Edward F. Gilman, Dennis G. Watson, Ryan W. Klein, Andrew K. Koeser, Deborah R. Hilbert, and Drew C. McLean
Introduction
Reaching a height of 75 feet or more with a 30-foot spread, silk-oak is pyramidal to oval in shape, eventually developing a few heavy horizontal limbs and a thick trunk. The light, ferny, green leaves, silvery-white beneath, are accented by large clusters of yellow orange flowers in spring. A great quantity of leaves fall in the spring immediately preceding the emergence of new growth, and leaves also fall sporadically throughout the year, creating quite a litter problem to some people. Silver gray to brown, leathery seed capsules follow the flowers.
Figure 1. Full Form—Grevillea robusta: Silk-oak
General Information
Scientific name: Grevillea robusta
Pronunciation: grev-ILL-ee-uh roe-BUS-tuh
Common name(s): Silk-oak
Family: Proteaceae
USDA hardiness zones: 9B through 11 (Figure 2)
Origin: native to coastal eastern Australia
UF/IFAS Invasive Assessment Status: not considered a problem species at this time, may be recommended (North, Central, South)
Uses: specimen
Figure 2. Range
Description
Height: 40 to 75 feet
Spread: 25 to 30 feet
Crown uniformity: symmetrical
Crown shape: pyramidal, oval
Crown density: moderate
Growth rate: fast
Texture: fine
Foliage
Leaf arrangement: alternate
Leaf type: odd-pinnately compound; made up of 7-19 leaflets
Leaf margin: parted, revolute
Leaf shape: lanceolate
Leaf venation: pinnate
Leaf type and persistence: evergreen
Leaf blade length: 6 to 13 inches; leaflets are 1 to 4 inches
Leaf color: green on top, silvery white underneath
Fall color: no color change
Fall characteristic: not showy
Figure 3. Leaf—Grevillea robusta: Silk-oak
Flower
Flower color: yellow orange
Flower characteristics: showy; emerges on one side of a raceme
Flowering: spring
Figure 4. Flower—Grevillea robusta: Silk-oak
Fruit
Fruit shape: unknown
Fruit length: ½–¾ inch
Fruit covering: dry or hard
Fruit color: silvery gray to brown
Fruit characteristics: does not attract wildlife; not showy; fruit/leaves a litter problem
Figure 5. Fruit—Grevillea robusta: Silk-oak
Trunk and Branches
Trunk/branches: branches don't droop; showy; typically one trunk; no thorns
Bark: gray to light brown, becoming furrowed in an interlacing pattern with age
Silk-oak works as a specimen in large, open landscapes but probably should not be located near houses due to their large size, messy habit, and the brittleness of the wood as it ages. Tops of trees are known to snap out of the tree in high winds. It is a valuable timber tree in its native Australia, growing to more than 125 feet tall.
Quick-growing silk-oak requires full sun and sandy, well-drained soils to perform its best, developing mushroom root rot in poorly drained, wet soils. Silk-oak thrives in heat and is quite tolerant of drought. It grows extremely well in southern California where it easily reaches 100 feet tall. Tall trees are often hit by lightning in Florida.
Propagation is by seed. For best results, extract seed from mature, unopened follicles and plant immediately.
Pests
Caterpillars.
Diseases
Mushroom root rot on poorly-drained soils.
Reference
Koeser, A.K., Friedman, M.H., Hasing, G., Finley, H., Schelb, J. 2017. Trees: South Florida and the Keys. Gainesville: University of Florida Institute of Food and Agricultural Sciences.
