As I was driving to the public library last week, I noticed a group of trees covered with orange blossoms. I couldn’t quite tell what they were from the car, but they looked unusual enough that I decided to go back for a closer look. These are tall specimens, towering above the two-story apartment buildings along F Street, but with my telephoto lens I was able to zoom in close enough for a positive identification.
The leaves and flowers were a dead giveaway: These trees are Grevillea robusta, commonly known as silky oak, southern silky oak or Australian silver oak although not related to the true oaks (genus Quercus).
Grevillea robusta is the largest species in the genus Grevillea and the only one growing as a true tree. All others are shrubs of varying sizes and growth habits.
Grevillea robusta is native to the east coast of Australia where it grows in environments that receive upwards of 40 inches of rain a year. However, in spite of that it is considered to be quite drought-resistant.
Since it hails from a temperate region, it doesn’t like excessively cold winters. Its hardiness rating of 18°F is no problem here in Davis (zone 9b). Our typical winter lows are in the mid-20s.
Grevillea robusta is fast growing and can reach 60-100 ft. at maturity. The five specimens I spotted in Davis were around 40 ft., if I had to venture a guess. I have no idea how old they are or who planted them. Grevillea robusta is definitely not a common street tree around here!
Grevillea robusta won the Royal Horticultural Society’s Award of Garden Merit in 1993 (click here for more info). I find the leaves and flowers to be very attractive although the overall growth habit is a bit lanky.
Detractors of Grevillea robusta cite the unsightly seeds that remain on the tree for several seasons, dripping sap in the spring, leaf and flower litter year round, and the presence of irritating chemicals in the flowers and fruits that can cause contact dermatitis. On Dave’s Garden a homeowner from Adelaide, South Australia, said:
We live in suburban Adelaide and have an 80ft Silky Oak in our front yard which was planted in 1959 according to the neighbours. It has now reached "significant tree" status in our council area and we are unable to remove it without an arborist's report and council approval. So far, 2 arborists have deemed it healthy. We have the plumber out twice a year to unblock the pipes at a cost of $500 a pop- not a general pipe clean! The whole sewer needs replacing now and both bathroom floors are cracked from the roots. The tree constantly drops leaves, sticky sap, branches and looks completely scruffy. The neighbour is moving because it is less than 10 mtrs from her front door and she can't stand it anymore! I would never plant this tree unless I lived in the country!
Nonetheless, a well groomed specimen of Grevillea robusta has the potential to become spectacular. I’m very happy I found these trees right here in town. Anything to break up the monotony of street-side plantings is a welcome sight in my eyes.
P.S. I just spotted additional five silky oaks in bloom in Davis (on Cowell near the Nugget Shopping Center) so they may not be quite as rare as I had thought.