To prepare for this month’s article, I met with the man who knows Cape St Francis and its problem plants like the back of his hand, Dave Bowmer, at the Anchor Garden, at the entrance to Cape St Francis.
The culprit this month is a native to Australia, the Port Jackson Willow (Acacia Saligna), often confused with the Rooikrans (Acacia Cyclops)
Port Jacksons can be viewed at many spots around town, for example on the Two Harbours Walk, at the bottom of Porto Cervo, on the Port side of the turning circle.
The Port Jackson is a lighter green, and the leaves are longer, and hasn’t been quite such a menace in St Francis as the Rooikrans. But the eradication of the Port Jackson requires poison, unlike the Rooikrans, which usually dies when cut below the lowest branch.

Description

Port Jackson grows as a dense, spreading tree with a short trunk, a weeping habit and yellow flowers. It grows up to 8m tall. A colonizer, it tends to grow wherever soil has been disturbed, such as alongside new roads. Its seeds are distributed by ants, which store them in their nests. Disturbance of the soil allows them to germinate. Seeds germinate readily, and hundreds of seedlings can sometimes be found beneath a single parent tree. It is also extremely vigorous when young, often growing over a metre per year.
10 Worst Trees for St Francis Bay

Uses

Port Jackson can be used for multiple purposes, as it grows under a wide range of soil conditions into a woody shrub or tree. It has been used for tanning , revegetation, animal fodder, mine site rehabilitation, firewood, mulch , agroforestry and as a decorative plant. Acacia saligna has been planted extensively in semi-arid areas of Africa, South America and the Middle East as windbreaks and for stabilisation of sand dunes or erosion.

Invasive potential

Port Jackson has become an invasive species outside its natural range due to the following contributing factors:
Widespread planting outside its native area
Rapid growth in soil with low levels of nutrients
Early reproductive maturity
Large quantity of seeds produced
Ability of seeds to survive fire
Ability to germinate after cutting or burning
Tolerance to many different substrates
Extensive root system
Taller growth (by more than 3 m in some places) than indigenous plants
It was planted in the northern suburbs of Sydney in the 1950s by well-meaning plant enthusiasts, and has subsequently become a major weed in Australia. Since 2019, the species is on the list of invasive alien species, and cannot be imported into the European Union. It has become illegal to plant it, breed it, transport it, or bring it into the wild.

Environmental impact in South Africa

In South Africa, it proliferated at an uncontrollable rate, having been introduced in the nineteenth century to produce tan bark and to stabilise the sands of the Cape Flats outside Cape Town after the indigenous bush had largely been cut down for firewood. In addition to replacing indigenous fynbos vegetation, it also hampers agriculture. It is listed as an invasive alien plant, where it has displaced native species through changing fire regimes.
The introduction of a fungus has proven to be effective at reining it in, reducing densities considerably.
Like most listed alien invasive trees, Port Jacksons:
a) are water guzzlers, the opposite of water-wise indigenous trees, and
b) spread easily, smothering our beautiful indigenous vegetation, and
c) are a fire hazard
Landowners are obliged by law to remove them .. so please do so, and replace them with indigenous trees which will be an asset to your garden, and will attract birds.
By the way, Dave pointed out that three indigenous plants, the Bitou, the Brachyleana and the Blombos have become pests due to their rapid growth in our town, and need to be treated with caution.
The background to this series is that we at Pam Golding asked a local botanist to list the Ten Best and Ten Worst trees for St Francis, and Foster agreed to publicize them. So far we have featured the following of the Worst: the Rooikrans, the Manatoka/Salt & Pepper, the Brazilian Pepper, the Beefwood (aka Casuarina) and the Port Jackson.
Please pick up a brochure on Trees For St Francis, available for free at our Pam Golding office.
by Richard Arderne
all photos: Richard Arderne