The problem
Bitou bush is a Weed of National Significance. It is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts.
Bitou bush threatens coastal dune vegetation along Australia’s east coast. It can outcompete and in many cases totally replace native flora, and it invades undisturbed as well as disturbed areas. Infestations of bitou bush drastically alter the environment for many native birds and animals. It can also create a favourable environment for other highly invasive weeds, such as asparagus fern, lantana and glory lily.
Ironically, one of the plants being choked by bitou bush along the New South Wales coast, golden wattle (Acacia longifolia) has become a major pest of coastal dunes in South Africa, where it was introduced to serve much the same purpose as bitou bush in Australia.
The weed
Bitou bush is a perennial, evergreen shrub, normally 1–2 m high although it can form canopies up to 10 m high. Unlike its close relative boneseed (Chrysanthemoides monilifera ssp. monilifera), which grows as an erect bush, bitou bush is a sprawling shrub. Its stems are branched and woody and the upper stems are often purple. The leaves, which are about 20–80 mm long, oval to oblong in shape and tapering at the base, alternate along the stems. Unlike boneseed, which has leaves with serrated edges, bitou bush has leaves with smooth edges. Bitou bush also has an extensive root system and appears more aggressive and more difficult to control than boneseed. The yellow, chrysanthemum-like flowers, up to 20 mm in diameter, are clustered at the ends of the branches. The small berries have green, fleshy skin that changes to brown and black on maturity. The fruit contains a single egg-shaped seed 5–7 mm long which is dark brown to black when dry.
Key points
- Prevention and early intervention are the most cost-effective forms of weed control. It is vital to keep clean areas free of bitou bush.
- Bitou bush infestations leave a large and persistent seedbank in the soil, so for longterm control infested areas must be treated repeatedly for several years.
- Integrated management using a range of control measures (eg mechanical, chemical and biological control, and the careful use of fire and/or grazing) is required to successfully control bitou bush.
- Monitor treated areas annually to detect and eradicate regenerating seedlings before they have a chance to produce seed.
Seeds germinate at any time of the year but mostly in autumn. Most seeds remain viable for at least two years. The seedlings grow rapidly during winter and a few plants may flower in the first year, particularly on burnt areas where there is little competition. However, plants are usually at least 18 months and sometimes three years old before flowering. Bitou bush can flower almost year round, but peak flowering occurs between April and June, unlike boneseed which forms flowers late winter and spring. Bitou bush fruits ripen during winter and the start of spring.
On the south coast of New South Wales, seedlings usually reach flowering age after three years (earlier if growing in burnt or favourable conditions), while on the north coast growth is much more rapid and flowering within one year is more usual.
How it spreads
Bitou bush spreads primarily by seed, with each plant producing up to 50,000 seeds a year. In established infestations, soil seedbanks can contain up to 5000 seeds per square metre. Soil disturbance (eg after bulldozing), fire and ingestion of seed by birds and animals promote seed germination but seeds can still germinate in undisturbed situations.
Birds are important in spreading seed as they readily eat the fruit and then pass the seed, sometimes many kilometres from the original source. Rabbits and foxes also eat the fruit and spread the seed in their droppings. Seeds are also spread by water, in ocean currents or through coastal creeks and waterways. Human activities can lead to the spread of bitou bush by vehicles and equipment. On a local scale, seeds may be spread in windblown sand.
Bitou bush can resprout after fire, slashing and herbicide application.
Where it grows
Bitou bush was first recorded in Australia near Newcastle, New South Wales, about 1908, probably introduced through dumping of ships’ ballast. From 1946 to 1968 it was planted along the coast to revegetate areas after sandmining. It was also planted on sand dunes near Menindee in western New South Wales where a small infestation still persists.
Bitou bush is naturalised in all states and territories except the Northern Territory. It is mainly restricted to coastal ecosystems with summer rainfall, similar to its range in South Africa.
Bitou bush grows in a range of environments – from open exposed dunes to shaded forests. It is tolerant of shade, salinity, strong wind, windblown sand and water, drought, low nutrients and, to some extent, disturbances such as fire. It grows poorly in wet or swampy soils and has a low tolerance to frost.
A recent survey recorded bitou bush on 900 km, or 80%, of the NSW coastline, with an estimated 36, 000 ha infested. Around Sydney there are mixed populations of boneseed and bitou bush. National containment zones for bitou bush have been established in New South Wales on the far north coast and the south coast. In Queensland bitou bush occurred on coastal sandmining areas but has been targeted for eradication for the past decade. Smaller isolated infestations occur on Lord Howe Island and near Melbourne.
Potential distribution
Bitou bush is spreading further into the understorey of forests and woodlands next to the coastal strip. Recent mapping has recorded infestations up to 10 km inland. There are still large areas outside its current distribution which are potentially at risk, including the whole of Tasmania and much of southern Australia.
What to do about it
Preventing spread
It is important to keep clean areas free of bitou bush. Once an infestation is established, preventing the spread of seeds into surrounding areas should be a priority. This means destroying established plants before they flower and produce fruit.
Any bitou bush plants in gardens should be destroyed since they represent a seed source and hence potential for further spread.
Much of southern Australia including the whole of Tasmania is climatically suitable for bitou bush Raising awareness amongst recreational vehicle users is important, particularly in coastal areas where seed may be spread by their activities.
Management strategies
In order to minimise the amount of seed produced, it is necessary to detect as many plants as possible. In areas where access is difficult, tracks may need to be cut to make control efforts easier. The greatest difficulty is the large area infested and the rapid reinvasion of an area after initial attempts at control. A sustained control effort is required for up to ten years. The sprawling bitou bush plants have branched, woody stems, which are often purple near the top.Bitou bush responds much more quickly than native plants after burning or land clearing, and with a large seed bank in the soil it will quickly form a dense growth of seedlings, swamping native species.
Shallow roots make mechanical control easier
Unlike many other woody weeds, bitou bush has a shallow root system with no distinct taproot, so pulling the bushes is possible. Seedlings and plants up to 1 m in height can be hand pulled.
Bitou bush does not persist when grazed or cultivated. Slashing alone is not effective as regrowth occurs from the stump, but applying herbicide to stems immediately after cutting should prevent regrowth. The removal of adult plants stimulates seed germination so new seedlings must be removed before they produce further seeds.
Herbicides are effective
Herbicides registered for bitou bush can be applied in winter at low rates that effectively kill the weed, yet have minimal impacts on coastal vegetation. However, in northern parts of the weed’s range where it matures more quickly, two spraying programs a year may be necessary to prevent seeding.
Herbicides registered for bitou bush can be applied from the air, from the ground or by a cut and paint method. Plants coated with dust or seaspray (eg those close to tracks or the beach) will be less affected by herbicides.
Isolated plants can be treated with herbicide applied by spot spraying. As infestations become larger, a strategically staged approach for removal is advisable to ensure that treated areas are not reinfested.
In New South Wales low dosages of herbicides have been applied from helicopters in winter, allowing large areas to be treated rapidly with minimum impact on native species. Reports indicate better than 95% control.
There are several effective biological control agents
The lack of natural enemies attacking bitou bush in Australia is one of the reasons it has become a serious weed. In classical biological control these natural enemies are introduced into Australia if they are shown not to attack Australian native species or other valuable plants. Biological control is a slow process and will not eradicate bitou bush. It is useful as part of an integrated approach and in areas where the application of conventional control methods is inappropriate due to economic, practical or environmental constraints.
Since 1987, when the first biocontrol agents for bitou bush and boneseed were released in Australia, seven control agents have been released. Two are well established and require no further distribution: the bitou tip moth (Comostolopsis germana) which destroys developing leaves, buds and flowers and reduces seed production; and the bitou seed fly (Mesoclanis polana) which destroys developing seeds. Leaf-feeding beetles (Chrysolina and Cassida spp.) were also released but have either not established or are colonising only slowly.
In 2001 the bitou leaf rolling moth (Tortrix sp.) was released on boneseed in the You Yangs in Victoria and on bitou bush in New South Wales. Although it has failed to establish on boneseed, it has established well at two sites on bitou bush. Another agent, the boneseed leafbuckle mite, is being tested for release.
Fire can kill mature bitou bush
Intense fire kills most mature bitou bush, although a small proportion resprout. Fire also kills bitou bush seeds in the litter and topsoil and stimulates germination of seeds from lower in the soil profile. These new seedlings must be removed before they produce further seeds.
Trials have shown that the seedbank in the soil is significantly reduced following burning of mature plants previously killed with herbicide. In areas where a large proportion of the remnant vegetation is known to be fire-sensitive, fire should not be used. Fire can also cause other problems such as increased erosion potential, increased traffic and access by humans and pest animals, and further invasion by weeds. Note that permission of the landowners and a permit from the relevant state or territory fire authority are generally required to authorise the use of fire, and that the fire should be undertaken by properly trained and equipped personnel.
Grazing
Cattle eat bitou bush and this limits its spread onto grazed properties next to heavily infested areas. But grazing in many bitou bush infested areas is not appropriate due to problems associated with stock, such as browsing of native plants, erosion from stock movement and the spread of other undesirable weeds.
