Dendrobium speciosum is widespread along the eastern coast of Australia and up to 180km (110-120 miles)
inland. It is also one of the ornamental flowers noted by the colonists and probably the convicts in Sydney.
It was taken back to England about 1800 and has been admired ever since .It has colloquial names depending where you are living. South of the New South Wales boarder it is know as the ‘Rock Lilly’, north of that boarder (Queensland) it is known as the ‘King Orchid’.
There seems to be a bit of a conflict as to how many varieties of there are with some saying that there are four and others saying there are six. The two main books that I am reading at the moment, both agree on there being six varieties.
The southern most variety is
D speciosum var Speciosum, with canes up to 60cm (2’), sometimes longer, but the average is 25-30cm (10 – 12 inches) tall. The flower spikes are about 30 – 45cm and carry 30 – 80 flowers. Its habitat is mainly on rock in open forest but also in some rainforest areas on trees. In parts of its habitats it is subjected to frost, snow, dry westerly winds in winter and searing heat in the summer.
The next one up to coast is
D speciosum var Hillii .The stems heregrow to 1m tall ,usually straight up and taper little through out. Cultivated plants seldom grow more than 45cm tall. It is quite common to have 100 or more flowers per spike. The habitat is the same as for
D speciosum var Speciosum but this time more grow on trees than rock.
Heading further up the coast we find
D speciosum var Grandiflorum .The stems are very similar to
Hillii
But the leaves are longer and wider. They normally have 60 – 70 flowers per spike but 100 or more is also not uncommon. Again the habitat is the same and wild specimen plants have been recorded at 2.5m across.
If we get into our vehicle again and drive north to the Tropic of Capricorn, we will find the next variety, and what its varietal name is. You guessed it
Capricornicum. This plant a smaller
Grandiflorum , smaller in all parts. Stems 7 – 25cm tall. These plants are normal found growing on sandstone rock faces near creeks and waterfalls. All known habitats of this Varity are dryer then that of
Grandiflorum .
The next variety that we come across in our northward travel is
Curvicaule .The stems here are from 7 – about 100cm tall, varying in shape from narrow at the base and widest at the top, to curved stems to squat straight stems. The spikes on these plants are only about 27cm long and the flowers last about 15 days.
The sixth plant that we come across in our travels is
Pendunculatum .The stems are 5 – 16 cm tall but are normally only 7cm,conical and dwarf. The spikes are held upright and are very rigid ,with the flowers bunched at the top. It also normally produces one spike per stem The seed pods of this one are deep purple in color. The habitat is rock faces in full sun.
Culture
Well that was a rundown on the varieties now we will consider the culture.
It is an ideal plant for well drained rockeries, preferring to be the top most plant. It also grows well in the forks of large trees or on the top of sizeable tree stumps. In these positions it likes lots of light, Full sun will not hurt it. If it is grown in a pot ,it should be put into a very very open mix. As growth is very robust, it will require frequent repotting.Do not start a small plant in a pot that is big. The best time to repot is just after flowering has finished. A second growth faze can occur in autumn and they will still mature before winter.
The minimum temperature for the southern forms is about 2degess C ,while the lower elevation ones from Queensland would like a temperature if 10C. The varieties
Grandiflorum and
Hillii like more shade than the others.It can take up to 12 years for one of these plants to mature to flowering size and will alternate flowering and growing years (some years it will only produce growth and the next year it will grow and flower). Water and feed the plants regularly over summer and allowed to dry off over autumn to help induce the flower spikes.

http://www.anbg.gov.au/gnp/interns-2...speciosum.html
http://www.orchidspecies.com/denspeciosum.htm
http://www.orchidspecies.com/denpedunculatum.htm