in cold (below 15°C) or very wet soil. Wait until soil temperature reaches at least 18°C before direct sowing, or start indoors 4–6 weeks before the last frost.

What mexican zinnia's hardiness rating actually means

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

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

Mexican zinnia hardiness — frequently asked questions

Is mexican zinnia cold hardy?

Mexican zinnia 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 zinnia 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 zinnia can survive?

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

What hardiness zone is mexican zinnia?

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

Can mexican zinnia 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 zinnia 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.