A Better Method for Controlling Wild Olive Trees in Australia

While the Australian olive oil indus­try con­tin­ues to thrive, increas­ing the demand for the olive plants to be imported and farmed, the olive tree has not been wel­comed in all areas of the coun­try.

When local birds and ani­mals con­sume its fruit, they inad­ver­tently spread its seeds into bush­land areas, where (if left unchecked) the seeds rapidly grow into ​‘feral’ woody weeds that invade and destroy local veg­e­ta­tion, as well as deprive sur­round­ing veg­e­ta­tion of ade­quate sun­light while they reach full height and breadth.

The feral olive trees are also cre­ated in instances when inex­pe­ri­enced farm­ers and gar­den­ers attempt to grow their own trees, which are then aban­doned or improp­erly har­vested.

Wild olive trees can live for cen­turies and are known to pro­vide breed­ing grounds for insects such as the apple wee­vil and the anthrac­nose fun­gus — both of which can harm the sur­round­ing plan­ta­tion and spread to healthy olive trees. It can also destroy valu­able live­stock graz­ing land and shore up pesti­lent ani­mals such as foxes and rab­bits.

All landown­ers are required by law to con­trol any olive plants on their prop­erty, under the Natural Resources Management Act of 2004 and South Australian Declared Plant Policy.

Previous attempts at culling the wild olive tree (includ­ing cut­ting and poi­son­ing the stumps) as well as drilling and fill­ing stumps with a Glyphosate or Triclopyr mix­ture have proved unsuc­cess­ful, lead­ing to the need for a more effec­tive con­trol method — one that the Adelaide and Mount Lofty Ranges Natural Resources believe that they have found in basal bark spray­ing.

The basal bark method has been found to be cheaper and time effi­cient in terms of hours of labor required and the amount and type of tools needed. It involves spread­ing a her­bi­cide directly to the bark of a plant using a sprayer or sim­i­larly hand­held device, for­go­ing the usual power drills and chain­saws required by ​‘fill and drill’ meth­ods.

Preliminary find­ings show that this method takes roughly 3.5 hours to exe­cute at a cost of $386, com­pared to the fill-and-drill method which takes 44.5 hours and costs $2,913. It also can be done stand­ing up, can be sprayed with­out a spe­cific tech­nique and elim­i­nates the need to trim branches or sur­round­ing debris.

Experts are care­ful to point out that the basal bark method is not a replace­ment for other meth­ods of con­trol, but can pro­vide a lower cost alter­na­tive in areas with low wood­lands and juve­nile trees. In addi­tion, it can take up to a year to com­pletely elim­i­nate leaves, allow­ing these leaves to reshoot in the future.

The suc­cess of the basal bark method comes as wel­come news, with no end in sight yet to Australia’s past few months of record-level tem­per­a­tures and high winds.

Several areas of the coun­try have already started to insti­tute plant clear­ing mea­sures, and a fire ban has been issued in parts of New South Wales, the Greater Sydney and Hunter regions, parts of the Central Ranges, the Southern Slopes and the North West.

Wild olive trees pose a par­tic­u­lar threat to spread­ing bush­fires, as its leaves are rich with oil, retain­ing heat and fuel­ing flames when alight. It’s removal — even tem­porar­ily — could have a sig­nif­i­cant effect on bush­fire risk.

• The the Adelaide and Mount Lofty Ranges Natural Resources Department


• Plant Health Australia