July 2017 Plant of the Evening - Dendrobium cacatua

Dendrobium cacatua (Den. tetragonum var. cacatua)

It is a well- grown plant with 10 canes, seven of which were bearing flowers. The flowers had the characteristic spidery shape of those of the tetragonum complex, but with attractive pure apple green tepals and pure white labella, the latter a characteristic of this species (variety).

Dendrobium cacatua is a northern species of the tetragonum complex, occurring in north-eastern Queensland from Mount Windsor Tableland to Eungella. Its geographical distribution overlaps with another probably better known northern species, namely Dendrobium capitisyork. However, the two species are physically segregated altitudinally, while Dendrobium cacatua is found at altitude 900—1200m, the other species is found at altitude 0-900m. Apparently, DNA data show that the two species are very similar and it is interesting to note the difference in morphology existing between them. This orchid occurs in dense rainforests and rainforest margins where it grows on trees in shady sites beside streams.

Because of its high altitude origin, this orchid grows quite well in Sydney area. In culture this orchid is usually mounted on fern slab and hung vertically to simulate its natural conditions and for better display at flowering. According to Bryan Spurrs, his plant is hung on the south facing side of his shade house which has a polycarbonate roof. It is misted everyday using a FOGG-IT green nozzle attached to the hose throughout the year. Fertiliser is applied weekly using Peter Excel at 1 gm/litre concentration.

Because of the uncertainty associated with the classification of this orchid, namely its status whether as a species or a variety of Den. tetragonum, it is difficult to determine the extent this orchid has been used in breeding. Currently in orchid hybrid registration with RHS, only the record of species (not details of variety) is required.

(Dendrobium cacatua - Photo by Sau-wan Chan)