The spread of commercial olive cultivation across Australia in the 1990s poses a major weed threat, if the spread of feral olives from former South Australian groves is any indication, according to studies published by two Charles Sturt University researchers.

"The South Australian experience shows that olives can become major woody weeds in suitable areas if left uncontrolled," says Dr Dirk Spennemann, from the University's Johnstone Centre.

The European olive, grown commercially in the 19th Century, became a major woody weed in the Adelaide Hills above the SA capital in 1930s. They have since been reported as far as Kangaroo Island, Eyre Peninsula, and in the Mt Lofty Ranges in the 1980s. Feral olive populations are also now found around Victoria, WA, NSW and even Norfolk Island.

Most temperate climate areas receiving over 500 mm of rainfall each year are suitable for growing olives. Commercial olives now cover 5 000 ha, with a further 7 000 ha planned, in all Australian states and territories.

Dr Spennemann and Mr Allen have found that native and naturalised birds and mammals spread seeds readily and extensively - the emu can spread olive seeds up to 50km! The oil in olive fruit is very attractive to birds and mammals as it provides them with a substantial source of energy.

The researchers maintain that containing feral olives should not be left to public agencies or Landcare groups.

"Strategies to prevent its escape into the surrounding environment should be part of planning for new and existing commercial olive groves. Some government regulations may also be needed to prevent the spread of olives," said Mr Allen.

Australia has a long history of intentional and accidental introductions of destructive plants and animals into the Australian environment, including rabbits, foxes, camels, pigs, blackberries, prickly pear and Patterson's curse.

Dr Spennemann's and Mr Allen's work on feral olives was published in the latest issue of the prestigious Australian Journal of Experimental Agriculture and in four languages in the international journal for olive growers, Olivae.