Legume weeds

Tree legumes

Legumes do very well in these tropical environments and many have become significant weeds. Leucaena, or coffee bush, is a legume originating from Central America and known in some circles as the "miracle bush" because of the amount of weight that cattle gain when feeding on it. For this reason, there are now more than 50,000 ha of Queensland planted with this bush and graziers are being encouraged to grow more and more. Another half a million hectares are suggested for planting. In the absence of grazing, this plant can form dense thickets that exclude native plants, especially along watercourses.

Prickly acacia

Prickly acacia covers more than 7 million hectares of north Queensland  Photo: Joel Brown

From a potential problem now to a recognised disaster: prickly acacia (Acacia nilotica). This spiny tree originates from Africa and was introduced to Australia as an ornamental and shade tree and as fodder for cattle. The plants escaped into the bush and this weed now covers a staggering 7 million ha of the dry regions in Queensland, mostly in the Mitchell Grasslands.

There are also coastal infestations around Home Hill and Bowen. Not only does this plant suppress native vegetation, but it also severely restricts the movement of wildlife and cattle. There are attempts to find biological controls for this weed. In the meantime, it is a declared weed which is generally controlled using chemical and mechanical techniques.

Mimosa

Another prickly Acacia species found in north east Queensland is the mimosa bush (Acacia farnesiana)—not to be confused with Mimosa pigra in the NT. Although some believe this is a native plant, it was originally introduced for the perfume industry because of its beautifully scented flowers and is also favoured as stock feed by graziers.

Mesquite

Mesquite (Prosopis spp.) is another invasive woody plant and though not as widespread as prickly acacia, is estimated to have the potential to spread over more than 60 per cent of Queensland's grazing lands.

They were introduced to Australia as ornamentals in station homesteads or town gardens and used in mine dumps and other soil stabilisation programs.

Unfortunately mesquite has a tendency to form dense thickets which out-compete other vegetation, interfere with mustering and block access to water. The sharp thorns can also injure animals and puncture tyres.

Prosopis spp. are native to North and South America and have caused severe economic losses in these countries. There are three species of mesquite in Queensland: P. pallida, P. velutina and P. glandulosa and a hybrid, with P. pallida being by far the most widespread. A number of control techniques have been developed including mechanical, chemical and biological control options and the use of fire. Fire is particularly effective for mesquite. See link to Savanna Links article below, and see Mesquite page in this section on using fire to manage mesquite.

Jerusalem thorn, or Parkinsonia, can form thickets along bores and dams, preventing water access to stock and wildlife Photo: Greg Calvert

Jerusalem thorn

The final plant in this group of weedy tree legumes is the Jerusalem thorn, also known by its scientific name Parkinsonia aculeata .

This prickly weed was introduced in the 1800s as a shade tree and is now a declared weed.

Small thickets occur on the coastal areas of north east Queensland, but inland it forms impenetrable thickets along bore drains and around dams, preventing water access to stock and wildlife.