Hoya carnosa (L.f.) R.Br. 'Compacta'

Where it comes from

Neither India nor Australia, despite being called the Hindu rope plant and despite Robert Brown publishing the name in a flora of New Holland. Kew gives the native range as south-central and south-east China, Hainan, Japan, Laos, Myanmar, Taiwan and the Nansei-shotō.¹ Bangladesh, Fiji, Korea, the Solomons, Sumatra and Trinidad-Tobago are all introduced.

The family is Apocynaceae. NC State and both relevant ASPCA pages still print Asclepiadaceae, which has not been current for a long time.²³

Naming and description

‘Compacta’ is unregistered and unpatented — and it could not be registered, since there is no International Cultivar Registration Authority for Hoya at all.⁴⁵ It does have a botanical shadow, though: Hoya compacta C.M.Burton was published at species rank in 1990, and Kew sinks it as a heterotypic synonym of H. carnosa.⁶¹

What causes the twisted leaves is not documented anywhere. I would like to say it is a known mutation; nothing at any level of source explains it. The nearest thing is a US plant patent that describes the curled-leaf geometry precisely — six to ten dimples per leaf, folding along the long axis, margins curling upward — and confirms such forms arise as sports propagated by cuttings. But that patent is for ‘Krinkle 8 Variegata’, not ‘Compacta’, and the mutation it actually claims is the variegation rather than the curling.⁷ So: undocumented, rather than “a genetic mutation”.

Fun facts

• Never cut off the old flower stalk. This is the single most useful thing anyone can tell a hoya owner, and two independent sources say it: the stalk is a peduncle, and the plant uses it to produce its next round of blooms.⁸ The RHS puts it as an instruction — “resist the temptation to deadhead hoyas”, because more flowers often sprout from the stumps of previous clusters.⁹ Note the hedging in both: “often”, “may”. It reflowers from the same peduncle; it does not promise to do it annually.
• The scent peaks around midnight, on a clock rather than in response to anything. Nectar secretion in H. carnosa follows a circadian rhythm with maxima near midnight, and it free-runs at about 29 hours in constant conditions — so it is genuinely an internal clock, not a response to darkness.¹⁰
• Twenty-one years later, somebody found out who it was for. Work on hoya pollination found the pollinia attach to the legs of night-flying moths that settle on the flower, with the slippery corona giving them only restricted footholds.¹¹ A midnight scent peak and a nocturnal moth, found independently and an archipelago apart.
• It is succulent but it is not a cactus, and the difference shows up in the watering. H. carnosa does use CAM — but published work finds substantial daytime carbon dioxide uptake as well, so it is not strictly CAM.¹² Let it dry between waterings; do not treat it like a desert plant.