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Publication #ENH444

Grevillea robusta: Silk-Oak1

Edward F. Gilman, Dennis G. Watson, Ryan W. Klein, Andrew K. Koeser, Deborah R. Hilbert, and Drew C. McLean2

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


[Click thumbnail to enlarge.]

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


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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


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Flower

Flower color: yellow orange

Flower characteristics: showy; emerges on one side of a raceme

Flowering: spring

Figure 4. 

Flower—Grevillea robusta: Silk-oak


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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


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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

Pruning requirement: little required

Breakage: susceptible to breakage

Current year twig color: brown, gray

Current year twig thickness: medium

Wood specific gravity: unknown

Figure 6. 

Bark—Grevillea robusta: Silk-oak


Credit:

Gitta Hasing


[Click thumbnail to enlarge.]

Culture

Light requirement: full sun

Soil tolerances: clay; sand; loam; acidic; slightly alkaline; well-drained

Drought tolerance: high

Aerosol salt tolerance: low

Other

Roots: not a problem

Winter interest: no

Outstanding tree: no

Ozone sensitivity: unknown

Verticillium wilt susceptibility: unknown

Pest resistance: resistant to pests/diseases

Use and Management

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.

Footnotes

1.

This document is ENH444, one of a series of the Environmental Horticulture Department, UF/IFAS Extension. Original publication date November 1993. Revised December 2006 and December 2018. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.

2.

Edward F. Gilman, professor emeritus, Environmental Horticulture Department; Dennis G. Watson, former associate professor, Agricultural Engineering Department; Ryan W. Klein, graduate assistant, Environmental Horticulture Department; Andrew K. Koeser, assistant professor, Environmental Horticulture Department, UF/IFAS Gulf Coast Research and Education Center; Deborah R. Hilbert, graduate assistant, Environmental Horticulture Department, GCREC; and Drew C. McLean, biological scientist, Environmental Horticulture Department, GCREC; UF/IFAS Extension, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.