Black Wattle (Acacia mearnsii): Listing, Identification and Why It Is a Water Problem
Black wattle — Acacia mearnsii — is listed under NEMBA as Category 1b in riparian zones and Category 2 where it is grown under permit for tannin or timber. It is the one major Cape invader with a real industry behind it, which is why its listing is split. Unlike Port Jackson and rooikrans it carries true feathery bipinnate leaves, not phyllodes, and it reaches 10–15 m on a single dark trunk.
NEMBA listings are area-specific. The same species can sit in Category 1b in one part of the country and Category 2 in another, because Category 2 exists for plants grown under permit for a commercial purpose such as timber or tannin. Always read the listing for the area the tree is standing in.
How to tell it apart
| Feature | Black wattle (Acacia mearnsii) |
|---|---|
| Leaves | True bipinnate leaves — fine, feathery, dark green. This alone separates it from Port Jackson and rooikrans. |
| Flowers | Pale cream to light yellow globular heads, spring |
| Bark | Dark, hard and fissured on mature trunks; the tannin source |
| Form | Single-stemmed tree, commonly 10–15 m — much taller than the Cape’s phyllode acacias |
| Where | River banks, wet lowlands, forestry margins, disturbed slopes |
| NEMBA | 1b in riparian zones; 2 under permit for tannin or timber |
Why it is treated as a water problem
What does black wattle do to a river?
Black wattle colonises the riparian strip — the band of ground either side of a watercourse — and replaces low, seasonal indigenous vegetation with a tall, dense, evergreen stand. It transpires more water year-round, and it does it exactly where the water is. That is why riparian populations are listed 1b even where plantation stands are permitted.
Why does it spread after a fire or a flood?
Its seed is hard-coated and long-lived, and heat and soil disturbance break that coat. A fire or a flood scours the bank and then triggers mass germination from the seed bank. Cleared riparian ground needs follow-up in the seasons after any disturbance event, not just after the initial fell.
Can the timber be used?
Yes. Black wattle is grown commercially for tannin from the bark and for pulp and charcoal from the stem, and it makes decent firewood — denser than Port Jackson, not as good as rooikrans. On a clearing job the usable stem wood is normally cut to length and the brush chipped.