Hoya undulata

Unrooted cutting of Hoya undulata.
One node.
One or two leaves.

Hoya undulata is a remarkable Hoya species native to Borneo, documented from West Kalimantan, Indonesia. It was formally described in 2015 by Sri Rahayu and Michele Rodda following its discovery near Putussibau by Sulaiman Hasim.

One of the most unusual characteristics of Hoya undulata is that it can produce two distinctly different types of foliage. Leaves growing along climbing stems are usually nearly flat and may be ovate, obovate, or oblanceolate, measuring approximately 10–20 cm long and 5–10 cm wide. Other leaves are smaller, more rounded to elliptic, strongly convex, and can cluster together to form enclosed structures known as domatia.

These domatia are particularly interesting because they can provide shelter for ants. At the type locality, Hoya undulata was observed growing in association with ant nests, with ants occupying the spaces formed by the clustered leaves. This myrmecophytic behavior is relatively unusual among Hoyas and was one of the important characteristics highlighted when the species was described.

The leaves are dark green above, sometimes marked with grey to purple spotting, with a lighter green underside that may occasionally become entirely purple. The margins are finely undulate, sometimes appearing almost minutely toothed toward the base of the leaf. Plants exposed to intense light were also observed to develop pronounced purple coloration.

Hoya undulata produces downward-facing, convex umbels containing typically 2–5 flowers in the wild material studied, although cultivated plants were recorded with as many as 12 flowers. Individual flowers are comparatively large, with a rotate, slightly concave corolla approximately 2.5–4 cm across.

The corolla is white with purple-red spotting on the outside and cream to light pink on the inside. The corona has an especially distinctive structure: each corona lobe develops a pair of lateral hooked appendages, a feature the authors considered unique among the Bornean Hoya species known at the time of publication.

The flowers were recorded as unscented and lasted approximately four to five days in cultivation.

The type plant was collected in February 2014 near Putussibau in West Kalimantan at approximately 300–380 meters elevation. It was growing epiphytically on a small tree trunk on a slope above a stream in lowland heath forest. The habitat was reported to have approximately 80% humidity and about 50% sunlight.

At the time Hoya undulata was published, it was known from only this documented locality. The authors also noted that a second collection of unknown locality was already widely circulating in cultivation. This distinction is important because cultivated Hoya undulata plants cannot automatically be assumed to descend from the documented Putussibau collection.

The name “undulata” refers to the characteristically undulating or wavy margins of the leaves.

Botanical name: Hoya undulata S.Rahayu & Rodda
Native range: Borneo (Kalimantan, Indonesia)
Documented type locality: Near Putussibau, West Kalimantan, Indonesia
Elevation at type locality: Approximately 300–380 m
Type collection: Sulaiman Hasim s.n.
Holotype: BO
Published: Gardens’ Bulletin Singapore 67(1), 2015
Notable features: Wavy leaf margins, dimorphic foliage, ant-associated leaf domatia, purple coloration under strong light, and distinctive hooked corona appendages

A note about this plant: The botanical and historical information above is based primarily on the original published description of Hoya undulata. The authors noted that, in addition to the documented Putussibau collection, a second collection of unknown locality was already circulating in cultivation. We do not possess original collection documentation for our particular plant and therefore cannot claim that it is directly traceable to the Putussibau type collection or another documented wild accession.