Acacia - Blackwood, Acacia melanoxylon
Blackwood (“Tasmanian blackwood”) is a relatively fast-growing Acacia species usually planted for high quality hardwood timber. However, because blackwood tends to have poor form it requires repeated form pruning for timber production.
Grown for erosion control in pasture, blackwood becomes stock resistant after 4-5 years and will grow into an attractive, spreading evergreen tree. Because it can grow on poor soils it is well suited for erosion control in gullies and steep slopes. Where used for gully stabilisation, blackwood develops an extensive root system that suckers into new trees. It also fixes atmospheric nitrogen into the soil, improving fertility.
Because it can spread by seed and root sprouts, blackwood is not suitable for riparian planting.
On favourable sites blackwood will reach up to 40 metres high and 1 m in diameter, living for up to 200 years. Although it tolerates skeletal soils and exposure, when grown for timber production it requires shelter and deep moist soils.
Commercial return: Medium
RainfallHigh rainfall; Moderately low rainfall;
AltitudeModerate altitude; Low altitude;
WindStrong wind; Moderate wind;
Soil drainageFree draining; Moderately free draining; Moderately poor draining;
Soil depthDeep; Moderate depth;
RainfallModerately high rainfall;
For timber production blackwood requires a sheltered site with good soil depth and even rainfall.
Prepare grass sites by spot spraying a one metre circle for each planting site. Use glyphosate herbicide with spray dye a few days before planting as this will not leave chemical residue in the soil. Prepare spots using straight lines and accurate spacings between spots so that trees can be easily found later. Cut down woody weeds and flatten these on the ground so they decay and don't support weed growth.
Container-grown is available from some commercial nurseries; Price per tree should range from $0.80 - $2.00 for 1 year-old contract-grown forestry stock.
Plant as soon as possible after you receive the plants. Make sure the root plugs are moist and plant trees with a slow release fertilizer tablet or put a trowel of high nitrogen fertiliser such as DAP or urea in a spade slit above the seedling.
Dig a hole twice the size of the plant container, leaving some soft soil at the bottom. Tease out (straighten and trim) any pot-bound roots before firming the soil around them, ensuring there are no air cavities.
Plant from autumn to spring. Blackwood grows vigorously in the first year but will require some releasing from weed competition. Young trees are shade tolerant and prefer side shelter. Best form develops from planting in lightwells, for example by clearing lanes through regenerating scrub. The width of the canopy opening should be half the height of the scrub, and seedlings should be planted directly in this gap. Make regular visits to trim back the scrub where it is encroaching on the blackwood.
If planting in lightwell lanes use a spacing of 7 metres x 2 metres to get 700 - 800 stems per hectare. If planting in groups in lightwells, plant groups of four trees in 1.5 - 2 metre squares, with each square at 7 - 8 metre spacings. Examples are shown in the Blackwood Handbook: Best Practice with Farm Forestry Timber Species
- Prevent weeds from competing with the Blackwood seedlings for at least two years after planting.
- Protect seedlings from browsing by stock and wild animals.
Blackwood is shade tolerant but establishes much better if exposed to overhead light and side shade. Control competing weeds.
Blackwood foliage is palatable to browsing animals but the bark is not. Once the crown is out of reach of stock, cattle or sheep can be reintroduced.
Young blackwood seedlings are palatable to hares and rabbits. They will chew foliage and slice off trees near ground level at a 45 degree angle, killing them.
Blackwood is not palatable to possums.
Pests and diseases
If blackwood is planted in waterlogged ground it will die from root-rot. In New Zealand blackwood foliage is attacked by a range of insects, but not sufficiently to significantly affect growth. Juvenile stem damage can be caused by cicadas. Malformation caused by insect damage may require remedial form pruning.
Blackwood grows well in sheltered sites with good soil moisture. Under these conditions, if grown for timber it is important to ensure the trees retain good form and are given plenty of space for a large crown to develop. Management options are shown in the video Growing Blackwoods.
Grown for erosion control in pasture, blackwood becomes stock resistant after 4-5 years and will grow into an attractive, spreading evergreen tree. Where used for stabilising gullies, blackwood provides an extensive root system that suckers, forming new trees. It also fixes nitrogen into the soil, improving fertility.
Left alone blackwood will grow a short stem and wide crown. If grown for timber form pruning is essential.
When blackwood loses its shoot tip it responds by branching into several new ones. Its stem then becomes segments of straight growth, interrupted by nodes which contain double or multiple leaders. To grow a good 6 metre butt log, blackwood should be form pruned annually. The aim is firstly to prevent malformation in the stem and crown, and then to maximise clearwood. Form pruning, to remove competing leaders while they are still small (< 3cm diameter), can start as early as year 1 or 2.
At year 3 start gauge pruning, removing all branches fatter than 30 mm. The aim of clearwood pruning is to minimise defect core to produce wood free of knots. Larger, upward pointing branches should be removed as soon as possible as they will reduce height growth and increase defect core diameter. Pruning "lifts" must be regular and at a frequency that minimises branch size and diameter over stubs (DOS).
Keep pruning cuts close to the branch collar and as vertical as possible. To avoid bark tearing down the stem, undercut the bark with a generous cut before pruning. Clearwood pruning normally starts at about year 4 and continues during the formation of the 6 metre butt log. All pruning should be completed by age 8 at the latest with a pruned butt of 4-6m.
At each intervention leave a green crown of around 3 metres for best growth rates as described in the leaflet Australian blackwood (Acacia melanoxylon).
Suitable pruning tools include loppers and a pruning hand-saw, a battery-operated reciprocating saw, or battery-operated loppers. Form pruning can be undertaken by using a pole pruner.
When grown for timber, blackwood should be thinned to about 200 stems per hectare to allow large crowns to form with large buttlogs. Thin as soon as the crowns begin to compete, so the remaining crowns can expand without interference. Planted at 700 - 800 stems per hectare blackwood can be thinned to 400 stems per hectare by age 7 - 8 years, and thinned again by 10 years to a final crop of 200 stems per hectare (i.e. about 7 m apart).
When to harvest
Surveys suggest that a stand of 200 stems/ha would have a mean diameter of around 57 cm at age 34, and a rotation age of 35 is suggested. If the logs are to be sold for veneer where large diameter is important, some trees may be held for up to 40 years.
Blackwood has been accepted internationally as one of the world’s premium decorative timbers, comparable to walnut, mahogany and teak. The timber is easy to dry with little degrade. It is medium-weight and fairly easy to work, has an even texture, with a straight or wavy grain, is strong in compression, stiff and resistant to impact. It turns and bends well and usually dresses to a smooth finish. It is used for high-quality furniture where darker colours are preferred, and for panelling and timber frames, cabinet-making veneers, turnery and knobs. It is also suitable for panelling, carving, flooring and boat-building.
The heartwood is moderately durable but unsuitable for ground contact and not reliable in situations where it is fully exposed to the weather. The sapwood is perishable. End uses are available in the blackwood showcase.
Markets and demand
A small Marketplace for blackwood timber has developed in New Zealand. The timber is extremely variable in colour, ranging from cream to red and black, and colour matching of boards can be a problem. However, the species sells for around three times the price of furniture-grade radiata pine.
A series of blackwood trials on five North Island sites showed the best growth occurred in a sheltered valley bottom, and the poorest on an exposed ridge with clay soil. At age 14, mean top height across the five trials ranged from 7.2 m on the poorest and most exposed site to 19.9 m on the best. Grown on to a diameter at breast height of approximately 60 cm and a height of approximately 38 metres, blackwood will produce a range of recoverable sawlogs. Total stem yield will be around 500 m3/ha but recoverable yield will be less: 300 m3/ha of sawlogs. The top logs of wide spaced trees with large crowns may have little value for timber; however blackwood is a premium firewood. Growth models are given in the following handbook: No. 4: Blackwood Handbook: Best Practice with Farm Forestry Timber Species.
Practical hints for measuring trees offers some advice on monitoring growth.
Carbon sequestration rate over time, and relevant Look-up Tables
If the land is registered under the Emissions Trading Scheme and the trees are planted so as to comply, i.e. at least one hectare, with tree crown cover of more than 30 percent in each hectare and an average width of tree crown cover of at least 30 metres, then they should earn carbon credits. The relevant look-up tables for determining how quickly they store carbon and earn credits, and guides to how to use the tables, may be downloaded here.
As blackwood is an exotic hardwood tables A2.2 and A2.4 apply, suggesting that over its first 30 years it stores carbon at a similar rate to radiata pine.
Timber return on investment
Studies of the economics of blackwood are few and variable. In general blackwood incurs higher silvicultural costs than radiata pine and produces less timber, but of higher value. In a hill country study it ranked behind radiata but was more profitable than sheep and beef farming. From such studies an estimate of the IRR, ignoring land costs but including direct costs, overheads and management fees (at 15% of costs) is from 5 to 8%.
The Acacia Melanoxylon Interest Group Organisation may put you in touch with growers in your area.
The Blackwood Handbook can be downloaded as an authoritive reference.
Scion has published a bulletin on growing blackwood, called Properties and utilisation of exotic specialty timbers grown in New Zealand, part II: Australian blackwood Acacia melanoxylon
Specialty Timbers New Zealand gives information on milling, drying and usage under Blackwood ».