Download Article 62 Acacia melanoxylon - Botanical Society of South Africa

Weeds in our Area (Part Sixty Two)
By Bob and Ena McIntyre – Garden Route Branch
Acacia melanoxylon
Still in pursuit of the various invasive Acacia species, our choice this week is Acacia melanoxylon,
commonly known as Australian Blackwood. Australian Blackwood was introduced into South Africa
around 1850 and planted in the Knysna forests as substitute species after logging of indigenous
species. Grown within the confines of our indigenous forests, Australian Blackwood produces high
quality timber that is referred to as ‘INDIGENOUS AUSTRALIAN BLACKWOOD’ in the timber
industry and is used extensively in the production of good quality furniture. However, when grown
outside of the indigenous forest environment the timber produced is of a significantly lower
standard that fetches only a fraction of the price of Indigenous Australian Blackwood. The
Department of Forestry is involved in a long-term project to eliminate the species from our
indigenous forests. Contained within the forest the species is not particularly invasive and is
mainly a problem in disturbed areas. In the latter situation large mono-specific stands develop
that displace indigenous vegetation.
Identification: A. melanoxylon is a large evergreen tree
up to 30m in height. The leaves, as with many Australian
Acacias, are greyish phyllodes (leaf-like petiole with no
blade) that turn to a dark, dull-green with 3-7 prominent
longitudinal veins and fine net-veins between. The leaves
of young plants and new coppice growth are often
bipinnate. The pale yellow flower-heads are typically
globular (little round puffballs), and appear during August
and September. The reddish brown seed pods are
narrower than the leaves, slightly constricted and twisted.
Seeds are almost encircled by pinkish-red seed stalks.
Control: These plants coppice readily and also sucker
from roots if felled and the stumps are not immediately
treated with herbicide. Garlon and diesel at 2% should be
used to treat stumps after felling. Another method, where
felling is impractical and the tree can be left standing, is
to frill the stem and to apply the same mix of Garlon to the frill. A. melanoxylon produces large
quantities of very long-lived seed necessitating regular follow-up. During 1986 a seed-feeding
biological control agent by name of Melanterius acaciae was released with good results.
Substitutes: These indigenous trees will do the trick - Croton sylvaticus (Forest Fever Berry),
Kigelia africana (Sausage Tree), Kiggelaria africana (Wild peach), Harpephyllum caffrum (Wild
plum)
Illustrations: “ALIEN WEEDS AND INVASIVE PLANTS”: Lesley Henderson. Copyright © 2001
Agricultural Research Council.