February Tree of the Month: Cork Oak

By David Gress, February 2020

The Cork Oak’s most notable and unique characteristic is its thick bark - a beautiful texture of the pale-gray trunk mixed with deeply fissured dark-brown vertical furrows.

Most famously, this bark is the material for wine bottle corks but is also used in a multitude of other products, including cork flooring, cork gaskets, cork bulletin boards, and sports equipment.

On trees over 25 years old, the bark can be up to 12 inches thick - thick enough that it can protect the tree from fire.  The bark is commercially harvested by very carefully removing it with hand tools - so carefully that there is no damage to the underlying living layer of the trunk, called the cambium.  Remarkably, a shorn tree will regenerate its bark every 9 to 11 years, after which it can be re-harvested.  Since it can live to over 200 years of age, it may be harvested some 12 times or more over its life span.

Cork Oak is a medium- to a large-sized evergreen tree with a broad rounded crown.  Most reach 35 to 40 feet in height with a spread of 25 to 30 feet - very old trees can exceed 60 feet in height with a spread of 60 feet.  The attractive leaves (1 ½ to 3 inches long and 1 inch wide) are leathery and toothed along the margins, dark-green and shiny on the top, but a pale-green and densely hairy on the underside.

It is monoecious, meaning it bears both male and female flowers on the same tree.  In the spring, the flowers appear male flowers on catkins, 2 to 3 inches long; and, small female flowers form in clusters.  In the fall, a copious number of acorns (oblong capped nuts, up to 1½ inches long) ripen and fall to the ground.

The botanical name for the Cork Oak is Quercus suber.  The genus name, “Quercus”, comes from the Latin word for oak trees; the specific epithet, “suber”, is the Latin word for cork.

This remarkable tree is native to Portugal, Spain, and Northern Africa.  It has been increasingly planted here as an ornamental tree since it is perfectly adapted to our Mediterranean climate.

In our area, successfully growing a Cork Oak is like growing our own native Coast Live Oak.  Once established, each species can easily survive on normal rainfall.  In a severe drought, they both benefit from additional irrigation.

However, for both, it is critically important to keep the area around the trunk dry - and to keep irrigation for a mature tree at least four to six feet from the trunk.  This is necessary because both species are susceptible to “oak root fungus” (Armillaria) and to “root crown fungus” (Phytophthora).  These are ubiquitous soil-borne fungal diseases that can damage roots and trunk bases - and are often fatal to the tree.

Like Coast Live Oak, Cork Oak is also prone to be completely stripped of all its leaves, over the course of just a few days, by a horde of oak moth caterpillars with voracious appetites!  But, do not, in your despair, cut down your naked looking tree.  This defoliation will not significantly harm an otherwise healthy tree - and may provide some surprising benefits - by removing old spent leaves and by providing fertilizer!  A healthy tree will bounce back quite soon enough with luxurious new foliage.

Cork Oak has been planted in Santa Barbara for at least 160 years.  Several exceedingly old specimens can still be found; the oldest is thought to have been planted in 1857 and still stands behind the Winchester-Trussell Adobe at 417 W. Montecito Street.  Two trees over 140 years old can be seen: one in the 200 Block of Natoma Avenue; and, one in front of the Sexton House on Hollister Avenue in Goleta.  Younger trees are located on Orella Street, on the west side of Pershing Park, in the City parking lot on the 500 block of Anacapa Street, and on the 100 block of Garden Street.