Wax plant

Identification and origin

Hoya carnosa belongs to the dogbane family, Apocynaceae, which now includes the former milkweed family, Asclepiadaceae. In the wild it is an evergreen epiphytic vine of the forests of southern China, the eastern Himalaya, Myanmar, Laos, Hainan, Taiwan and southern Japan down through the Ryukyu Islands. The genus honors Thomas Hoy, gardener to the Duke of Northumberland in the late 1700s; the epithet carnosa means “fleshy.”

Its opposite, oval, thick and leathery leaves are 3–6 in long; their glossy, wax-like surface is often flecked with silver. The stems climb by twining around a thin support or trail from a hanging basket; indoors the plant usually reaches 2–4 ft. The flowers, white to pink with fuzzy petals and a red, star-shaped central corona, come in rounded umbels of ten to thirty, from spring into fall. Most plants don’t bloom until they are two or three years old. There are many cultivars: ‘Variegata’ (cream margins), ‘Krimson Queen’ (white to pink margins), ‘Krimson Princess’ (variegated center) and ‘Compacta’ (‘Hindu Rope’), with twisted leaves packed tightly like a rope.

Light

Bright light, ideally indirect, all year. A spot by an east- or west-facing window works well; behind south-facing glass, acclimate it gradually, since strong sun can scorch or redden the leaves. Light is the key to flowering: in a dim corner the stems stretch and thin out, variegation fades and flowers never come. Once buds appear, don’t turn or move the pot.

Watering

Let the mix dry almost completely between waterings, then water thoroughly and empty the saucer. Lifting the pot is a good guide: when it feels light, it’s time to water. The plant drinks more in warm, bright seasons and much less in winter, when you water sparingly. Use room-temperature water, since cold water can shock the roots. Both soggy soil and a long dry spell cause leaf drop.

It copes with ordinary household humidity, but grows and blooms best at 40 to 60% relative humidity; a tray of damp pebbles helps when the air is very dry.

Potting mix

A chunky, airy, fast-draining mix that favors root oxygen over water retention: for example one part potting soil, one part orchid bark and one part perlite. The pot must have drainage holes. Feed lightly in spring and summer with a liquid fertilizer at half or quarter strength, and stop in winter to let the plant rest.

Temperature

It is comfortable between 64–81 °F, with nights a little cooler than days, which encourages flowering. Don’t let it drop below 55 °F: pressed against a cold window in winter, it suffers. While resting in winter, a mature plant will take nights around 50 °F, but no lower. It can spend the warm season outdoors in bright shade and should come back in as soon as nights fall below 50 °F. Year-round outdoors, it only suits USDA zones 10 to 12.

Repotting

Repot rarely: it blooms better when slightly pot-bound and doesn’t mind snug roots. Move it on only when it has truly filled its pot or when the mix has compacted and broken down, going up just one pot size. An oversized pot stays wet too long and invites root rot.

Pruning

Prune in early spring, before growth picks up, to shorten overlong stems and keep the plant compact. Above all, never cut off the flower peduncles, those short leafless spurs: they rebloom year after year, and removing them can delay the next flowering for a long time. Let spent flowers drop on their own. Cuts ooze a milky sap.

Propagation

From stem cuttings, the most reliable method. Take a piece with at least one or two nodes and plant it in a light mix somewhere warm and bright: roots form within a few weeks. A single leaf with no node will not grow a new vine, so always keep a piece of stem on the cutting.

Common problems

• Mealybugs: cottony clusters in leaf axils and along stems; dab them off with a cotton swab dipped in rubbing alcohol or treat with insecticidal soap.
• Scale: small, immobile bumps on stems and leaves.
• Spider mites: leaves stippled with pale dots, sometimes fine webbing, especially in dry air.
• Fungus gnats: drawn to potting mix that stays too wet.
• Root rot: the most common problem, caused by overwatering; yellow leaves and mix that never dries.
• Leaf spots: wet foliage that dries slowly for lack of air movement.
• Poor growth or no flowers: too little light, cool temperatures, old compacted mix, or a plant that is still too young.

One symptom can have several causes: check light, watering, temperature and pests before drawing conclusions.

Toxicity

No toxicity has been reported: it is listed as non-toxic to dogs, cats and horses, and there is no known record of poisoning in people. As with any plant, a pet that chews the leaves may still vomit or have a mild, short-lived stomach upset.