In the south, the dividing line between var. hillii and var. grandiflorum is indistinct and blurred. This cross-over in the distribution can vaguely be described as occurring in a line from the Bunya Mts, (between Dalby and Kingaroy) to the vicinity of Gympie (100kms north of Brisbane). The line is not a thin one and can be up to 30 or 40 kms deep. If in this zone, you may encounter typical examples of small flowered var. hillii plants or large flowered var. grandiflorums. . . a complete lucky dip if you like.
Most likely you will see something in between, such as small variety hillii sized blooms but with yellowish colouration. Yellow is a colour that does not occur in var. hillii. Whites do not occur in var. grandilflorums (extremely rare anyway, I have seen 2). So yellowish flowers in this zone are a dead set indication of cross over breeding. So is size; bigger then typical var. hillii blooms but rarely as large as typical var. grandiflorum flowers. Assigning specimens to any particular basket is not always easy.
To the north of its range, var. grandiflorum is more or less finished once you get to the Fitzroy River catchment in the vicinity of Rockhampton. Once you are north of the Gladstone district very few specimens of any D. speciosum are observed, until the zone of the next variety of D. speciosum, var. capricornicum, is reached near Rockhampton.
Broadly speaking, var. grandiflorum is quite similar in appearance to var. hillii. They each have aerial roots and they are the only varieties of D. speciosum to do so. My observations suggest that var. grandiflorum will generally be slightly more robust however. Pseudobulbs are slightly thicker and the leaves a bit thicker and stouter. Provided you know the providence of your plant then you can work out the variety you have as a rule . . . keeping in mind the intermediate zone in the south of the range.
Now we come to the real identifying characteristics that separate var. hillii from var. grandiflorum. Where hillii blooms rarely exceed 25mm, grandiflorum would rarely be that small. Blooms over 80mm deep, and occasionally reaching over 100mms, are quite common and can be either very heavy and robust or thinner and feathery, looking Dendrobium tetragonum like to some degree. Colour is generally mid yellow to dark yellow, then all the way through to gold tones. Colour deepens as the blooms age. They are at their deepest when the flowers are verging on going off.
A lot of enthusiasts consider var. grandiflorum to be the most attractive variety of Den. speciosum. I wonder why? Perhaps it is the colour, which is nearly always some shade of yellow and often intense yellow to gold. Perhaps it is the flower size, which can be called big. Maybe it is the length of the raceme which can be up to 750 mms and as wide as 150mm in diameter through the flowers from side to side. To sum up: var. grandiflorum is just plain gaudy and pretentious.
A bad habit of var. grandiflorum is leg crossing; the lateral sepals are quite subject to crossing completely over and totally destroying the good looks of a specimen. To be considered good, a bloom has to have tough, robust texture and lateral sepals that do not come anywhere near crossing over.
Var. grandiflorum is equally at home on tree or rock. It just loves old man hoop pines but a whole variety of other hosts will suffice. I have seen it on the non-shedding fibrous trunk of Eucalyptus species in open forest. In tall rainforest it is completely at home on any of the giant Flindersia species. That is, the more typical huge buttressed tree species of the rainforest.
As a general rule, plants from the north of the distribution are much more impressive from the horticultural point of view. They frequently have heavy texture, as well as much superior shape. Southern plants of var. grandiflorum are often the equal in depth of colour and some even superior in brightness, if not darkness of colour.
I have never seen a single specimen that could hold a flag to the northern forms when it comes to shape and substance and texture. Size can be equally large but the southerners are inevitably feathery. To be brutally honest, I have had numerous offers of swaps of southern specimens for my good northern ones and have politely declined and found excuses to not do the swap. The last thing I need in my orchiderium is a sub-standard speciosum.
Yet, some of you will tell me one day that you have a fantastic, ripper of a grandiflorum, from near Gympie or Bundaberg, and I will hope you do. But I would want to see it in the flesh before I offered it a home. I am pretty much a grumpy, doubting stick in the mud on this point. I would not budge easy . . . I can assure you of that!