Acacia mearnsii Seeds | Black Wattle
LATIN NAME: Acacia mearnsii
COMMON NAME: Black Wattle
Plant Profile
Acacia mearnsii is a fast, short-lived wattle of the wet forests of Victoria and New South Wales, and it is grown around the world for its bark. The leaves are bipinnate and fern-like, dark green, and the flowers are pale yellow balls in long sprays that open in spring. The tree reaches fifteen metres with a straight trunk in a good site, and it lives for perhaps thirty years.
The species has been planted in South Africa, Brazil, Kenya and China for tannin, for pulp and for fuel. It is also one of the most invasive trees in the world: it seeds heavily, the seed lasts for decades in the soil, and it colonises grassland, fynbos and riverbanks, where it uses water and changes fire regimes.
Science & Biotechnology
The bark contains up to forty per cent tannin, mostly proanthocyanidins, and it is one of the main commercial sources of vegetable tannin for leather. The bark is stripped from the trunk and from the branches, dried and chipped, and the extract is sold as a powder. The tannin gives leather a firm, pale finish, which is why it is used for sole leather and for belting.
The seed has a hard coat and a long life in the soil, and it germinates in pulses after fire or after soil disturbance. That is the mechanism behind its invasiveness: a stand builds a seed bank of thousands of seeds per square metre, and every fire or clearing event releases a new cohort. Control therefore has to deal with the seed bank, not just with the trees.
Uses & Benefits
The bark is harvested for tannin, and the wood is used for pulp, for charcoal and for fuel. The tree is also planted for shelter and for nitrogen fixation on poor soils, and the flowers are a source of nectar for bees.
In Australia the wood was used for boomerangs and for spears, and the bark was used as a medicine and as a fish poison. In South Africa the tree is a serious problem in catchments and in fynbos, and it is the subject of long-running control programmes, including biological control with seed-feeding weevils.
Growing from Seed
Seed needs hot water treatment: pour boiling water over it, leave it overnight, and sow the swollen seed at 20 to 25 degrees. Germination is in one to three weeks, and the seedling is very fast, reaching a metre in the first year.
The plant grows on acid, poor soils and needs full sun. It tolerates frost and drought once established, and it fixes nitrogen. The warning is its seed bank: do not plant it where it can spread into natural vegetation, and if it is already present, remove the trees before they seed and follow up for several years.
Our Take
A useful tannin and pulp tree, and one of the world’s worst woody invaders. Plant it only where the spread can be controlled.
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