Blackwood for Farm Forestry

Updated: May 2008

AG1149

Shane Lavell, Traralgon

This Agriculture Note describes the characteristics, uses and site requirements of Blackwood (Acacia melanoxylon) as a farm forestry species.

Acacia melanoxylon, commonly known as Blackwood, is one of the largest of the acacias. Blackwood varies in size from a large shrub of 2-3m in height to a medium sized tree to 30m tall. It is prized for its appearance grade timber qualities, used for many internal applications such as furniture and mouldings, and is highly sought after timber for wood-turning and other craft uses.

Natural occurrence

Distribution of Acacia melanoxylon in south-eastern Australia illustrating its preference for the higher rainfall zones of Australia (Costermans, 1985).

Blackwood occurs naturally from the Atherton Tablelands in north Queensland, along the east coast to the southeast of South Australia, and in Tasmania. The best natural form is found in moist gullies and in cool temperate rainforest on deep soils. The annual mean rainfall throughout its range varies from 700mm up to 1500mm. Blackwood prefers mild to warm summers and cool winters with no more than 40 frosts per year. It occurs from altitudes at near sea level up to 1500m.

Blackwood occurs in swampy areas in Tasmania (Blackwood Swamps), to lower valley slopes and even exposed hill tops. The best specimens are found in good quality forest podsols, though it does occur as a shrub and smaller tree on a range of other soil types. Related to Lightwood (Acacia implexa), being a similar species.

Provenances

There is considerable variation in form and size between Blackwood specimens from different areas. Trials were established in Tasmania in 1988 using 22 provenances from Tasmania and mainland Australia. The main variation was found to be due to frost effects, this result was also found when five provenances were planted near Canberra. Provenance trials were also established near Hamilton in 1993 and 1994 using 20 different provenances and there is great variation evident. The best performer in the Hamilton trials was from Cressy in Tasmania and the NSW provenances were consistently the worst.

A third provenance trial established in north-east Victoria has shown significant difference in growth and form between Victorian and Tasmanian Blackwood and their NSW and Queensland cousins (Bandara and Stackpole, 2003). The southern Australian provenances performing better than those from northern Australia. Four provenances (Welshpool, Silver Creek, Redpa and Lancefield) showed consistently good volume growth and stem form and the authors of the report suggested that these provenances could form the base of a tree improvement program.

Experience in New Zealand has shown that the Otways and Tasmanian provenances exhibit excellent form and good growth under suitable site conditions and correct silvicultural practices. Variation in wood colour between Tasmanian and Victorian Blackwood is believed to be because of provenance and not age or other factors.

Site requirements

Blackwood has good potential for farm forestry in higher rainfall areas on sheltered sites. It will tolerate flooding for part of the year, although it does not do well if the water is still. It will grow in areas where the annual average rainfall exceeds 600mm per year, however best results are achieved on sites where the mean annual rainfall is greater than 1000mm per year. Blackwood naturally occurs on a wide range of soils but the best growth for farm forestry will occur on welldrained deep fertile sandy loams over clayey subsoils.

Growth rates

Blackwood initially grows fairly quickly, but from about 5-8 years growth slows and a final crop of sawlogs would not be expected before 35 years. In lower rainfall areas the time to final harvest may be at least 50 years.

Management

Blackwood occurs naturally as an understorey species and predominantly on sheltered sites. Any management regime to grow Blackwood for timber production should endeavour to mimic these conditions.

Open grown trees tend to be heavily branched and shorter, and are not usually suitable for timber.

If grown in a single species woodlot Blackwood should be planted at 1600 (2.5m X 2.5m) to 2500 (2m X 2m) stems per hectare. Close spacing encourages the trees to grow in competition for light and discourages branching.

Blackwood is often planted with a faster growing species as a nurse crop, in alternating rows or between plantings and as an external buffer on exposed sites or aspects. This encourages the Blackwood to grow straight up to compete for light and reduces branching. The trick in this planting regime is to manage the nurse crop so that it does not overtop the Blackwood and suppress growth altogether, or that the Blackwood does not out compete the nurse crop in the first few years. The nurse crop is eventually harvested to permit final canopy closure of the Blackwood stand.

Blackwood usually needs form pruning in the first few years to ensure one straight stem. This stem should then be lift pruned to produce a stem clear of branching for the first 2.4m - 6m. Care should be taken when pruning to leave adequate leaf area to maintain growth. It is recommended that no more than half the canopy is removed with each pruning. Branches should ideally be removed before they reach 2.5cm in diameter, and competing leaders and acutely angled branches need to be removed as soon as possible to ensure a straight knot free log.

Thinning should be undertaken to minimise competition for light, nutrients and moisture; the final crop will depend on the site but will generally be between 100 and 250 stems per hectare. Thinning should be undertaken in several steps as Blackwood is fairly shallow rooted and can be subject to windthrow on exposed sites.

For more detailed information on establishment techniques, pruning and thinning of Blackwood see the references Bird (2000), Nicholas and Brown (2002) or Race (1993).

Pests and disease risks

Blackwood is fairly free of pests and diseases in Victoria. It can be susceptible to wood borer on sites where it does not naturally occur. Blackwood is not affected by the rusts and galls that affect many other Acacias.

Blackwood is one species that is highly tolerant of Phytophthora cinnamomi and can therefore be used in wetter areas where this fungus is thought to be a problem.

Blackwood foliage is particularly palatable to many browsing animals (rabbits, wallabies, wombats and cattle), therefore measures need to be taken to protect the young seedlings from these animals. Rabbit proof fencing and tree guards can be effective in protecting the seedlings from browsing.

Possums will also eat the juvenile foliage; a method for combating this is to plant slightly older seedlings that have predominantly adult foliage.

Uses

Blackwood Office furniture (photo: Lifestyle Furniture)

Blackwood is known world wide as one of the best furniture and cabinet timbers. It is used as solid wood for furniture and suitable logs are sliced for decorative veneer. Blackwood has also been used for panelling, gunstocks, in boat building and as backs for guitars and violins. Smaller pieces are used extensively for craft wood, carving and turning

Blackwood is also a high quality firewood but far too valuable to grow for this purpose alone.

In drier areas Blackwood can be an effective windbreak tree because of its branching habitat when planted in exposed sites, although the growth rate will be adversely affected in these conditions.

Wood quality

Blackwood is golden brown to darker brown and may have reddish streaks or tint; the sapwood is very light in colour. The grain is predominantly straight but may occasionally be wavy. It is an excellent timber to work and dresses to a smooth finish that takes a high polish. It machines very well and is easy to fix and glue.

The green density is 870 kg/m3 and the air-dried density (approximately 12% moisture content) is 640 kg/m3.

Blackwood has moderate durability both above ground and in the ground, although outdoor use is not usually recommended for Blackwood as many other species can do the job better. Timber is graded SD4 for strength of seasoned timber. The sapwood of Acacia melanoxylon is susceptible to Lyctus borers.

Drying characteristics

Blackwood can be kiln-dried but air-drying is usually recommended (Bird, 2000). The timber is fairly stable, it should be stacked and measured during drying; "spring" may occur in some back-sawn boards. It is easy to dry, with little checking and negligible collapse, shrinkage is about 1.5% radially and 4% tangentially (Bootle,1985).

Timber User Advice

The sanding dust from this species can cause irritation to the skin and bronchial tubes of some people, similar symptoms can be experienced when handling seed material. (Bootle, 1985).

Summary

Blackwood is a valuable furniture and cabinet timber that has promise as a farm forestry species. It performs best on protected sites in fertile sandy loams with an annual average rainfall above 1000mmm. Early form pruning and lift pruning to obtain a straight log clear of knots will afford the best returns. The timber is easy to finish and work, and presents very few problems in drying.

References

Russel O'Shea in his Blackwood plantation on his property at Brucknell in 1998 (photo: Liz Hamilton)

• Bandara, G. and Stackpole, D. (2003) unpublished. Variation in Growth of Blackwood (Acacia melanoxylon) Provenances in North Eastern Victoria, Department of Sustainability and Environment (Victoria), Forest Science Centre, Heidelberg.
• Bird, P.R. (2000). Farm Forestry in Southern Australia, Department of Natural Resources and Environment (Victoria), Pastoral and Veterinary Institute, Hamilton.
• Boland, D.J., Brooker. M.I.H., Chippendale, G.M., Hall, N., Hyland, B.P.M., Johnston, R.D., Kleinig, D.A., & Turner, J.D. (1997) Forest Trees of Australia, CSIRO Australia, Collingwood.
• Bootle, K.R (1985) Wood in Australia, McGraw-Hill Book company , Sydney
• Costermans, L. (1985). Native Trees and Shrubs of South -Eastern Australia. Rigby Publishers, Melbourne.
• Cremer, K.W., Ed. (1990) Trees for Rural Australia, Inkata Press Pty Ltd, Melbourne.
• Nicholas, I and Brown, I (2002). Blackwood: A handbook for growers and endusers. Forest Research Bulletin 225, Forest Research, Rotorua
• Race, D., Ed. (1993). Agroforestry: Trees for Productive Farming. Agmedia, East Melbourne.
• Sewell, A. J. (1997). Australian timbers, Volume 1: Commercial Timber Species of Eastern Subtropical Australia, Department of Natural Resources (Queensland). Foremost Printing, Palmwoods.
• Tame, T (1992). Acacias of southeast Australia, Kangaroo Press, Kenthurst.