Oak trees are an integral feature in California. Numerous cities are named for them, such as Oakland, Del Rey Oaks and Oakdale. Acorns from the oak trees were a main source of food for Native American tribes, who ground the seeds into flour and made various foods from the paste. Later, early settlers in the region found that acorns also made good hog feed. The hardwood makes beautiful, strong furnishings and other implements. Oak burns slowly and makes hot coals, perfect for cooking over a fire. Several specific characteristics distinguish the highly useful oak tree from other types of trees in California.
Examine the shape of the plant. Oaks can grow as shrubs or trees. In both cases, California oaks grow about as wide or wider than their height, often with branches that extend as low as the ground before bending upward again.
Check the ground and branches for acorns. These are the most easily identified part of an oak tree. Acorns change from green to brown as they dry and drop from the tree. They are ovoid in shape, with a detachable woody cap where the acorn connected to the branch. Oak trees all produce acorns, but may mature for 20 to 50 years before the first flowering. The absence of acorns therefore doesn't rule out the possibility that the tree is an oak.
Examine the branches. Don't discount shrubs; some of the 20 or so varieties of oak native to California grow like bushes close to the ground. Oaks are often listed as deciduous, but a few California oaks are evergreen. In both cases, oak leaf stems grow out of the branches in a spiral pattern.
Look closely at the leaves. California oak leaves come in a huge variety: shaped like spear heads, rounded, branched, glossy and smooth, dry and rough, and two different colors for the top and bottom. A single tree can even sport a few different shapes of leaves. Their only commonality is that California oak leaves grow on short stalks from the branch and tend not to be completely symmetrical in shape.
Check a field guide of California oak varieties, using the shape and color of the leaves and acorns for help in identification. Generally, pictorial guides divide the oaks into tree and shrub subsections. For instance, a coastal shrub oak has narrow acorns ending in sharp points, and small, serrated dark-green grayish leaves. The island oak tree is characterized by large, long, smooth-edged leaves, green on top, brownish below, with small round seeds.