The Alien Invaders Column 11 Long-leafed wattle: fire and water hazard
Long-leafed longifolia)
(Category 1b in South Africa – must be controlled by the property owner, and may not be cultivated or sold). wattle
What does it look like?
A scruffy, dense shrub on rocky ground; up to a 10mtall tree in deeper soils along rivers. In early spring (late August onwards) it bears finger-like spikes of yellow fluffy flowers, which turn into rounded galls.
The galls are a result of the flowers being stung by a wasp that was introduced as a biocontrol agent.
The bark is a pale grey, and the “leaves” dark green (actually phyllodes – the real leaves fall off early and are replaced by a modified leaf stalk).
It is also called Port Jackson willow around Grahamstown; the‘real Port Jackson willow is actually Acacia saligna – it is a similar species which is common in and around Port Alfred but not this far inland, and is under effective biocontrol by a fungal pathogen. Where does it come from? South-eastern Australia and Tasmania.
Why is it bad?
(Acacia
Although long-leafed wattle is one of our great biocontrol success stories in South Africa, it is still a menace because of the prolific seed bank that was generated before the biocontrol agent was introduced. Although seed set is now reduced tremendously – thanks to the wasp – there are still millions of long-lived seeds in the soil throughout the upper Kowie catchment.
New seedlings and saplings can be seen every year even on Mountain Drive, which has been under alien invasive management for 20 years. Areas like Stones Hill are still under domination by extensive stands of long-leafed wattle.
Long-leafed wattle transforms our species-rich grassland and fynbos into alien mono-stands, resulting in local extinctions of ecotourism draw card species such as striped flufftail and our Grahamstown version of Garden Route species such as erica, protea, brunia and leucodendron that would be more evident otherwise.
Dense stands of longleafed wattle also increase fire risk exponentially, lower ground-water levels, and make it an expensive exercise to return land to usefulness.
Simply leaving this species, in our fire-prone environment, results in a complex mix of burnt and dead adult trees and a mix of young and old growth that becomes extremely time-consuming and expensive to address; this is especially so in mountainous terrain like Mountain Drive and Stones Hill.
Long-leafed wattle is still probably the most prolific invasive species in the upper Kowie catchment, and remains one of the main targets for local Working for Water operations.
How can I control it?
Because of the dominant seed bank, controlling this species is a long-term project on any property.
The adult trees are fortunately easy to kill, as – unlike numerous other wattles (eg black wattle Acacia mearnsii) – mature trees die once felled, and don’t coppice; single-stemmed trees can also be ring-barked.
For smaller trees, particularly on rocky slopes, cut stump treatment using a suitable herbicide is advisable.
Clearing the adult trees is only the first step: their removal generally results in a carpet of seedlings, which if left can become an impenetrable jungle of wall-to-wall saplings.
Young saplings can be easily dealt with using a brush-cutter once they reach 1m tall, or can be sprayed with a broad-leaf herbicide.
Annual follow-ups to address new emergence will be necessary in perpetuity.
Fortunately, the wasp means we are only dealing with existing seeds and not also new seed-set.
Three to five years of concerted effort can see a 90 percent reduction of this species on a badly infested property.
What can I replace it with?
Because this species is primarily a problem on farmland and small-holdings around town in grassland and fynbos, the ideal vision is to return invaded areas to natural vegetation. On sites where longleafed wattle has been the dominant tree for decades it will be necessary to manually sow replacement grass to curb erosion, reduce new seedling emergence, and encourage indigenous species to recolonise.
Numerous indigenous pasture grasses are good options, including broad-leafed bristle grass ( Setaria megaphylla), broad-leaved panicum ( Panicum deustum), common bristle grass ( Setaria sphacelata var sphacelata), common finger grass ( Digitaria eriantha), couch grass ( Cynodon dactylon), Guinea grass ( Panicum maximum), Rhodes grass ( Chloris gayana) and weeping love grass ( Eragrostis curvula).
Hand-harvesting and sowing the seeds of local fynbos species can increase this diversity, slowly returning the area to the beauty and diversity it once was.
For alien invasive species advice, property assessments, alien control plans and practical implementation of control actions, contact The Alien SWAT Team: Dr Ralph Clark 072 439 8981; Timothy Mattison 060 527 7816; thealienswatteam@gmail.com.