Hoya is a genus of approximately 300 tropical and subtropical, evergreen lianas and subshrubs. Native to Asia, Australasia and the Pacific islands, Hoya species are found in rainforest, coastal and cliff habitats, where they may be epiphytic or terrestrial in habit.
Hoya leaves are simple, often succulent, and range in length from about one centimetre (Hoya engleriana) to twenty-five centimetres (Hoya latifolia). Leaf shape also varies, ranging from round (Hoya obovata) through linear (Hoya linearis) to heart shaped (Hoya kerrii). Leaf colour can vary from light green, sometimes with darker veins (Hoya vitellinoides), to dark green, sometimes with lighter veins (Hoya cinnamomifolia) or silvery white markings (Hoya pubicalyx).
The waxy appearance of the flowers give Hoya species their common names, waxplant or waxflower. Flowers can be single but are more commonly arranged in spherical or hemispherical clusters. Each flowerhead is produced on a stalk, called a spur, which is perennial. With each flowering cycle the spur extends in length. Many species have fragrant flowers, whilst some produce nectar, often in copious quantities. Individual flowers range in diameter from approximately five millimetres (Hoya bilobata) to nine centimetres (Hoya macgillivrayi) and in colour from white and yellow through pink to brown.
Each flower is typically star-shaped with five thick, triangular petals, topped by a star-like structure called the corona which includes the male and female parts of the flower. The flower of Hoya bella, which is a popular houseplant, was described by William Hooker, first director of the Royal Botanic Gardens, Kew, as ‘resembling an amethyst set in frosted silver’. Hoya pollen is packed into sacs known as pollinia, which are sticky and can attach to pollinators such as moths, flies and ants. Pollinia of the east Asian Hoya carnosa attach to the legs of moths as they walk around flowerheads searching for nectar.
Pollination is not the only association Hoya species have with insects. Some species are myrmecophiles – they have specific associations with ants. Hoya imbricata, from the Philippines and Sulawesi, is an epiphytic climber whose stems cling to tree bark using adventitious roots. Its round, mottled green-and-purple leaves, which are up to ten centimetres in diameter, slightly overlap each other like the tiles on a roof. Beneath the somewhat concave leaves, ants find shelter, creating hidden pathways to move up and down trees. The plant benefits from such an association through organic material deposited beneath the leaves by the ants.
Kleijn D and van Donkelaar R 2001. Notes on the taxonomy and ecology of the genus Hoya (Asclepiadaceae) in Central Sulawesi. Blumea 46: 457-483.
Mochizuki K et al. (2017) Pollinia transfer on moth legs in Hoya carnosa (Apocynaceae). American Journal of Botany 104: 953-960.
Wanntrop L et al. (2014) Wax plants (Hoya, Apocynaceae) evolution: epiphytism drives successful radiation. Taxon 6: 89-102.