Dendrobium mortii

A BIT ABOUT . . . Dendrobium mortii

© Gerry Walsh

Dendrobium mortii – Colourful flowers and the very thin diminutive leaves

In both pics the reflexing nature of the petals and sepals is evident

Dendrobium mortii, what’s that look like? Well, we used to call it  D. tenuissimum. That’s what D. mortii looks like. But be cautious, because strictly speaking, there is no such animal as D. tenuissimum any more. It is now calledD. mortii, which is the old name that we used to call the species that we now know by the new name of D. bowmanii . . . am I making myself understood? I don’t know how to make it any simpler actually. Botanist change names of many plants and usually with good reason. Not always mind you.

D. mortii is the daintiest and the least conspicuous member of the Rhizobium section of the Dendrobium genus. And while it certainly is an unobtrusive little devil when seen in its natural surrounds, it really isn’t very little at all. For those among you who aren’t familiar with it, D. mortii has the form and growth habit of D. teretifolium, except that it’s stems are only about 1.5 to 2 mm thick. Its leaves are cylindrical, averaging about the thickness of a match, and anywhere between 4 cms and 11 cms long and faintly grooved longitudinally.

The average size of a D. mortii plant is difficult to define. In some situations it can be very compact with squat spacing between the leaves. In other situations it may be very long in stem but only carry five or six leaves. I suppose an average kind of plant would hang pendulous for about 40 cm and carry around. a dozen leaves. Under favourable bush house conditions this species can really romp away forming clumps that may hang down for a metre or so and develop a mass of branching stems. However, I have never observed a wild plant reaching the dimensions that a well cultivated bush house specimen can attain.

D. mortii plants can bloom at a very small size – a size at which other section Rhizobium species would be considered as only seedlings. The flowers are generally around the 20 to 25 mm span but occasional giants can be seen reaching the 50 mm mark. The most appealing flower colour is vivid lime green but a somewhat less vivid green would be the normal I guess. Some forms will open a sort of avocado yellowy colour. Regardless of the colour on opening, the blooms of D. mortii will age to this same yellowy hue. The  labellum is white and always displays some degree of bright  purple blotching, or edging if you like, along the labellum  side lobes. The combination is quite pleasing.

The flower is basically your typical D. teretifolium type  shape, but with one very distinct difference. When the  flower first opens it will stay this typical shape for only a  short time. After a. day or so it’s segments will suddenly  reflex quite severely, to the extent that the tips of the petals  and sepals will actually touch behind the mentum – literally turning itself inside out in the manner of Liparis reflexa. This peculiarity of D.  mortii is natural for the species . . . this  is what D. mortii does! The flower would  be more appealing if it didn’t reflex but it  can’t be considered a fault when it  inevitably does.

There is no such thing as  a D. mortii that doesn’t reflex as it ages. So judges take note – this species cannot  be marked down because it reflexes – but  you can give more pixie points to one  that hasn’t begun reflexing at the  commencement of judging. The next day  it will surely be reflexed, particularly if  the weather warms up. But it can’t be dismissed because it is doing what nature  programmed it to do. It is definitely to be  considered even when it is reflexing. Many times I have seen inexperienced new judges, and some well established ones as well, completely ignore a D. mortii plant with a waving hand at best. ” Too reflexed” they might utter. As I have said, it is what D. mortii does. To ignore reflexing D. mortii is equivalent to  ignoring a white D. kingianum because it isn’t pink. It’s stupidity to do so.

Here in Sydney, late September seems to  be the peak flowering time, usually  missing the bulk of the spring shows. It  is not that frequently seen at monthly meetings either. D. mortii is not shy to  flower at all. It produces a goodly number of blooms when compared to the  sparseness of the plant itself. It sprouts  many more flowers proportionate to its bulk then do similar species, D.  pugioniforme and D. teretifolium var . fairfaxii being good examples of what I mean. Flowers are virtually always  solitary and are highly scented.

D. mortii has never been a common  orchid in captivity. In the heady days of  rainforest logging, it was occasionally  procurable from select native nurseries if you really searched it out. But since  1983, when rainforest logging in NSW  was officially wound up, the availability of D. mortii was  crucially affected. This, combined with its general  uncommonness in the wild, is the reason for its non appearance at shows and meetings. Double-U Orchids was listing flasks of D. mortii in the mid 1980s. Unless you are  lucky enough to find a fallen branch on the ground, you’ll have trouble acquiring a plant of D. mortii.

D. mortii is distributed from the southern slopes of the Barrington Tops in the south, then northwards to at least the Lamington Plateau in extreme southern Queensland. Places where I have made personal observations include the Patterson River, Allyn River, Williams River, Dingo Tops, Ellenborough River, Forbes River, Hastings River, Cockerawombeeba Creek, Wilson River, Dorrigo Plateau, Little Rosewood Creek, Richmond Range, Mt Lindesay, Tweed Range and the Lamington Plateau.

The one aspect that links all these far flung locations together is ‘rainforest’. Not just marginal rainforest and semi wet brush country, but tall and towering rainforest, be it sub-tropical (Wilson River) or cool temperate Coachwood-dominated (Cockerawombeeba Creek). In the lower altitudes, 500 metres or so, it is seldom found far from creeks. But up over the 1000 metres mark it will colonise ridge tops where the affects of mist and fog are intensified. Around the higher parts of the Tweed Range you will readily see dainty curtains of D. mortii hanging from mossy limbs just above the road.

In 1987 I observed several plants growing on rocks high up on Mt Lindesay – but still under well developed rainforest. I have not heard of another instance of D. mortii occurring lithophytically so it must be a very rare habit. One of these rock-dwellers was even flowering – in July would you believe. Most of the older Hoop Pines about Mt Lindesay carry copious quantities of D. mortii, along with a host of other species. But if you plan to arrange an outing around this beautiful area be prepared for a long and hard day’s work – Mt Lindesay is not for the inexperienced.

Around Dorrigo it can be very common on Hoop Pines in undisturbed rainforest. Not far from the hamlet of Megan, there is a gnarled, stunted old Hoop Pine which has forked about three metres above the ground – very unusual indeed. In it there are clumps of D. speciosum, D. falcorostrum (even more unusual), Bulbophyllum weinthalii and literally dozens of plants of D. mortii – most of which are advanced seedlings. The leaves are strange because they are no longer than three cm and about as thick as a drinking straw. I can offer no explanation for this extra-short/extra-thick leaf habit. Similar looking plants are found about the whole district, but only on Hoop Pines. Flowers are typical D. mortii however.

As with most of the species in its section, D. mortii will hybridise naturally with other Rhizobium members. Over the years natural hybrids have been recorded between D. mortii with D. pugioniforme, and with D. schoeninum. In 1992 I was up in the Border Ranges picking up Hoop Pine limbs while searching for (and finding) B. globuliforme. This was a great locality for a wide range of species and at one point I found a small plant of D. schoeninum on the ground. Its flowers were very colourful for that species but it wasn’t until I examined it more carefully in full sunlight that I realised it was totally green, a colour that D. schoeninum never is. There was a good deal of D. mortii about the place and this orchid was in all ways intermediate between the  two. It has now grown up and puts on a great display every Spring. You can get lucky – or you can get observant.

Under cultivation, D. mortii is slightly out of step with its terete relatives. Whereas D. schoeninum and D. teretifolium etc. are generally fairly quick to establish on a new mount, D. mortii is pretty slow by comparison. Because of its overly thin stems, roots and leaves, it has no capacity to store up an energy supply. It relies instead on a habitat pledging exact and totally reliable doses of shade and moisture. And so it is in your bush house. You have to give it what it wants or you can bid it farewell.

No other host material that I’ve tried even comes close to a good slab of Cyathea tree fern. D. mortii loves to let its roots run riot through the spongy fibre. And it is very advantageous to use a good handful of epiphytic moss around the base of the plant as well. This is one species where I allow that creeping Pyrosia fern (rock velvet fern) to establish itself on the mount. I generally remove Pyrosia from most orchids because once it gets a go on it eventually works to the detriment of many species. It can become excessively thick and overwhelm the orchid after four or five years. But not so with D. mortii. The best clumps I’ve ever seen had Pyrosia all around the roots.

I’ve used natural cork with good success too but always with the moss around the base of the plant. I don’t like using timber slabs at all. Because of the amount of moisture demanded by D. mortii, the surface of the wood soon becomes slimy and the orchid roots inevitably relinquish their grip. This looks totally unsightly in my opinion, even though they may continue performing their function of nurturing the orchid plant. Ironbark totems are really good as well, but they will break down after five or six years unfortunately and it should be your goal to avoid remounting if possible. No doubt about it . . .  a slab of tree fern is the host you should use.

It goes without saying that you mustn’t shy away from watering your plant – particularly if it hasn’t established itself fully. Actually, the more I think about it, I doubt it would be possible to over water D. mortii, provided you haven’t packed sphagnum moss around the roots and excluded oxygen from the equation. Fertilise it like all your other orchids of course. You really do need to fertilise fairly regularly with any orchid if you expect to get a first prize at the next meeting. Fertiliser is so often the difference between the plant that comes first and the one that comes eleventeenth .

Shade should be at least 50% or preferably 70%, but D. mortii is one of those uncommon orchids that would certainly flower in almost total shade. The number of flowers would be reduced dramatically of course. Your only problem is obtaining a plant in the first place. If you ask around you’ll probably pick up a small piece from one of the other members in your society. Treat it well and it will perform for you over many years. D. mortii isn’t subject to any particular pest that comes to mind. So you really should be growing it.

From Australian Native Orchid – Jones