June Tree of the Month: Silk Oak

By David Gress

The Silk Oak is perhaps the oddest-looking but most amazing tree to be grown in our community.  When its canopy is covered with flowers – which are colored bright orange, yellow and red and are quite curiously shaped – it is a tree that demands a second or third look, being both strangely weird and uniquely stunning.

Because of its attraction as an ornamental specimen tree for landscaping and because it is so well suited to the climate of the central California coast, it has been planted in the Santa Barbara area since the late 1800s.

Its flowering begins in April and can continue into July, depending on the weather.  The flowers form in “racemes” (a cluster with separate flowers attached to and evenly spaced along a central stem), which appear at the ends of short, leafless, branches.

Each raceme contains two or more horizontal flowers (5- to 6-inches in length).  The flowers are “perfect”, in that each flower contains both female parts and male parts.  As with other proteas, the flowers do not have petals but, instead, have two rows of brightly colored orange-yellow carpels (female flower parts) curled up on 1-inch styles.  The carpels gradually uncurl and straighten out reaching skyward, until they resemble the stiff bristles of an upward-facing hairbrush – I told you they are curiously shaped!  The anthers (male flower parts) emerge from the orange-yellow perianth at the bottom of the flower.

The flowers produce copious amounts of nectar that are eagerly enjoyed by visiting bees and birds who act as pollinators.  Pollinated flowers produce fruits that are brown, boat-shaped, follicles (½ inch long) on 1-inch stalks.  Each follicle contains two dull-brown flat seeds; each seed is surrounded by a papery wing, which aids in its dispersal in the wind.

The canopy is covered with fern-like, pinnately-compound, leaves (4- to 10-inches long and 4- to 6-inches wide), each with 11 to 31 lanceolate-shaped leaflets with up to four lobes at the ends; this leaf shape development gives the leaves a delightfully lacy texture.  In the spring, the tree will shed a large number of older leaves, just before the appearance of flowers followed by the emergence of new leaves.

The gray-brown bark is hard and furrowed, often with a lace-like pattern. The trunk is normally single and straight.

It is a medium- to large-sized tree that can reach over 100 feet in its native habitat – but usually only attains 40 to 60 feet in our local growing conditions.  It is fast-growing, with a narrow pyramidal form when young and a broad oval crown at maturity.

Silk Oak is endemic to limited areas in Australia’s coast in southern Queensland and northern New South Wales, in subtropical and dry rainforests.

In its homeland, Silk Oak is harvested for its rot-resistant wood, which is used for fine carpentry and in the manufacture of furniture, cabinets, fences, and musical instruments.

Native stands of Silk Oak are now legally protected in Australia, due to deep concerns over the dwindling of native tree populations – the result of centuries of overharvesting.  Fortunately, Silk Oak is not threatened with worldwide extinction, because, in addition to its use as an ornamental in urban landscapes, it is being planted in large numbers in commercial tree plantations and for use in agroforestry.

It is now found in all the mild climates of the world.  In wetter climates, it can be an invasive species; however, in our Mediterranean climate, this has not been a problem.

Silk Oak’s common name is a bit of a misnomer, because it is not even closely related to true oaks, which are in the genus Quercus in the Fagacea Family.  Actually, Silk Oak is the largest tree in the genus Protea in the Proteaceae Family; this Family first appeared over 200 million years ago!

The common name of “Silk Oak” is sometimes thought to be in reference to its soft foliage. In Australia, its other common name is “Silky Oak”; this latter name is said to relate to the silky-smooth texture of its milled wood that is similar in color and hardness to true oak wood.

Whatever its common names, its botanical name is Grevillea robusta. The genus name, Grevillea, honors Charles Francis Greville (1749-1809), a member of the British House of Commons and a co-founder of the Royal Horticultural Society.  The specific epithet, robusta, is a Latin word meaning “strong” or “robust”.

Silk Oak is very easy to grow, being very adaptable to various types of soil – but it does best in deep, well-drained, loam soils.  Once established, it tolerates drought well but will thrive with regular irrigation during dry periods.  It is remarkably resistant to insect pests and diseases.  When young, it is frost sensitive; within a few years, it can tolerate temperatures down to 18 degrees F.  It can be propagated by seeds and by cuttings.  Seeds should be collected when fresh and soaked for 24 hours before planting.  From planting out, it can take two to four months to germinate and up to six years before flowering.

Both beautiful and utilitarian, Silk Oak can be planted as either an interesting ornamental tree or as a protective windbreak.

Note:  It does have rather brittle branches – these will require some regular pruning on mature trees to avoid wind damage.  Care should also be taken to avoid the contact dermatitis that can result from the handling of its flowers and/or sap.  The main consideration for its use in the landscape is to provide sufficient room for its size at maturity.  It basically grows too large for smaller residential lots, so it is better suited for planting in parks and other larger parcels.

The characteristics that make Silk Oak an excellent ornamental tree include its dazzling floral display, its luxurious evergreen foliage, and its rapid growth.  These outstanding qualities have earned it the prestigious Royal Horticultural Society‘s Award of Garden Merit.  Despite its few drawbacks, Silk Oak adds interest and beauty to parks, estates, and streets.

Mature Silk Oaks can be seen in several locations in our community as street trees: in the 1200 block of Anacapa Street; in the 1400 block of Garden Street; in the 2900 block of Las Positas Road (along the municipal golf course); and, in the 3000 block of Eucalyptus Hill Road.   There is a magnificent specimen on Arguello Road.  A large tree stands near the front of Mission Santa Barbara, and 2 in Willowglen Park.  They are very prominent on the sides of Highway 101 at Mission, Carrillo, Arrellaga, and Castillo Streets.    

Tree-of-the-Month articles are sponsored by Santa Barbara Beautiful, whose many missions include the increase of public awareness and appreciation of Santa Barbara’s many outstanding trees and, in a long-time partnership with the City Parks & Recreation Department, the funding and planting of trees along the City’s streets.

Those who wish to honor a special someone can do so with an attractive commemorative marker that will be installed at the base of an existing street tree in the City of Santa Barbara.  Because Santa Barbara Beautiful has participated in the planting to date of over 13,000 street trees, there are plenty of trees from which to choose!  Application forms are available on the Santa Barbara Beautiful website, www.sbbeautiful.org.

Article and Photos by David Gress