Management of Indigenous and Alien Malvaceae on Islands near Perth, Western Australia

Malva dendromorpha, European tree mallow, has not previously been described as a serious environmental weed. Over the last two to three decades tree mallow has invaded seabird nesting islands off the West Australian coast, growing in dense clumps to 3m tall and outcompeting native perennial species including Malva australiana, the Australian native hollyhock. As a result of excessive growth of tree mallow on small islands there has been a serious loss of biodiversity. The stands of biennial tree mallow have no understorey and die back in the hot, dry summer, exposing the soil to erosion by the strong sea breezes and also rendering it vulnerable to invasion by annual weeds. The habitat may then be less suitable for nesting bird colonies. Control measures on Seal Island over two years included regular mechanical removal and stump treatment with Roundup (glyphosate). Subsequently weedmat was laid down and some native species were planted. While eradication was not possible, control produced an ecologically-desirable outcome. There has been a 70% reduction in the cover of tree mallow and the native hollyhock has re-established itself locally. Other planted native species failed to survive amidst heavy growth of annual invading alien grasses and herbs which included Malva parviflora (marshmallow). Similar invasions by tree mallow have occurred on islands in South Australia and Victoria. In South Australia, management options were investigated but adequate resources to put them into practice were not available. In Victoria, regular hand removal of tree mallow over a seven year period has virtually eliminated tree mallow and the native hollyhock is flourishing. The problems we encountered are summarised and future directions outlined.