Wax plant (Hoya carnosa)

Meet the plant

A twining vine with thick, glossy, leathery leaves in pairs along the stem, and, on a happy plant, clusters of small star-shaped flowers that look as if they were made of porcelain and smell of honey in the evening (RHS profile, Missouri Botanical Garden). In the wild it climbs trees in the forests of southern China, Taiwan and Japan, rooting into bark and leaf litter rather than soil, which explains most of what it wants indoors: air at the roots, light, and a dry spell between drinks.

It is a slow starter and a long companion. Penn State calls it a lesson in patience; Missouri Botanical Garden calls it long-lived; every extension page that rates it calls it easy (Penn State Extension, Clemson HGIC, NC State Extension). The leaves store water, so a forgotten watering costs little; a pot that stays wet costs the roots. It grows as a vine: a bare runner reaches out first and leafs up later, and it wants something to climb. The ASPCA lists it as non-toxic to cats and dogs (ASPCA); the milky sap can irritate skin, so wear gloves when you cut it (RHS growing guide), and one botanical database lists it as toxic if eaten (NParks).

What to expect in a year: a few new pairs of leaves and a runner or two in spring and summer, very little in winter, and probably no flowers yet. Flowers usually begin when the plant is two or three years old, often later (NC State Extension).

Light

Bright light, close to an east or west window, and a few hours of gentle direct sun is a gift rather than a danger once the plant is used to it (Clemson HGIC, Penn State Extension). Sources range from "bright indirect" (Iowa State Extension, NC State Extension) to "at least half a day of direct sun" (Missouri Botanical Garden). The cautious path is bright indirect light with morning or evening sun, moving any closer to a south window over a couple of weeks (Pistils Nursery).

Too much sun shows as a red flush, which is harmless and fades, and then as bleached or brown crispy patches, which do not (Iowa State Extension, RHS growing guide). Too little shows as thin stretched growth, long bare runners that never leaf, and no flowers. Light is the first thing to change when a hoya will not bloom (UF/IFAS Gardening Solutions, Iowa State Extension).

Water

Check first. Push a dry finger as far into the mix as it will go, and lift the pot: learn what it weighs wet and dry (Vermont Hoyas, watering, Clemson HGIC). Dry all the way down and light means it is time. Any dampness means wait. The leaves tell you too: the thick leaves hold more water than you would think, and experienced growers wait until they pucker slightly before watering (Penn State Extension).

The rule: water thoroughly, until it runs out of the bottom, once the mix is almost completely dry, then let it dry again (Iowa State Extension). Do not water in sips (Vermont Hoyas, watering). Use room-temperature water; cold water shocks the roots (NC State Extension). Sources differ on how dry is dry: some say keep the compost moist in summer (RHS growing guide), the American extension pages say nearly or completely dry between waterings. This guide takes the dry side, because overwatering in a heavy mix is the quickest path to root decline (Iowa State Extension).

Thirsty: leaves puckered or wrinkled, soft and dull, on a dry light pot. A soak brings them back over a day or two. Overwatered: leaves yellowing or dropping while the mix is still wet, or wrinkled leaves on wet mix, which means the roots are damaged and can no longer drink (Iowa State Extension, NC State Extension). If the leaves say thirsty and the soil says wet, believe the soil and look at the roots.

How often that lands depends on the pot, the mix, the light and the room. In a chunky mix in bright light in summer it often comes round every week or two; most hoyas do not need water more often than that (Vermont Hoyas, watering). In winter the plant rests and wants much less, sometimes a month between drinks (UF/IFAS Gardening Solutions), though a pot by a radiator in dry heated air can dry faster than you expect (Vermont Hoyas, watering). It is easier to overwater when it is cool, so in a cool room err dry. Treat the numbers as a reminder to check, not as the instruction.

Soil and pot

A chunky, fast-draining mix, because the roots want air: equal parts orchid bark, peat-free compost and coarse perlite (RHS growing guide, Clemson HGIC). Hoya growers go chunkier still, mostly perlite and bark with a little compost and a handful of charcoal (Vermont Hoyas, medium). Ordinary potting soil on its own stays wet too long. Mixes break down over two or three years into fine material that holds water, so refresh the mix on that rhythm even if the pot stays the same.

The pot needs a drainage hole and should be snug. A hoya does not mind being pot-bound and flowers better that way; a pot too large keeps the mix wet and rots the roots (Iowa State Extension, NC State Extension). When you do move up, go only one size, 2 to 5 cm wider (Vermont Hoyas, repotting). Growers disagree on material: Logee's likes clay for a drier root zone (Logee's), Vermont Hoyas prefers plastic because you can judge the weight and bare terracotta dries a chunky mix too fast. Either works if you check before you water.

Repot in spring, never while the plant is in bud or bloom, and only when roots have filled the pot or the mix has gone fine and soggy (NC State Extension, RHS growing guide). A carnosa can live and bloom in the same pot for years. The two months after repotting are when overwatering does the most harm (Vermont Hoyas, repotting), so water with extra care then, and hold the feed for five or six weeks (RHS growing guide).

Feeding

Lightly. A balanced liquid houseplant or orchid feed at half strength every four to six weeks from spring to early autumn (Iowa State Extension, Logee's). Sources disagree about winter: the RHS keeps feeding once a month through autumn and winter (RHS growing guide), while Iowa State and Logee's stop, because the rest helps flowering (Iowa State Extension). This guide stops. Every few months water through with plain water to wash out salts (RHS growing guide). Sources range from a quarter-strength feed at most waterings to a single dose a year; the weak monthly rule sits inside all of them. Nothing for five or six weeks after repotting.

Flowering

This is why people keep a hoya. The flowers come in rounded clusters of ten to thirty small waxy stars, pale pink with a darker red centre, about a centimetre and a half across, most fragrant in the evening (Missouri Botanical Garden, RHS profile). They usually come in summer, sometimes spring or autumn, and they drip nectar, so put a cloth under a plant in bloom (NC State Extension, The Houseplant Guru).

What it takes: a plant two or three years old or more, bright light with some direct sun, a snug pot, and a cooler, drier, unfed winter (NC State Extension, Logee's, UF/IFAS Gardening Solutions). Most plants that never bloom simply are not getting enough light (Iowa State Extension).

Two things never to do. The flowers grow from short woody spurs, and the same spur flowers again year after year, lengthening a little each time. When the flowers fall, leave the spur alone: cutting it off is cutting off future flowers, and it can set the plant back for years (NC State Extension, Missouri Botanical Garden, RHS growing guide). Let spent flowers drop by themselves. And once buds appear, do not move, turn or fuss with the plant until the flowers are over; a moved plant drops its buds (Missouri Botanical Garden, Iowa State Extension).

When something looks wrong

Look at the soil first; most hoya trouble starts there.

• Puckered, wrinkled or soft leaves. Check the mix. Dry and light: soak it through and the leaves plump up over a day or two. Wet: the roots are damaged; let it dry almost completely and slide it out to look (Iowa State Extension).
• Yellow leaves on wet mix. Overwatering, a pot too large, or a mix that has broken down. Stop until nearly dry, check that water runs freely out, and repot into fresh airy mix in a snug pot if the mix is compacted (Iowa State Extension, Clemson HGIC).
• Leaves dropping. Too wet, too dry for too long, cold water, or cold in winter on a windowsill (NC State Extension, RHS growing guide). Correct the water, move it off cold glass, keep it above 13 C.
• No flowers. Light first, then patience, then the spurs: were they cut? Stop feeding in winter and give it a cooler rest (Iowa State Extension, Logee's).
• Buds drop before opening. The plant was moved, the watering changed, or the temperature swung (Iowa State Extension, RHS growing guide). Leave it where it is and keep the rhythm steady until it has finished.
• Red leaves, or bleached and scorched. A red flush in bright light is harmless and fades. White or brown crisp patches are sunburn: move it back or filter the light, and introduce stronger light over a couple of weeks (Iowa State Extension, Clemson HGIC).
• Long bare vines. Normal. A hoya sends a runner out first and leafs it later; in low light the runners stay thin and bare. Wind them onto a support, give more light, and do not cut them off (Penn State Extension).
• Black or dark soft patches. Wet leaves without airflow, waterlogged mix, or winter cold (Iowa State Extension, RHS growing guide). Remove the worst leaves, keep water off the foliage, let air move, check the roots.
• Mealybugs. White cottony tufts in the leaf joints: the hoya's most common pest (Clemson HGIC, Logee's). Dab each one with a cotton bud in rubbing alcohol, wash with insecticidal soap, repeat weekly for several weeks, and keep the plant apart until clear.
• Scale. Small hard brown bumps with sticky residue (Iowa State Extension). Wipe or scrape them off, then wipe with insecticidal soap or horticultural oil and repeat.
• Spider mites. Pale speckling and fine webbing in dry warm air, often by winter heating (Clemson HGIC). Shower the plant, undersides included, every few days, spray with insecticidal soap or neem weekly, and raise the humidity.
• Sticky drops in flower. Nectar. Normal. A cloth under the plant, and wipe the drops so mould does not grow on them (The Houseplant Guru).
• Black sooty film. Sooty mould growing on honeydew from mealybugs or scale, or on dried nectar. Treat the insects first, then wipe the film off with mild soapy water (UC IPM).
• Black mushy base, wilting on wet mix. Root rot. Cut away every dark soft root, repot in fresh dry mix in a snug pot, water only when nearly dry, and take cuttings of healthy vine straight away: a plant this far gone often does not recover (RHS growing guide, Clemson HGIC).

Propagation

By stem cuttings, in spring or early summer. Take a piece of mature but not woody vine with two or more nodes and at least a couple of leaves; a leaf on its own may root but will never make a vine (Iowa State Extension, NC State Extension). Wear gloves for the sap. Stand it in water, where it roots in two or three weeks, then pot it up (RHS growing guide); or push it into moist perlite up to the first leaves, keep it warm and humid in bright indirect light, and expect roots in four to six weeks (Vermont Hoyas, propagation). Pot the rooted cutting into the chunky mix in a small pot, and give it the same light and the same dry spells as its parent. A vine can also be layered: pin a length of stem to a pot of mix beside the plant and cut it free once it has rooted (RHS growing guide).