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The rare and remarkable species of Australian Discaria

Ian Menkins

Original article first published by Attila Kapitany at www.australiansucculents.com and reproduced here with permission.


With their roots in ancient Gondwanaland, two bushland wonders continue to surprise and captivate our senses...

Discaria is a genus in the family Rhamnaceae, comprising eight (possibly more) species of mostly shrubby, often spiny perennials. The species are found only in the temperate zones of the Southern Hemisphere and share a common Gondwanan ancestry. The majority occur in South America, with one species endemic in New Zealand, and two species endemic in Australia.

Many of the world's Discaria species qualify as xerophytes in the true sense of the term, and the Australian species are no exception. They frequently occur on porous or well-drained sites and on soils of low fertility. Some species can fix nitrogen from the atmosphere with the help of symbiotic bacteria (Frankia) that form nodules in their roots. A study of the roots of Australian Discaria in the wild state was conducted by Hall and Parsons in 1987. They found profuse nodulation on the roots of the four D. nitida plants that they examined. However, the roots of 25 mature specimens of D. pubescens were also examined and none contained nodulation. Discaria may also grow in association with beneficial soil fungi (Mycorrhizae), but at this time no studies have been conducted to this author's knowledge.

In Australia, the genus is represented in the more temperate zones of all eastern mainland states and in Tasmania, usually within 300 km of the coast, and most often on the western sections of the Great Dividing Range. They are found in valleys, on rocky ridges or steeply eroded hillsides, and along gully lines, from 220 to 1,400 m altitude, and in the 520 to 800 mm rainfall zone. The substrate is often gravelly, sandy, calcareous or limy. If conditions are suitable, they can sometimes form dense colonies and may be locally common.

Figure 1 provides a key to the species in Australia.

Figure 1: Key to the genus Discaria in Australia
1 Young stems cane-like; spines usually > 2 cm long, prominent, occurring at every node; leaves sparse, shed early, often absent; flower petals presentDiscaria pubescens
1* Stems never cane-like; Spines usually only 1-2 cm long, often scattered; plants leafy throughout summer, deciduous in winter; flower petals absent Discaria nitida
Adapted from: New South Wales Flora Online (19 October 2008), Genus Discaria in PlantNET - The Plant Information Network System of Botanic Gardens Trust, Sydney, Australia (1999 - 2008).

The two Australian species are rare and protected by legislation. However their spiny appearance has undoubtedly caused them to be mistaken for weedy exotics of poor reputation, such as African boxthorn (Lycium ferocissimum). It is possible that many colonies of both species have been inadvertently destroyed in the past due to ignorance or misidentification. It should be noted that the spines of both native species of Discaria are quite innocuous. The spines do not have barbs nor are they needle-like as you find in plants like cacti. Still, they are sharp enough to poke the eyes or scratch sensitive skin, so should be handled carefully. Discaria do not contain caustic sap like African Euphorbia (which they superficially resemble) and are not considered toxic in any way.

Discaria nitida Tortosa (Shining Anchor Plant, Leafy Anchor Plant) is a perennial, sub-alpine shrub, 2 to 5 metres tall. The spines measure 1-2 cm and are widely scattered along the reddish-brown, leafy stems. The leaves are entire, 10-25 mm long. D. nitida is truly deciduous, losing all of its leaves in winter. The flowers appearing in spring are tiny (less than 3 mm diameter), white or cream in colour, and lack petals. They form in clusters of up to 10 per node on short stalks.

Discaria  
Discaria pubescens
Photo: Ian Menkins
  

The plant occurs in rocky situations, usually along streams in sand or gravel, and often on limestone in the Snowy Mountains above 1000 metres altitude. The species occurs from the Cooleman Caves area through to Cobungra in Victoria. It is a threatened species listed as Vulnerable (classification ROTAP: 3RC-).

Discaria pubescens (Brongn.) Druce (Hairy anchor plant, Australian Anchor Plant, Hooky Bush) is a rigid, much branched, perennial shrub growing 40 cm to 2.5 metres tall. The plant is dominated by sturdy, spreading spines up to 5 cm long. These are positioned on opposite sides of the green, mostly leafless stems at regular intervals. The ovate to oblong leaves are 3-15 mm long and often finely toothed. They are shed early so that the plant usually appears leafless, however the tiny leaves may persist on new growth even through the winter. The flowers have white-cream petals, are 3 mm long, and appear on stalks in clusters of 2-10 in the nodes. Plants may flower prolifically in spring and are reputed to be highly fragrant and very attractive to native bees. Despite its name, mature plants are not noticeably hairy. In fact the spiny canes have a smooth, almost waxy surface texture, and often have the appearance and flexibility of green plastic.

The species occurs in woodland and open forest, often in rocky situations, at around 220 to 1200 metres altitude, from south-eastern Queensland to Tasmania. It is widespread and can be locally common, but considered endangered (classification ROTAP: 3RCa). Plants may produce dense colonies under ideal conditions. The species is not thought to reproduce vegetatively by suckers, as excavations conducted by Hall and Parsons in 1987 revealed that most plants thought to be suckers were in fact seedlings. Plants often produce horizontal underground stems from their thickened rootstock that can emerge up to 40 cm away. But these stems always remain attached to and dependant on the parent plant (Hall and Parsons, 1987).

The more interesting of the two endemic species is arguably Discaria pubescens; however both species qualify as true xerophytes and occur in similar habitats. Discaria pubescens is probably the hardiest and most widely distributed of the two species. It occurs as far north as the south-eastern Darling Downs in southern Queensland and as far south as Tasmania.

Near the township of Allora on the Darling Downs in Queensland, this species is located within a reserve where it grows on an eroded, basalt hillside that is dissected by numerous deep gully lines. The colony is restricted to the soils containing numerous calcareous nodules, but is absent from the nearby basalt ridges. However, eroded basalt silts, clays and gravel still make up a large proportion of the substrate in which the plants grow. At first glance the habitat appears quite arid, but this is deceiving due to the eroded nature of the landscape. The gully lines and slopes can retain moisture for lengthy periods following good rainfall events and support other endangered plants of temperate Australia, such as Stemmacantha australis (Austral cornflower) and the cryptic, semi-succulent root parasite Thesium australe (Austral toadflax). The latter species is an ephemeral parasitic herb, gaining much of its nutrient from the roots of kangaroo grass (Themeda triandra) and possibly other grasses and herbs. Its yellow appearance is its characteristic feature - a symptom of its persistent chlorosis - and often betrays its location deep within the grass tussocks.

Discaria pubescens habitat
Discaria pubescens - habitat at Allora, Queensland
Photo: Ian Menkins

Some preliminary experiments with propagating Discaria suggest that seed is difficult but not impossible to germinate. The seed is often produced in abundance in late spring or summer. The plants produce three-lobed fruits that measure about 3-5 mm diameter. The pods look similar to Euphorbia fruit pods (for the sake of comparison, as Discaria is not related to that genus). Each pod contains one to three ovoid, leathery seeds that are brown-black in colour. These are expelled explosively from the capsules upon drying.

Hall and Parsons (1987) and Wright and Briggs (2000) experimented with different seed raising methods, such as scarification ("nicking' the seed coat to expose the white embryo); stratification (keeping seed at a set temperature for a set period); imbibition (soaking in water at a specified temperature); acid treatment (soaking in HCl solution); and smoke treatment (soaking in smoke impregnated water).

Their combined results showed that germination was highest when fresh seed was soaked in water at room temperature for 1 hour, then cold-moist treated in a refrigerator at 4-8C for a lengthy period prior to sowing. The cold treatment times that produced the highest germination rates were as follows:

  • 30-90 days (60-90 days optimal) for D. pubescens
  • 30-146 days (60-90(-146?) days optimal) for D. nitida.

Smoke treatment and acid treatment had negative to neutral effects on the germination of D. nitida seed, while scarification had a neutral to minimal effect on the germination of both species. Scarification did not appear to be vital for promoting strong germination. In most cases involving cold treatment, scarification actually resulted in lower germination rates than for non-scarified seed, suggesting that scarification of cold treated seed is neither beneficial nor necessary. We can assume that any nicking of the seed coat by predatory mammals, birds, or insects produces little or no increase in germination rates. The low effectiveness of scarification and smoke treatments on both species suggests that fire is not intrinsically linked with the ecology of Discaria (Wright and Briggs, 2000). In 2005 a native plant enthusiast tested the effectiveness of heat by soaking some D. pubescens seeds in hot water that had just gone off the boil. This method produced no germination after 12 months, by which time most of the seed had rotted away.

  Discaria fruit
 

Discaria pubescens - fruit
Photo: Ian Menkins

Discaria species usually occur in hilly or sub-alpine areas that are prone to low nocturnal temperatures in winter. They are therefore likely to be frost germinants - a term used to describe those plants that rely on the seasonal freeze and thaw cycles to germinate their seed. In this case, treatment of the seed at sub-zero temperatures for several weeks, followed by soaking for 3-12 hours in room temperature water, could be worthwhile.

In the wild state, Discaria may be threatened by development, mining, agriculture, and other human activities. Mature plants could probably survive translocation to areas of similar soil and geology. However moisture levels would need to be maintained at least until plants have re-established. The plants would need to be carefully monitored for many months, even years, after translocation. Populations of plants tend to favour riparian areas, gullies or other drainage lines, suggesting that they prefer a slightly sheltered aspect where fire frequencies and intensities are generally much lower than the surrounding landscape.

Propagation can be achieved from tip cuttings dipped in Clonex PurpleTM hormone rooting gel. This method may prove to be the most reliable and consistent way to reproduce many Discaria species in horticulture. Unofficial experiments on cultivated specimens have shown that Discaria pubescens propagates readily from tip cuttings. A new growth point should first be selected, preferably after flowering and fruiting has ceased. It is advisable to reduce some of the surface area of the 3-6 cm cutting by removing any leaves and by chopping off the growing tip. Also, some of the spines can be trimmed back by about half for easier handling. The end to be dipped in rooting gel should be cut cleanly and sharply on a slight angle about 1-2 mm below a node. Heel cuttings can also be successful. The cuttings can be inserted gently into the propagating mix using a toothpick, and the tray positioned in a moist, warm and well-lit location. The cutting tray should be enclosed in glass or plastic to maintain humidity, and may be misted with a hand held mister from time to time. Once new growth appears this is usually evidence that the roots are starting to form, and any covers can be slowly removed over several weeks. When roots can be clearly seen coming through the bottom of the tray, the plants can be pricked out into individual pots and gradually hardened off. The cuttings often grow rapidly (about 20 cm growth was noted on one cutting, only 3 months after pricking out).

To date, results in garden culture have been poor, even when considerable quantities of lime or dolomite were added. The plants appear to require a great deal of moisture with excellent drainage, which is often difficult to maintain in a garden situation, especially on flat, poorly drained sites, or on steep, very dry slopes. Heavy clays or deep sands may also present a problem for cultivating this species. Gardens that are heavily mulched and possess high fertility levels could be most unsuitable for Discaria. In the wild, both species of Discaria occur on geology of low fertility and low to moderate alkalinity (e.g. basalt, slate, metamorphics, and limestone). In Victoria, plants were all reported as growing in topsoil composed of sandy clay loam (Hall and Parsons, 1987). However in southern Queensland D. pubescens occurs on basalt or trachyte geology with gravelly, loamy-clay topsoil. Here the topsoil and substrate often contain numerous calcareous nodules no larger than 10 mm diameter.

The cutting grown plants seem to survive best when retained in potted culture, if provided with ample moisture in the growing season. Ideal drainage is also essential. Plants seem to prefer semi-shade rather than full sun, but may tolerate more extreme conditions once mature. Plants often fail to recover if pots are allowed to dry out for even a single day in hot weather.

D. pubescens has a well developed taproot. The rootstock forms a gnarled, woody swelling just below the soil surface. This compound structure supports the main trunk(s), as well as any young stems and horizontal shoots. Excavation of the root system of D. pubescens reveals a bunched and somewhat tangled growth habit, composed of numerous canes and shoots attached to the thick rootstock (Hall and Parsons, 1987). The thickened axis of the rootstock can be loosely defined as a caudex and is similar to the lignotuber found in other native plants like banksias and eucalypts. However the term "lignotuber" is most often associated with hardwood species that are both fire promoting and fire resistant. In those species, the lignotuber stores numerous dormant buds and so functions to promote rapid regeneration after bushfires. Discaria on the other hand is generally found in places where the frequency and severity of fires has been reduced e.g. along otherwise poorly vegetated gully lines (Wright and Briggs, 2000). The plants do not have the appearance of being highly flammable. Furthermore, the rootstock produces and replaces new shoots regularly (Hall and Parsons, 1987). Cutting-grown plants produce their own compound rootstock and secondary shoots after several months.

Hall and Parsons (1987) sectioned one woody stem of a mature specimen of D. pubescens in the wild and determined it to be around 25 years of age, while other plants were estimated to be more than 30 years of age. The green canes they sectioned had between 1-6 annual growth rings. This compares favourably with my own observation that canes often die off after several years, especially during dry periods, and new ones emerge from the woody trunks, branches, or rootstock following good rain events. Occasionally old trunks and branches will also die off completely. This process of shedding vegetative material during periods of stress is found in many Australian plants and in xerophytes in general. It is possible that some very old specimens of Discaria pubescens in Australia could have rootstocks and trunks well over 100 years old, but most of the young stems coming off those branches may be no more than 6 years old.

D. nitida on the other hand, does not have cane-like stems, or a taproot, or a thickened, centralized rootstock. Its stems are typically erect and tree or shrub-like. The stems lose their green colour and become woody after only one season's growth. The stems are generally more persistent than those of D. pubescens. The few stems of D. nitida examined by Hall and Parsons (1987) revealed ages of 6-18 years. However they noted much thicker stems were present at the site.

Both species of Discaria should be suitable subjects for grafting. We need only consider that the genus is placed in the family Rhamnaceae, which includes many species that could prove to be hardy and long-lived rootstocks. The extremely tough, drought resistant Vine tree or Supplejack (Ventilago viminalis) and the widespread and equally hardy Red Ash (Alphitonia excelsa) are two likely candidates. Rhamnaceae also includes such hardy exotic contenders as European Buckthorn (Rhamnus alaternus) and Indian Jujube (Ziziphus mauritiana). The latter is a declared noxious weed in Australia, where it is known commonly as Chinee Apple. It would be ironic if this species proved useful as a rootstock for one of our endangered species. Hardy exotic species of Discaria, such as New Zealand's D. toumatou, could also be trialled as rootstock, depending on availability of plant material and AQIS restrictions. However, rigorous testing would need to be carried out to ensure that any exotic species do not sucker. D. pubescens may prove problematic as a graft unless part of the bud-bearing rootstock is mounted on to the host plant. Green canes may initially take as a graft, but die off suddenly or fail to reshoot. Trials need to be conducted to test the ability of canes to form a permanent graft.

Grafting may prove useful for the establishment of these rare species in gardens, even on compacted clay soils. Cultural methods like topiary (try twisting two or more branches together as they grow!), coppicing, and bonsai culture should all be investigated. There is little doubt that Discaria would look at home in any small or large garden where the use of xerophytes is a prevailing theme, e.g. in native gardens, dry plant gardens, succulent and cacti gardens, and Japanese gardens. Moreover, such a rare and curious plant would provide an excellent conversation piece. Strategic placement of such a spiny customer under windows may also prove useful in the prevention of crime!

I am certain that these remarkable species could survive in garden culture with a bit of care and effort, coupled with a good understanding of their requirements. If anyone has had success with cultivating any species of Discaria, I would be very interested to hear from you (imenkins@NO-SPAMoptusnet.com.au - remove "NO-SPAM" from the address).

References and Further Reading:

Briggs, J.D. and Leigh, J.H. (1996). Rare or Threatened Australian Plants. 1995 Revised Edition. pp. 466. CSIRO Publishing, Melbourne.

Hall K.F.M. and Parsons R.F. (1987). "Ecology of Discaria (Rhamnaceae) in Victoria", in Proceedings of the Royal Society of Victoria. September 1987, vol. 99, no. 3, pp. 99-108.

New South Wales Flora Online (1999-2008). "Genus Discaria", in PlantNET - The Plant Information Network System of Botanic Gardens Trust. 19 October 2008. Sydney, Australia.

Wright, G.T. and Briggs, J.D. (2000) Population survey and notes on the ecology of the Leafy Anchor plant (Discaria nitida) in New South Wales. pp. 49. NSW NPWS unpublished report, Hurstville.



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