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Publication #ENH444

Grevillea robusta: Silk Oak1

Edward F. Gilman and Dennis G. Watson2

Introduction

Reaching a height of 75 feet or more with a 25-foot spread, silk oak is pyramidal to oval in shape, eventually developing a few heavy horizontal limbs and a thick trunk. The light, ferny, grey-green leaves, silvery beneath, are accented by large clusters of bright 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. Black, leathery seed capsules follow the flowers.

Figure 1. 

Middle-aged Grevillea robusta: Silk Oak


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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 (Fig. 2)
Origin: not native to North America
Invasive potential: little invasive potential
Uses: specimen
Availability: somewhat available, may have to go out of the region to find the tree

Figure 2. 

Range


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Description

Height: 60 to 100 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 (Fig. 3)
Leaf type: odd-pinnately compound
Leaf margin: parted, revolute
Leaf shape: lanceolate
Leaf venation: pinnate
Leaf type and persistence: evergreen
Leaf blade length: 2 to 4 inches
Leaf color: green
Fall color: no color change
Fall characteristic: not showy

Figure 3. 

Foliage


[Click thumbnail to enlarge.]

Flower

Flower color: orange, yellow
Flower characteristics: showy

Fruit

Fruit shape: unknown
Fruit length: unknown
Fruit covering: dry or hard
Fruit color: black
Fruit characteristics: does not attract wildlife; not showy; fruit/leaves a litter problem

Trunk and Branches

Trunk/bark/branches: branches don't droop; showy; typically one trunk; thorns
Pruning requirement: little required
Breakage: susceptible to breakage
Current year twig color: brown, gray
Current year twig thickness: medium
Wood specific gravity: unknown

Culture

Light requirement: full sun
Soil tolerances: clay; sand; loam; acidic; slightly alkaline; well-drained; occasionally wet
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.

Footnotes

1.

This document is ENH444, one of a series of the Environmental Horticulture, UF/IFAS Extension. Original publication date November 1993. Revised December 2006. Reviewed February 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Edward F. Gilman, professor, Environmental Horticulture Department; Dennis G. Watson, former associate professor, Agricultural Engineering Department, 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.