Cold hardiness & minimum temperature

Is Merrill's Wax Plant (Hoya merrillii)cold hardy? Hardiness zone & min temp

Also called Merrill's wax plant, Merrill's hoya, Philippine wax plant.

More about merrill's wax plant

About Merrill's Wax Plant

Hoya merrillii · also called Merrill's wax plant, Merrill's hoya · tropical

Hoya merrillii is a rare epiphytic vine native to the Philippines, first discovered in humid lowland forests on Mindoro Island, and named in honour of American botanist Elmer Drew Merrill who extensively documented Philippine flora. It is prized for its compact umbels of up to 25 small, star-shaped, golden-yellow flowers with red-orange centres that emit a sweet, honey-and-caramel fragrance most intense in the evenings. The most important care point is to provide warm temperatures with high humidity and avoid cold draughts, as it cannot tolerate temperatures below 15°C for any length of time. The ASPCA lists the Hoya genus as non-toxic to cats and dogs.

Cold limit: USDA 11-12 (indoor in most climates) · RHS H1a (18–32°C)

What merrill's wax plant's hardiness rating actually means

Merrill's Wax Plant 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 H1a means: Tropical — needs a heated room or greenhouse; no frost tolerance whatsoever. On the US scale that maps to USDA 11-12 (indoor in most climates) — 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 above about 15 °C (warm, never cold). Merrill's Wax Plant has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for merrill's wax plant as it gets too cold:

Below about above about 15 °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 merrill's wax plant go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above above 15 °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 merrill's wax plant can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1a figure above.

Merrill's Wax Plant hardiness — frequently asked questions

Is merrill's wax plant cold hardy?

Merrill's Wax Plant 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. Merrill's Wax Plant can only live outside year-round in genuinely frost-free climates (roughly USDA 11-12 (indoor in most climates)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature merrill's wax plant can survive?

Minimum survivable temperature is roughly above about 15 °C (warm, never cold). Merrill's Wax Plant has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

What hardiness zone is merrill's wax plant?

Merrill's Wax Plant is rated USDA 11-12 (indoor in most climates) and RHS H1a — Tropical — needs a heated room or greenhouse; no frost tolerance whatsoever.

Can merrill's wax plant survive winter outside?

It can holiday outdoors in summer once nights are reliably above above 15 °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 merrill's wax plant below its minimum temperature?

Below about above about 15 °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.