Those of us who reside in the foothills are blessed to be in the presence of so many ancient and beautiful oak trees.  They provide us with cool shade (very important during our current heat wave) clean our air and give us something graceful to study and wonder about.  Oak trees have historically been used for lumber, fuel, feed, medicine, tanning and dye production.  A little closer to home is its use in the wine industry – specifically, barrels and corks – and how about that oak wine rack the bottles are stored in?  Our lives are affected daily by oak trees and the products we receive from them; have you ever thought about how our community affects them in return?  As development extends farther into the foothills, respect and proper care become necessary to support the population of oak trees, ensuring that our future generations will be able to benefit from our current variety and quantity of these majestic trees.

The genus (family) of oak trees is named Quercus (Latin for "oak").  There are over 600 different members in this family, all unique in their appearance and location.  Our foothill woodlands are naturally dominated by blue oak trees existing in a forest cover zone defined as "Blue-Oak – Digger Pine."  The blue oak is also known as iron oak, mountain white oak or mountain oak.  In stature it is short, averaging 30 feet, but ranging from 20 to 66 feet.  The canopy is rounded and open with many crooked branches covered in twiggy growth.  Thin, scaly, light-grey bark gives way to bluish-green leaves that are 1-3 inches in length and have smooth to lobed edges.  The thick leaves are blue-green above and yellow-green below, making identification possible. Its scientific name is Quercus douglasii in honor of Scottish biologist, David Douglas, who named it in 1831 after its blue-green foliage.  Blue oak is one of the largest ancient forest types in California, covering about 3 million acres.  It is neighbor to black oak and Ponderosa pine at higher elevations and valley and live oak at lower elevations.
 
What makes this oak tree so unique is its natural ability to thrive in our harsh foothill conditions.  As most backyard gardeners are well aware, our soil here is neither fertile nor easy to cultivate.  Seldom will you find blue oak in deep, fertile soil because it cannot compete with the taller pine and more aggressive oak species.  Instead, it specializes in undesirable areas; blue oak is well adapted to dry climates, grass fires and drought.  It can survive temperatures above 100° F for several weeks at a time with no supplemental irrigation.  Fatal drought stress is resisted several ways by different parts of the tree and its growth habit:

Leaves:  A thick, waxy layer is produced on the top surface of the leaf, reducing water loss and providing the bluish cast.  If the heat continues for too long, the tree will protect itself by dropping all its leaves and entering early dormancy; thus, conserving moisture.

Roots:  The root system is extensive, growing through fractured rock to a depth of 80 feet or more to locate groundwater reserves.  As much as 50% of the soil surface may be covered with rock or stones.  Blue oak acorns germinate earlier than those of other species; all available energy is initially channeled toward root production, resulting in 73% of the tree's weight existing underground after the first growing season.

Growth rate:  Blue oak is one of the slowest growing oak trees.  Many mature trees are less than 25 feet tall and may require 50 to 100 years to obtain a trunk diameter of only 10 centimeters.  The growth rings on a stump are so tightly packed that it's easier to lose your place than attempt to count them all.

Unfortunately, the trait that has made this tree a survivor is now its main cause of death.  Excess irrigation is the leading killer of established oak trees in the landscape.  Please, if you are fortunate enough to have a blue oak, do NOT give it extra water.  Summer irrigation activates a large amount of decay-causing fungi that exist in our soil and helps them attack the roots and base of the tree.  For this same reason it is best not to landscape underneath your trees; instead, simply leave the existing native grasses.  If you desire a more polished appearance, spread mulch or wood chips, keeping the layer away from the tree trunk and root flare and disturbing the soil as little as possible.