Australian Blackwood (Acacia melanoxylon), also known as Tasmanian blackwood, black wattle, Sally wattle, hickory, mudgerabah, Paluma blackwood or simply blackwood, is one of the best known of Tasmania’s wattles. It is native to eastern Australia and very commonly found throughout this region, but is now cultivated all over the world for its stunning and highly sought after wood.
Blackwood is a fast growing successional species that lives for 15 to 50 years and reproduces by prolific seeding. It can vary in appearance, from dense, twisted undergrowth through to straight-trunked trees up to 35 metres in height and 1.5 metres in diameter. Smaller plants have many branches and dark green, leathery elongated leaf-like stems that make them appear shrub-like, while older trees often have thick, bare trunks beneath a dense crown. The bark on older trunks is dark greyish-black, fissured and somewhat scaly, often with a partial covering of lichen.
Blackwood flowers are pale yellow to white in colour and grow in clusters of 30 to 56 flowers. Each flower has multiple stamens, giving them a fluffy appearance. Flowering can occur throughout the year but most frequently occurs in spring. The fruit is a long, smooth, flattened pod that may be curved, twisted or coiled. The pods contain black oval seeds surrounded by a red fleshy covering known as an aril. Young pods are green, but they become darker and redder as they mature over summer to autumn.
Blackwood is a common understorey species native to eastern Australia, ranging from the Adelaide Hills, through Victoria and Tasmania and up the east coast to the Atherton Tablelands in northern Queensland. It grows best at low altitude in a cool climate with moderate to high rainfall and the best populations are found in the moist, sheltered swamps and lowlands of north-western Tasmanian eucalypt forests.
However, populations have been successfully established throughout New Zealand, South America, Africa, Asia, Europe, and the United States both as ornamental trees and in plantations. It thrives in deep, moist, fertile soil, but can be grown in almost all soil types and at a range of altitudes, with good tolerance to drought, frost, wind, sun and shade.
Though blackwood can tolerate a range of conditions, it can be more susceptible to borers when exposed to the elements or deprived of water. It also tends to grow much more slowly when areas become overstocked, as the trees are forced to compete for nutrients.
Ecological and cultural importance
In South-east Queensland blackwood plays host to many native butterfly larvae, including the tailed emperor, silky hairstreak, imperial hairstreak, stencilled hairstreak and large grass-yellow butterflies. Its dense foliage provides shelter for many birds and shade for a variety of animals, and it can be very useful in helping restore damaged, eroded or unstable areas of forest, as it can take root and grow quickly in most conditions.
Blackwood has been useful in various farming applications. It can be useful as a shelterbelt, blocking the gaps between taller eucalypts and blunting the force of fierce winds as well as bushfire. Its bark is resistant to damage from stock and it provides an excellent source of shade for them. Blackwood also hosts nitrogen-fixing Rhizobium bacteria, which improves soil fertility and has beneficial effects for surrounding trees. Finally, apiarists may grow them as a good source of pollen.
Blackwood also has a rich history of use by indigenous Australians. Aboriginals found that blackwood twigs and bark stunned fish, making them easy to catch. They also found a useful analgesic effect from infusing the bark in hot water and using it to bathe sore joints. They used its sap as an adhesive and its wood to craft weapons and tools.
Blackwood is prolific across south-eastern Australia and is not considered to be threatened. It is generally planted and harvested sustainably, and about a third of total Blackwood forest types are reserved.
Blackwood grows quickly and regularly produces large numbers of strong seeds, which float on water and are attractive to browsing animals, and are therefore dispersed widely. The seeds germinate prolifically after fire. Blackwood also sprouts vigorously from root suckers, particularly when the roots are damaged. The suckers can spread widely to form dense clumps.
The sustainability issues surrounding blackwood are actually centred upon its potentially invasive nature. Because of its rapid growth, regeneration from seed and regrowth from root suckers, it can be very difficult to control. It is included in the Global Invasive Species Database, and is now considered a pest species in many countries, particularly in South Africa where it is endangering many native plants.
Though blackwood has myriad uses, it is most prized for its beautiful, naturally lustrous timber, which is commonly used in cabinets and other furniture and veneers, as well as flooring, boats, musical instruments and decorative pieces.
Blackwood owes its popularity not only to its appearance but also the fact that it seasons well, and has excellent stability, compressive strength and resistance to impact. The timber is also easy to stain, nail and screw and bends well with steam treatment.
Blackwood consists of heartwood and sapwood, which have very different appearances. Sapwood ranges from straw-coloured to pale grey, whereas heartwood is golden to dark, sometimes reddish brown with deep brown to black rings. It is usually straight-grained but can occasionally be wavy or interlocked. This can produce a particularly striking curly ‘fiddleback’ figure when quartersawn.
The main negative aspect of blackwood is that it is susceptible to infestation with some beetles and termites, and available treatments are often ineffective when the problem arises.