An Update on Dendrobium section Rhizobium

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.

Selected Species

Dendrobium striolatum

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.

Dendrobium fuliginosum

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.

Dendrobium bowmanii

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.

Culture

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.

Pots

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.

Fertiliser

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.

Potting Mixtures

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.

Mount Culture

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.

Standout Hybrids

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 and Den. striolatum the shape of the flowers and the colouration can be very attractive. Even when Den. striolatum was used as the pollen parent it was evident that substantial amounts of colour were transmitted to the progeny. Dendrobium striolatum ‘Ruffles’ FCC/AOC-OSNSW AM/ANOS provided its progeny with broad labellum’s and large frills while still maintaining rounded segments and good colour saturation, mainly of the petals. Dendrobium striolatum ‘Tasmanian Gold’ created an intensity of gold with a burgundy ‘spray can’ affect, the most pleasing colours by far.

We purchased a flask of Den. Nutmeg from Phil Spence (registered 2009) in the early 2000’s and grew about 50 plants on to flowering. This year several plants were identified to be used in the breeding with potential in their high flower counts, vast range of colour variations and great shape. We have had quite a number of successful crosses in 2015, for example using Den. Grumpy, Den. Australian Freckles and Den. Weatherly.

Dendrobium Lauren ‘Holly Chief’ (registered 2009) was awarded HCC/AOC-ANOS at the 2015 Kempsey Speciosum Spectacular. Brian saw the capabilities of this plant and since then we have flowered Den. Lauren x Den. Australian Ginger. The results have been exciting. The shape, flower count and ease of growth have all carried through to the progeny. We have used it in 2015 for several hybrids, and feel the wide labellum and the expansive size of the flowers will produce worthwhile results.

ANOS Judging Standard

Shape

Optimum shape for the style of hybrid being judged, in relation to its parentage. Flowers should be balanced and segments should be well proportioned.

Colour and Attractiveness

Shall include white or any other colour or combination of colours. It should be clear glistening and fresh, and all markings and shadings should be well defined.

Size

Points to be allocated based on the optimum size for the style of hybrid and the exhibit’s genetic background. Flowers should be consistent in size along the inflorescence.

Substance and Texture

The flower or flowers should be of good substance and texture, and should be firm, fresh, lustrous and without blemish.

Floriferousness

The exhibit should carry a good floral display in proportion to its size. Credit should be given for clones displaying more flowers than is considered average for the hybrid style being judged.

Habit of Inflorescences

The flowers should be displayed to best advantage and be evenly distributed along the inflorescence. It is also important to consider how you will display plants of this section for judging.

Awards

ANOS have so far awarded 17 hybrids from Dendrobium section Rhizobium. The following are a sample of those 17 awards:

Den. Grey Ghost ‘Tumbi’ AM-AD/ANOS

Den. Oliver Jack ‘Gerard’ HCC/ANOS

Den. Lauren ‘Holly Chief’ HCC/ANOS

Den. Australian Freckles ‘Best Blue’ AM/ANOS

Den. Bronze Belle ‘Deejee’ HCC/AOC

Den. Tweetas ‘Violet Lip’ AM/ANOS

Den. Tweetas ‘Mustard’ AM/ANOS

Den. Tweetas ‘Albie’s gift’ HCC/ANOS

Den. Australian Ginger ‘Wow’ HCC/ANOS

Den. Australian Ginger ‘Golly’ FCC/ANOS

As we look through photographs of this selection of awarded hybrids the following similarities can be observed to form a general judging standard. This standard can be translated into breeding objectives:

• Shape: wide, mostly rounded, proportioned and balanced segments. No signs of reflexing or flowers that do not fully open.

• Colour: clear, distinctive colours, and markings/colouration are consistent.

• Size: size is probably the most variable within this sample, however all flowers are of consistent size on the exhibit. I think because hybrids of this section have quite diverse species composition, size is something judges may need to spend a little more to on refining the way points are allocated.

• Floriferousness: Plants are usually medium – large in size. I have observed plants with a more compact growth habit are favoured in award judging. I believe this is due the fact that the flowers are closer together and therefore form a visual illusion that the plant is more floriferous. Individual inflorescence flower count is also quite variable and is dependent on the species composition of the hybrid.

Directions

Registered Hybrids

The following Dendrobium section Rhizobium hybrids demonstrate what we have been striving to achieve in the latest generation of breeding.

Den. Wickersham (Den. Lauren x Den. Australian Ginger)

We have recently flowered remakes of this grex using Den. Australian ginger ‘Wow’ HCC/ANOS-AOC. So far examples have displayed large flowers with outstanding shape and colouration. The segments of this next generation have been wider, the flowers are open, plants display a high flower count and the growth habit is slightly more compact. So far, the Den. Wickersham we have flowered as seedlings have been wining against mature awarded plants and I believe they will for part of the future bench mark, especially representing modern Den. teretifolum style hybrids. We commenced breeding with this grex in 2018 with crosses aiming to once again increase flower count and segment width, while maintain size and colour.

Den. Sharday (Den. chordiforme x Den. Tweetie)

Den. Sharday is possibly the most experimental we have been thus far, apart from the recent use of Den. wassellii. The first release of these were made by Brian Gerhard in 2009. I decided to remake this hybrid in 2017, to make them available to a new generation of growers interested in this section. Den. chordiforme increases flower count dramatically, while Den. Tweetie allows these hybrids to repeat flower throughout the year, with two main flowerings being in autumn and late spring. Therefore, this hybrid also extends the flowering period from April until late October, with the peak for this section being in mid-August to mid-September. While Den. chordiforme does reduce flower size, Den. Sharday produced a vast array of colour combinations. I believe this grex directly supports my opinion that species diversification and when used logically can contribute to achieving much within the breeding od section Rhizobium.

Den. Hunter Doll (Den. Rosemary Jupp x Den. teretifolium var. aureum)

This is most likely one of the last crosses we will register using a variety from the Den. teretifolium complex. This hybrid was made to try and gain more breeding plants with the interesting blue/grey colouration that is produced using Den. teretifolium var. aureum and pinkish hybrids. These cornerstone hybrids will be used with compact growing plants in the next generation of our breeding.

Den. Hunter Legend (Den. Australian Ginger x Den. teretifolium var aureum)

Alike the above cross this hybrid was made to gain plants with increased flower size and flower count to be used future hybridisation. I believe we did achieve what we had predicted, with the first flowering seedlings carrying 7 – 10 flowers per inflorescence. The colours produced have mainly been yellows and golds and flower size has been consistently large thanks to the pollen parents influence.

Den. Hunter Hope (Den. Aussie Phil x Den. Australian Ginger), Den. Hunter Horizon (Den. Tweetas x Den. Weatherly), and Den. Hunter Sand Dune (Den. Tamara x Den. teretifolium)

These three registrations are described together as they represent the main aim of our breeding program. They all have achieved improvements in shape, slightly increased flower size, and produced vast amounts of colouration. These hybrids have further created new or built on existing colour combinations. I believe these hybrids unlock a very exciting future and I look forward to their progeny. There is still work to be done, especially in regards to shape. Further, I believe we should aim to increase flower count.

Den. Hunter Shadow

This is one of the first crosses to flower that represent our line of D. schoeninum and D. fuliginosum influenced hybrids. D. Hunter Shadow has produced many different colour variations with many having an intense ‘spray can effect’ on all segments. This generation display a D. teretifolium flower shape, so the next generation will hopefully aim to overcome this, while still maintaining attractive colouration.

The Next Generation

The following Dendrobium section Rhizobium hybrids demonstrate what we have been producing through our breeding.

Den. Glenn Wall ‘Orange’ x Den. Tweetas ‘Red Lip’

This cross was made due to the potential it holds to add exciting new colours and/or colour combinations to section Rhizobium. The main aim of this cross is to achieve orange flowers and if not deep gold flowers. Further, because of the use of Den. Tweetas ‘Red Lip’, colourful labellum’s may also be produced. We expect to flower the first of this cross in 2019.

Den. Australian Freckles ‘Best Blue’ AM/ANOS-AOC x Den. Australian Sunblessed ‘Judy’

Den. Australian Freckles ‘Best Blue’ AM/ANOS-AOC is the parent of Den. Oliver Jack and many other quality hybrids. Awarded in 2016 it displays rounded segments and is bursting with colour. The particular Den. Australian Sunblessed used is the darkest I have seen (our thanks to Ron Young for allowing us to use this plant in our breeding program). The aim of this cross was to produce plants with dark colouration, while maintaining quality shape.

Den. wassellii x Den. Hot Coals ‘Best’

This is possibly the most experimental cross I have produced in almost six years of breeding with section Rhizobium. I am aiming to produce compact growing plants that repeat flower throughout the year. I’m hopeful that the colour and ‘spray can effect’ of Den. Hot Coals will be apparent in this generation. However, no breeding with Den. wassellii that moves past a primary hybrid level has been carried out so it is difficult to predict how much colour will be maintained. However, I believe the use of Den. wassellii will most likely increase flower count. We have also produced Den. Harmon ‘Mark’ x Den. Hot Coals ‘Best’ to achieve similar outcome, in case Den. wassellii does reduce colour in its progeny.

Den. Australian Ginger ‘Wow’ HCC/ANOS-AOC x Den. Kalon Byrne-Dimos ‘Newbold’

This hybrid uses possibly two of the largest flowers within our collection that have a Den. striolatum style flower. Again, large and colourful flowers are expected. However, we are hoping to maintain the quality shape and proportioned nature of the clear yellow Den. Kalon Byrne-Dimos ‘Newbold. This cross should produce plants that can be used in future breeding to achieve a more circular flower shape.

Den. Tweetie ‘Huge’ x Den. Australian Ginger ‘Big Guy’

Here we are searching for large and colourful flowers similar to Den. Tweetas, however, this time with an extra dose of Den. fuliginosum colouration should be improved. Den. Australian Ginger ‘Big Guy’ is the largest flower I have seen of this grex. Den. striolatum ‘Ruffles’ FCC/AOC was used; therefore, this hybrid has a large ruffles labellum. While producing hybrids with large, ruffled labellum’s is not a current priority, it could indeed add to the attractiveness of hybrids with section Rhizobium.

Den. Lauren ‘Holly Chief’ AM/AOC x Den. Martin ‘Gold’ HCC/AOC

This cross represents a very simple objective, to produce large flowers of yellow tones. While maintaining size, we aim to increase segment width. Den. Martin ‘Gold’ HCC/AOC also has the attractive feature of being a compact plant with a flower count of four to six flowers per inflorescence.

Den. Martin ‘Royal’ x Den. Tamara ‘JD Spots’

These plants represent the best of our ‘spotted’ hybrids. Therefore, this cross aims to make available plants that have a high possibility of being densely spotted. While still maintaining flower size and a compact growth habit.

Den. Tweetas ‘Old Gold’ x Den. Grey Ghost ‘Tumbi’ AM/ANOS-AOC AD/ANOS

Using Den. Grey Ghost ‘Tumbi’ HCC-AD/ANOS (Den. striolatum x Den. Rosemary Jupp, registered DUNO 2006) can achieve further progress in our breeding program. It has already produced large almost blue-wash hybrids in Den. James Garner (registered DUNO 2013). Its colouration seems to produce interesting combinations in its progeny. Therefore, this cross could possible widen the range of colours existing within the section.

Den. Nutmeg ‘Mars Shadow’ x Den. Martin ‘Royal’

Den. Nutmeg ‘Mars Shadow’ has a high flower count, and great shape and colour. Most importantly it has a very compact growth habit. Therefore, this cross aims to increase flower count and decrease plant size, while still maintaining the positives of shape, colour and flower size.

Den. Rocky Carroll x Den. Australian Freckles ‘Best Blue’ AM/ANOS-AOC

This is the next generation that represents our line of Den. schoeninum and Den. fuliginosum influenced hybrids. For many reasons the ‘spray can effect’ has been a very popular aspect for many breeders. However, limited hybridisation has been made available that also aims to improve flower shape, size and floriferousness. As previous discussed this generation aims to slightly move away from Den. teretifolium flower shape, along with reduce plant size and increasing floriferousness.

Den. Tweetas ‘Neville’ AM/AOC x Den. Martin ‘Gold’ HCC/AOC

Den. Tweetas ‘Neville’ AM/AOC has excellent shape and colour, we used it with Den. Martin ‘Gold’ HCC/AOC to increase flower size and floriferousness. Both parents already display quality flower shape and open very well, hence these qualities should be maintained.

Future Breeding

Flower count, shape and size are crucial elements which in the past have been overlooked. Looking to the future, some outstanding colour combinations and more shapely flowers can be expected by further breeding. We are also endeavouring to increase the flower count by the use of Den. Nutmeg, which have been carrying 10-20 flowers on their second flowerings. Our biggest challenge is to create complex growing hybrids without jeopardizing the previously mentioned focuses of our breeding program.

Individuals should strive to maintain correct records of parentage, this may require updates as we learn more about past issues/mistakes and scientific reviews of Dendrobium section Rhizobium species are made available. This will ensure that the wider community can better understand the outcomes of their role in hybridisation. This would also aid in achieving correct parentage knowledge, and therefore correct nomenclature.

Further, it would prove foresightful to commence line breeding of species with in Dendrobium section Rhizobium. This would ensure a sound collection of quality species to reintroduce into breeding programs in the case that developments in hybridisation fail or halt.

In my opinion, there is a very exciting future ahead for this particular section of Dendrobium. If careful decisions are made in terms of their breeding these pleasant native orchids will continue to improve and develop.