Classification of the genus Acacia (in the wider sense) has been subject to considerable debate. It is generally agreed that there are valid reasons for breaking it up into several distinct genera, but there has been disagreement over the way this should be done. As of 2017, it is widely (but not completely) accepted that the section that includes the majority of the Australian species (including this one) should retain the name Acacia, whilst other sections of the genus should be transferred to the genera Acaciella, Mariosousa, Senegalia and Vachellia[].

Common Name: Black Wattle

General Information

Acacia mearnsii is a small to large, spine-free, evergreen, single-stemmed or multi-branched tree with a low, rounded, spreading crown[]. It usually grows from 6 - 15 metres tall, but can reach 25 metres. The bole is usually straight, and can be up to 50 cm in diameter but is more likely to be 10 - 35cm[, ].
This species is the world's premier source of tannins and it is widely cultivated in warm temperate to tropical areas for its wood and tannins[]. It is also widely grown to provide shade and shelter for plantation crops, as a soil conditioner and stabilizer and has excellent potential for use in restoring native woodlands.
Acacia mearnsii is a vigorous, nitrogen fixing species that has precocious and prolific seed production, with a high density and accumulation of long-lived seeds in the soil, and a variety of potential dispersal mechanisms including water, mammals and possibly birds. It causes a number of mainly environmental problems and is hard to control because of its ability to form root suckers. This species is a highly invasive species, considered to be one of the World’s 100 Worst Invaders, and has become a problem in countries as diverse as South Africa, Portugal, Hawaii, Zimbabwe and New Zealand[].

Known Hazards

Especially in times of drought, many Acacia species can concentrate high levels of the toxin Hydrogen cyanide in their foliage, making them dangerous for herbivores to eat.

Botanical References

Range

Australia - New South Wales, Victoria, South Australia, Tasmania.

Habitat

An understorey tree of tall open forests, in fringes of closed forests or in dense thickets on recolonized lands[].

Properties

Weed Potential	Yes
Medicinal Rating
Other Uses Rating
Habit	Evergreen Tree
Height	15.00 m
Growth Rate	Fast
Pollinators	Bees, Insects, Self
Self-fertile	Yes
Cultivation Status	Cultivated, Wild

Cultivation Details

Acacia mearnsii is a plant of warm temperate to subtropical areas, it can also be grown at moderate to high elevations in the tropics. It is found at elevations from 300 - 2,440 metres in areas where the mean annual temperature is in the range 9 - 20°c and the mean annual rainfall is 500 - 2,050 mm[]. It prefers a moderate climate, exhibiting great intolerance to extreme heat or cold[]. Its lower altitudinal range is decided by the fact that trees cannot stand high summer temperatures, and the upper altitudinal limit is based on the fact that the tree does not tolerate temperatures below 0°c[]. Winter frosts and cold winds during the early part of the rainy season affect growth and survival rate, but older trees can withstand mild frost[]. Localities experiencing severe hailstorms and snowfall are generally unsuitable[].
Requires a sunny position[]. Flourishes in deep, well drained, light textured and moist soils[]. It thrives in well-aerated, neutral to acid soils, loamy soils, soils derived from shale or slate and is highly intolerant of alkaline and calcareous soils[]. Soils with lateritic pan close to the surface are most unsuitable[]. Adequate soil moisture is a prerequisite for satisfactory growth[]. Trees cannot withstand drought because of their superficial root system and high rate of transpiration[].
Trees are fast-growing, but short-lived, with a life span generally in the range 10 - 30 years[]. They begin to yield fertile seed from the age of 5 years, giving good annual crops[].
The tree regenerates naturally from seed after burning in clear-felled plantations[]. Seed may lie dormant in the soil for up to 6 years without loss of viability[]. A large proportion of the seed may become dormant in the soil and may remain viable for more than 50 years[].
The trees have strong light requirements and respond to thinning in the early years. Growth rate is comparatively slow for the first 1 or 2 years. Thereafter, both height and diameter increments are rapid up to the age of 6 - 7 years, after which they fall off gradually[].
The tree has low coppicing power[].
Taproot development largely depends upon the depth of the soil, but the tree has the general tendency to develop a superficial lateral root system. Because of this, trees are liable to being uprooted by strong gales during the monsoon season[].
This species has a symbiotic relationship with certain soil bacteria, these bacteria form nodules on the roots and fix atmospheric nitrogen. Some of this nitrogen is utilized by the growing plant but some can also be used by other plants growing nearby[].

Edible Uses

None known

Medicinal

The bark is rich in tannins and so can be used in the many ways that astringents can be employed such as to treat diarrhoea and dysentery, to treat haemorrhoids, stop internal bleeding, bathe cuts and abrasions, as a mouthwash to tighten the teeth in the gums etc[].
Many Acacia trees also yield more or less of a gum from the trunk and stems. This is sometimes taken internally in the treatment of diarrhoea and haemorrhoids[].

Agroforestry Uses:

Acacia mearnsii is a natural pioneer species, invading disturbed areas, grasslands, coastal scrub etc[]. With its fast rate of growth and ability to fix atmospheric nitrogen, it can play an important role in restoring native woodland. However, because of its tendency to invade other habitats, it should only be used in this way within its native range[]..
The tree grows well at high elevations, even on slopes with shallow or poor acid soils that are unstable and will not support agricultural crops[]. It can, therefore, be very effective in preventing soil erosion. Densely packed plantations have proved effective in preventing further erosion, even on hillsides of up to 50 degrees slope[].
The tree has also been planted as a shelterbelt, a firebelt and as a shade tree in tea plantations[, ].
An efficient nitrogen-fixer and good source of green manure, it thus can restore and regenerate soils[].

Other Uses

Wattle bark is the most widely used tannin material in the world[]. It contains 30 - 45% (dry basis) high-quality tannins[]. A powdered bark extract is also used to prepare tannin formaldehyde adhesives for exterior grade plywood, particleboard and laminated timber[]. Possibilities of using the bark in the production of biodegradable polyurethane foam are being tested[]. Bark harvested for its tannins should only be taken from mature stems, and only when the sap is rising at the beginning of the growing season - which is when the tannin content is highest and the bark is most easily removed from the wood[].

The heartwood is pale brown with a pinkish tinge; it is not sharply demarcated from the sapwood. The texture is moderately fine and uniform; the grain commonly interlocked; lustre is medium; there is no distinctive odour or taste.The wood is moderately hard to hard, heavy, fairly tough and strong, not very durable. It dries rapidly, but with pronounced warp, particularly cupping; shakes tend to open and knots to split slightly. It is moderately easy to work and polishes well. It is used for house poles, mine props, tool handles, cabinet work, joinery, flooring, construction timber and matchwood[, ].
The pulp productivity is about 320 kg/cubic metre[]. Testing has shown that it yields a pulp with good strength characteristics and is suitable for wrapping paper and hardboard. It is also used for rayon[].
Originally distributed as a source of tannin, black wattle is now recognized as a valuable fuel wood[]. The wood is moderately dense with specific gravity about 0.75, splits easily and burns well with a calorific value of 3,500 - 4,600 kcal/kg. The charcoal is extensively used in Brazil and Kenya, and in Indonesia the tree is extensively used as a domestic fuel and for curing tobacco[].

Propagation

Seed - best sown as soon as it is ripe in a sunny position in a warm greenhouse[].The dried seed of most, if not all, members of this genus has a hard seedcoat and may benefit from scarification before sowing to speed up germination. This can usually be done by pouring a small amount of nearly boiling water on the seeds (being careful not to cook them!) and then soaking them for 12 - 24 hours in warm water. By this time they should have imbibed moisture and swollen - if they have not, then carefully make a nick in the seedcoat (being careful not to damage the embryo) and soak for a further 12 hours before sowing. Sow the seed in Spring in a greenhouse. As soon as the seedlings are large enough to handle, prick them out into individual pots and grow them on in a sunny position in the greenhouse for their first winter. Plant them out in late spring or early summer, after the last expected frosts, and consider giving them some protection from the cold for their first winter outdoors.
Acacia seeds that have matured fully on the bush and have been properly dried have a hard seed coat and can be stored in closed containers without deterioration for 5 - 10 years or more in dry conditions at ambient temperatures. It is best to remove the aril, which attracts weevils and can lead to moulds forming. The arils are easilyremoved by placing the seeds in water and rubbing them between the hands, then drying the seeds and winnowing them[].
Cuttings of half-ripe wood with a heel, mid summer in individual pots in a frame[]. Overwinter in a greenhouse for the first winter and plant out in their permanent positions in late spring or early summer. Fair percentage[].