Invasive Grevilleas on The Ridgeway

Stephen Panter

The following article is included here as a warning that just because a plant is an Australian native, does not prevent it from becoming a nuisance when cultivated beyond its natural range. The potential problenm is not confined to grevilleas - a number of acacias, for example, have also become pests outside of their natural range.

In the Australian Capital Territory, exotic shrubs such as Cotoneaster have escaped from cultivation, becoming the obvious "environmental weeds" of public reserves. On "The Ridgeway" east of Queanbeyan, across the border in New South Wales, Grevilleas have seeded from native gardens and are now increasingly conspicuous in vegetation on road verges, nature reserves and private land.

"The Ridgeway", a rural residential subdivision, still retains much of its original flora; open woodland dominated by Eucalyptus spp. (E.polyanthemos, E.goniocalyx, E.macrorhyncha) and Callitris endlicheri. The main understorey plants range from medium shrubs (Cassinia spp.,Acacia spp.) to tussock grasses (Poa sieberi, Themeda triandra).

The invasive Grevilleas, which form dense, prickly shrubs one metre tall by two metres wide, appear to be derived from Grevillea 'Canberra Gem', a common cultivar derived from G.rosmarinifolia. They can smother patches of native grassland wherever the Eucalyptus/Callitris canopy has been thinned by past clearing or by fire. Often, these sites, such as the Molonglo Gorge railway verge, are the habitats for small populations of unusual indigenous plants, including Microseris scapigera, Thysanotus tuberosus, Tricoryne elatior, Ajuga australis, Linum marginale and about eight species of orchid. Two Grevilleas are indigenous to "The Ridgeway". A form of Grevillea lanigera occurs along the railway verge as a small shrub with ovate leaves and sparse red flowers. Grevillea ramosissima, a holly-leaved shrub with spikes of yellow flowers, occurs in a small population on private land.

The garden escapees have spread into sites of these indigenous grevilleas and may lead to an interbreeding problem. The invasive grevilleas have already crossed the railway line and may be a long-term ecological threat to the Molonglo Gorge Reserve. Closer to Canberra, grevilleas have escaped from cultivation in the Botanic Gardens and are spreading into adjacent woodland.

Native plant gardeners should be aware that some Australian plants from outside the local area can become as much of an environmental problem as exotics in natural areas. Therefore, care should be taken in selecting native species, especially of Grevillea and Acacia, for planting into gardens near remnant bushland.

According to Sandy Berry and Michael Mulvaney in "Environmental weed survey of the ACT" (SGAP Canberra Region Journal, Vol 10, No 5, September 1995), Grevillea rosmarinifolia has hybridised with the local Grevillea juniperina. The hybrid is the most common Grevillea on Red Hill in Canberra, and also occurs on the lower slopes of Mt Ainslie. Another consequence of the mixing of Australia's flora is that genetic differences between species are homogenised. For example, a flat-leaved Northern Tablelands form of Grevillea juniperina is naturalised in inner Canberra and probably breeding with the local needle-like leaved form.

The message is simple: think before you plant, and may your native gardens flourish - but not beyond the garden path!

From the newsletter of ASGAP's Grevillea Study Group, November 1995 issue.

Australian Plants online - 2007
Association of Societies for Growing Australian Plants