Dendrobium section Rhizobium: An Update on breeding, ANOS Award Standards and Culture
Introduction to Dendrobium section Rhizobium
Dendrobium sect. Rhizobium Lindl. was established by John Lindley in 1851 based on Den. linguiforme. This classification was used by Schlechter in his major revision of Dendrobium in 1911-14. Brieger proposed in 1981 that this group be separated as a new genus Dockrillia, but this was rejected. Section Rhizobium includes around 30 species, which are mainly centered in Australia (17 –18 species) and Papua New Guinea (10 species so far described), with others in New Caledonia, Fiji, Tahiti, Samoa, Aru Islands and Vanuatu. Like so many orchids, name changes have been common over the years – for example Den. striolatum Rchb.f. was described as Den. teretifolium (1839), Den. milliganii (1859) and Callista striolata (1891) (Clements 1989).
The species are very variable in leaf form and flowers, which are commonly non-resupinate, appearing upside down. Each growth typically carries only a single leaf, which in the majority of species is long, pendulous, and terete. Some such as Den. linguiform
e deviates from other species by having broad, laterally flattened leaves, which are very fleashy and leathery. This feature can also be seen in the other Dendrobium sections Conostalix and Lichenastrum. Plants are usually found at low to moderate altitudes, growing as epiphytes or lithophytes in rainforests and open forests, often in bright light. The brighter the light the redder the growths become, due to the presence of anthocyanins.
This species’ striated markings on its segments and widely opening flowers have made it attractive to breeders. Den. striolatum generally produces singular flowers, sometimes two are produced, in my experience this occurs inconsistently. Mainly forms with large shapely flowers have been used in breeding, four generations of line breeding were completed by Brian Gerhard and Neil Finch. Our collection of 50+ forms seem to flower in October or November, depending on the climatic conditions of that year, flowers typically last around 12 days. Plants achieve specimen size very quickly and I have found pot culture to be best as this does not allow the plants to dry out. Due to the lithophytic nature of this species we add extra scoria rock to the potting medium. While line breeding has ceased, we have recently (2018) began line breeding again. Den. striolatum has proved successful in breeding, while it does reduce flower count it produces superior shape, ease of growth and general floriferousness. Den. striolatum does tend to dominate colour saturation when it is present in the current generation, however has less of an effect than the Den. teretifoium complex.
Dendrobium teretifolium complex
Probably the most popular group of the section within cultivation, it has also been extensively used in hybridisation. The Den. teretifoium complex was most likely attractive to pioneer hybridisers because of flower size, floriferousness and clear colours. Usually 6-15 flowers per inflorescence are produced and plants flower very freely. Some minor variation in the shade of colour and flower size are reported. Some may regard varieties of the complex as separate species, however variation in size, shape and colour are small to moderate and most likely reflect typical epigenetic changes, along with geographical distribution. Den. teretifolium seems to do best on hardwood mounts, however other varieties within the complex have performed well in pots. This complex may continue to be used in modern hybridisation, however I believe it should be used minimally and logically. This is due to the fact the varieties within the complex reduce colour saturation and create large growing plants with reduced floriferousness.
This species typically produces 3-7 flowers which do not open widely and can flower sporadically, but the major flowering occurs in late spring. Flower size and shape has been quite variable in my experiences, this could be due to the fact most plants are in fact selfings. The flowers are very dark and colour does not vary dramatically. I’ve found these plants like medium to high levels of light and grow best in pots or tree fern pots, as these keep the roots moist and cool. Phil Spence completed very successful cornerstone breeding with Den. fuliginosum that has led to some of the best hybrids within the section to date (eg,. Den. Australian ginger and Den. Tweetie). Den. fuliginosum provides in hybridisation the desired ‘spray can effect’, sometimes repeat flowing and diverse colours. In my opinion when crossed with Den. striolatum based hybrids Den. fuliginosum generally has little influence on shape.
Generally, produces one – two flowers, occasionally has been sighted with 3-4 flowers (Upton, 1989). Flowers are green to yellowish- green, with purple brown striate markings on the rear of the segments. High light has proved best, along with frequent watering in warmer months, slightly reduced in winter, Den. bowmanii possibly may benefit from protection in southern states. Mount culture is suitable; however, we have had good results with pot culture. No extensive line breeding has been completed to my knowledge, but Den. bowmanii has been used recently in experimental breeding especially by George Dimos (VIC), with mixed and inconsistent results, usually intermediate between the two species involved. The green colours of Den. bowmanii could prove interesting in future breeding. Germination has proved difficult to achieve in my experience. May be more beneficial than Den. mortii as is easier to grow and flower.
Our nursery is in Newcastle, New South Wales, in a suburban area that is quite shaded and has a wide range of temperatures through the year. Temperature typically varies from 7–30° C and is rarely below 4.5° C or above 33° C.
We prefer to use various sizes of squat pots manufactured by the Orchid Pot Co. These quality pots come in an extensive range of sizes and are made of durable plastic. The free draining design is the reason these pots have been so successful.
Avoid high nitrogen fertiliser as it leads to soft, fungal-prone growth and few flowers. We use a high potassium fertiliser with a N:P:K ratio of 8:5:25, at a rate of one gram per litre of water fortnightly, all year round. We compliment this by using Amino Grow as a drench or foliar feed. This helps the uptake of potassium (K) whilst also maintaining a high level of sugars. To maintain optimum results, we use micro-fine lime at the rate of one gram per litre in early autumn and again at the beginning of spring.
House Coverings, Watering, Air movement and Humidity
Our main tunnel house is covered in 30% black shade cloth. This allows maximum light which most species and hybrids of this section require to produce masses of flowers. However, some species such as Den. pugioniforme may prefer less light. Species such as Den. chordiforme and Den. erythraeum may grow better with a covered roof over the winter months. In our conditions these plants prefer to remain consistently moist all year around. No dry spell is required for most species and hybrids, apart from Den. teretifolium var. fasciculatum, which will benefit from reduced watering in the winter. Constant and consistent air movement is crucial to growing this section. We have found that hanging the plants helps in achieving optimal air movement. With regard to humidity, while not necessary, I believe it is best to keep the plants consistently moist throughout the warmer months.
Propagation and Pruning
Plants can be propagated quite easily. It is best to take a cutting when a new aerial root forms outside of the pot. Cut above the new root when it is 5 cm long and pot into a 50 mm tube. If a cutting has no roots a rooting hormone may be useful. The pruning method is used to initiate new growths at the base of the plant. Older plants, especially those of the Den. teretifolium complex, or Den. schoeninum especially, may need pruning. These plants sometimes become full of ‘dead wood’ when growths are lengthy and don’t flower. We suggest pruning after flowering and before new growth is initiated. Be careful not to prune too hard. We usually take 2–5 growths from a mature plant.
Current Growing Medium
A medium starting with a quality treated Kiwi Pine Bark, Size 2 is recommended for 50–80 mm pots and Size 3 for larger pots. Pine bark makes up around 70% of the media, the remaining 30% is 10 mm scoria rock. A mixture of chopped Sphagnum moss and perlite may also be useful in growing cuttings with minimal or no root growth.
We are not strong advocates of mount culture, and most of our collection of around 1000 mature plants have been growing in pots for some years. However, some species require mounts to flourish. These are the Den. teretifoliumcomplex, Den. cucumerinum, Den. linguiforme, Den. pugioniforme, Den. racemosum and Den. rigidum, which have all grown well on cork mounts or aged hardwood. Some growers use ‘gutter guard’ mounts, we have found pants dry out too quickly in these.
Previous Growing Mediums
I think it is crucial to explore what mediums we have previously used and explain why they were not successful. Composted pine barks were the first used. These barks started out as high quality, affordable and long-lasting products. However, over time they became expensive, far too composted and contaminated with slugs and wood. This is the reason we started using coconut fibre, this product was used 50/50 with pine bark to reduce the cost of our medium. However, this product deteriorated to almost mud quickly in our conditions and held salts easily that led to loss of root systems. Major inconsistencies in product quality and PH ultimately led to its phasing out. Next came perlite, because again it reduced costs and aided in moisture retention, while also aerating the medium. In my opinion, perlite is also capable of retaining possibly harmful salts. Further, it seems to heat up in warmer weather and therefore can burn the plants root system.
Dendrobium Tweetas (registered by DUNO, 2005) is Den. Tweetie crossed with Den. striolatum. Several different Den. Tweetas were sown by DUNO. The most outstanding results were from cross 1877 using Den. striolatum ‘Tasmanian Gold’. These hybrids had intense colour. However, as usually the case when Den. striolatum is present in the immediate gene pool, flower count was reduced, and this is something to be considered in future breeding.
Dendrobium Australian Ginger (registered by Australian Orchid Nursery, 2003) was the hybrid that demonstrated what is achievable by knowledgeable individuals. DUNO has sown around 15 variations of Den. Australian Ginger using different forms of Den. striolatum. For a primary hybrid between Den. fuliginosum an