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Official Magazine of the Nature Society (Singapore)

Heavenly Hoyas
An introduction to the species
of Singapore and Malaysia

Text by Leong Lup San
Photos by K. F. Yap

Most people will associate Hoya with the common garden species H. carnosa from China, a vine with white to pink waxy umbels of flowers, as well as its cultivar known as "Indian Robe" or "Crinkled Curl". The genus Hoya is much more than these in terms of the forms, leaves, sizes and flowers.

This may come as a surprise for some but academics have named up to 200 species, with the actual number fluctuating up and down over the years as some species are merged while new ones are described.

Hoyas belong to the family Asclepiadaceae, which contain many horticulturally interesting plants like the Stapeliads. Usually, hoyas are twining vines, ooze milky sap when cut (there are exceptions), and more often than not, have interesting waxy blooms. The umbel of flowers are borne out of the peduncle, a thick flower stalk that does not drop off after flowering is over. In fact, it can grow bigger and more flowers can arise the next time round. In the wild, hoyas normally inhabit shady tropical forests but there are succulent forms found in the deserts of Australia.

The regions with the most hoya species are Southeast Asia and the island of New Guinea. According to RE Rintz (1978), Peninsular Malaysia has about 25 species out of which nine are also found in Singapore (Hsuan Keng, 1990). The nine species is inclusive of Physostelma wallichii, now reclassified as Hoya campanulata (Rintz, 1978). Even though some of the Singapore records are very old, I have seen H. verticillata, H. latifolia and H. diversifolia growing wild in mangroves and forests on the mainland and there is no doubt that more of the missing species are to be found here.

Local Species
Singapore and Malaysia house one of the species with the largest leaves—up to 25 cm long. This is called H. latifolia but also known as H. macrophylla and H. polystachya. It has thick, pretty heart-shaped leaves with prominent venation; alas, the individual flower is rather inconspicuous. Clumps can be found growing within and around the Zoological Garden in Mandai and it inhabits a drier habitat. There is an elegant long-leafed form found across the Causeway in Malaysia.

Hoya diversifolia

Hoya macgillivrayii

Hoya multiflora

Hoya verticillata

Hoya pubicalyx
from the Philippines

Hoya pachyclada
hails from Thailand

Hoya kerrii
from Thailand
Hoya lasiantha Korthals ex Miquel (1856)

Text and photo by K. F. Yap

On April 13-16 this year, I joined a group of 31 nature lovers on a trip to the Endau-Rompin National Park in Johor, Malaysia. The trip was organised by S. Rajamanickam on behalf of the Nature Society (Singapore).

On April 15, a group of us ascended Gunung Janing Barat (450 metres above mean sea level). It was a gruelling climb, even though I had hired a porter (Dolah, a young Jakun tribesman) to help carry my heavy camera equiprnent. We were the last climbers to reach the sandstone plateau, home to the endemic Livistona endauensis and peat swamp forests. L. endauensis, a fan palm species, was first found in Ulu Endau, Johor.

The descent was even more treacherous than the ascent. About halfway down, while catching our breath, I spotted, in the distance, a hanging patch of orangey-gold swaying in the afternoon breeze. On approaching closer, it turned out to be a Hoya species (Asclepiadaceae) that I had never seen before.

It was truly an exquisite encounter of the first kind. I have recorded the image for posterity.

The epiphytic plant of five branches, arising from a single stem, was growing out of a tree wound, at about eye-level. One of the pendulous branches carried an umbel (inflorescence) of four blooms. They were, as described by Rintz (1978), positively-geotropic (hanging earthwards). The pointed corolla lobes were strongly reflexed, densely pubescent with long straight hairs only at the base and golden orange. The corona lower lobes were upcurved and roundly blunt, white tinged with light pink.

Rintz (1978) recorded it as uncommon in the lowland and hill forests of Perak and Selangor, Peninsular Maiaysia. It was not recorded as having been found in the State of Johor, West Malaysia. Hoya lasiantha is, indeed, alive and well in its newly discovered habitat. Nature lovers do sometimes get unexpected and surprising rewards.

I am indebted to Chuah Ai Lin for digging up a copy of Rintz's publication from the NUS Library.
Another Hoya species, which one can find in the jungles of MacRitchie Reservoir, is a smaller vine which is very floriferous and gives off an intensely sweet smell in the evening. It grows very well in apartment balconies and deserves more attention. The plant has been called H. parasitica, H. ridleyi, H. globifera and H. acuta before settling into the present H. verticillata. This has a very wide distribution—from India, Indo-China to Malesia.

Near the coast and in mangroves, another large vine with pale green succulent leaves and typical waxy pink blooms can be found. Its name, H. diversifolia, remains unchanged over time.

There are even myrmecophilous hoyas; H. mitrata (not recorded in Singapore) has two types of leaves, the typical speckled hoya leaf and the "cabbage-type" leaves which function as ant nests. Another ant-plant, H. imbricata from the Philippines, has pairs of disk-shaped leaves cupped against tree trunks which act as ant "hotels".

According to Hsuan Keng (1990), H. coronaria is also found in Singapore. This is a very different plant—dark green hairy stems and leaves, with very large thick white waxy flowers. It belongs to the Eriostemma section which some taxonomists may place in a genus on its own, owing to features very distinct from the "normal" hoyas. It is certainly a shy bloomer, but very rewarding when it does.

Physostelma wallichii is also another plant which sits on the fence—it is more popularly called H. wallichii or H. campanulata. Vegetatively, it looks like a skinny vine with soft green leaves. Its flowers are like waxy bells instead of the usual "star" shape. It was recorded in Tampines years ago but I suspect it has gone extinct by now. Currently, this plant is placed in the section Physostelma with several other species, so it is still a hoya—for now at least.

Taxonomists have also threatened to remove H. multiflora (the shooting star hoya) from the genus and one can understand why—it is a shrub (not a vine) with a truly unique bloom. This plant has not been recorded wild in Singapore.

Personally, I think an introduction to hoyas cannot he complete without mentioning two other very interesting species. The first one, H. imperialis, is a large thick vine and is truly regal when it blooms. There are many colour variants, from bright to purplish red and even white. It is found in Malaysia and Borneo.

Another hoya is called H. macgillivrayii—from Northern Queensland and New Guinea. There are several similar species like H. onychoides and archboldiana—all equally impressive. Their blooms, among the largest in the genus and certainly one of the most perfectly formed, epitomise the beauty created via eons of evolution in the natural world

Literature cited

  • Rintz, R. E. (1978). The Peninsular Malaysian Species of Hoya (Asclepiadaceae). Malay. Nat. J. 30 (3/4): pp 467 - 522.
  • Keng H. (1990). The Concise Flora of Singapore (Gymnosperms & Dicotyledons). pp 148 - 149.

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