Download Article 77 Grevillia robusta

Weeds in our Area (Part Seventy Seven)
By Bob and Ena McIntyre – Garden Route Branch
Grevillia robusta
Australian silky oak (Grevillia robusta): Originally recommended for use as ornamental and shade
trees for large gardens, for planting groves on farms and a good windbreak. A popular
combination was to plant a Silky Oak alongside a Jacaranda. These two species flower at around
the same time and the contrasting soft blue flowers of the Jacaranda and golden-orange of the
silky oak made quite a spectacular colour combination. Grevillia robusta is member of the
Proteaceae family. In their country of origin, Eastern Australia, the wood is used in furniture
manufacture. They were popular as street trees and many fine examples can be seen along the
main street in Oudtshoorn and there are several old trees on pavements in Knysna. After many
years of popularity it has accustomed itself so well to local conditions that it has become
problematic – hence its Category Three classification (no new plantings or any form of trade) and
invasive status of potential transformer. Locally the species invades forest margins and riverbanks
and further north, moist savanna. The sap is a skin and eyelid irritant.
Identification: Grevillia robusta is large and
evergreen reaching between 18 – 30m in
height. The leaves remind one of coarse fern
fronds, being deeply incised and quite large
(between 150 – 300 mm in length). The leaf
margins are recurved and the leaves are dark
green above and grey-white and softly hairy
underneath. The tiny flowers are a rich
golden-orange colour and are borne in very
showy bottle-brush type sprays on stalks.
The flowering time is between September
and November. The fruits are brown-black
follicles that are about 20 mm long and the
flat seeds are winged, implying wind
dispersal.
Control: As no herbicide is specifically registered for this species, the best control option is the
physical removal and destruction of the plant material. Keep a close watch if you have a mature
specimen in your garden or nearby and weed out any seedlings as soon as they appear.
Substitutes: Kigelia africana (Sausage Tree), Croton sylvaticus (Forest Feverberry), Spathodea
campanulata (African Flame).
References: “ALIEN WEEDS AND INVASIVE PLANTS”: Lesley Henderson. Copyright © 2001
Agricultural Research Council, A to Z of Gardening in SA (W G Sheat).