Scientific: Grevillea robusta (Synonyms: Grevillea umbratica, Grevillea venusta, Stylurus robustus)
Common: silk oak, Australian silky oak
Family: Proteaceae
Origin: Southeastern Queensland and northeastern New South Wales, Australia.
Invasive Alert: Silk oak shows moderate levels of invasive potential in mild climate regions around the world. It has naturalized in South Africa, New Zealand, Hawaii, French Polynesia, Jamaica, Norfolk Island and south Florida.
Pronounciation: Gre-VIL-lee-a row-BUS-ta
Hardiness zones
Sunset 8, 9, 12-24
USDA 9-11
Landscape Use: A skyline tree with strong vertical accent, mesic and oasis landscape designs. This is NOT a desert or xeric landscape tree.
Form & Character: Strongly upright, rugged-looking, wispy and stately, yet messy.
Growth Habit: Evergreen, woody, broadleaf perennial tree, grows moderate to fast to 100-feet tall in coastal central and southern California, typically around 50-feet tall when mature in Phoenix.
Foliage/Texture: Alternately arranged foliage, fern-like to pinnate, entire or lobed, leathery, slow to decompose after abscission; medium to coarse texture depending on tree size.
Flowers & Fruits: Showy orange yellow flowers arranged in one-sided, 4-inch long raceme; fruit is a leathery follicle.
Seasonal Color: Sometimes spectacular floral display in the spring.
Temperature: Cold hardy, but can struggle when daily temperatures exceed 115oF.
Light: Full sun, though susceptible to trunk sun scald. Needs protection from western sun if not surrounded by landscape greenry.
Soil: In Phoenix, avoid hot, dry sites or soils that are poorly drained. Do not plant near reflective surfaces or on south and west exposures of large buildings. Can develop leaf chlorosis (yellowing) if grown in soils of moderate to high alkalinity.
Watering: Regular supplemental water needed during summer months.
Pruning: Elevate foliar canopy base only as needed.
Propagation: Seed
Disease and Pests: Root rot in poorly drained soils.
Additional comments: Silk oak is an ornamental and stately tree for mesic and oasis landscapes in Phoenix that casts a strong skyline silhouette. It is however a relatively messy tree that produces copious leaf and small branch litter. Moreover, silk oak leaves are unusually lignified and decay slowly, which can just compound the litter irritaions of most landscape neat freaks. Silk oak was once more frequently planted in Phoenix, but is not today as it doesn't conform to our prevailing present expectations for landscape water conservation, tidy landscapes free from bio-litter and debris (just plastic trash please), and environmental justice and sustainability. Silk oak wood from indigenous silk oaks in Australia is a valuable source of timber.
Minor taxonomic factoid: Grevillea is a robust genus of over 250 tree and shrub species from Australia.
