Valley Oak is considered the monarch among all oaks by virtue of its size, age and beauty. Descriptions of its remarkable stature appear in the diaries of many early visitors to California. A member of the Beech family, Quercus lobata is the largest of all oak trees reaching heights of more than 100 feet with trunks measuring 6 or 7 feet in diameter.

This magnificent tree is limited to California and is quite possibly the largest North American oak, making it very unique. Fertile soils, spring rains and intense sunlight produce the perfect conditions needed for the valley oak to achieve rapid growth. Many trees with trunks 3-4 feet in diameter are relatively young- only 150 to 250 years old.

Valley oaks are opportunists, meaning that they have mechanisms in place to allow them to live in environments which typically do not favor many other plant species. They do not require excessive water, but having the means to access a water source is favorable.

A young valley oak can have a taproot up to 60 feet deep as it searches for ground water in a landscape characterized by seasonal droughts. As the tree matures, the tap root sloughs off and the tree develops a tiered root system 2-4 feet below the soil surface. This allows the tree to avoid, rather than endure drought conditions.

Leaves and Flowers

Valley oaks have an easily identifiable leaf shape that we often associate with oaks. The dull green, pubescent leaves are alternate, simple and 2 to 4 inches long; margins typically have 9 to 11 deep, rounded lobes and the bottom of the leaf is a paler green.

Valley oaks are deciduous, meaning they lose their leaves in the fall. When spring arrives the branches are covered with bright green leaves to provide a dense canopy of shade during the long, hot summers common in the central part of our state.

Flowering for the valley oak is timed by temperature change as cold nights yield to warmer spring days and plant hormones known as auxins promote bud burst by increasing the tree’s intake of water. 

Male flowers are visible by the yellowgreen pendent like structures (see photo at right), 1 to 2 inches long and female flowers are borne on leaf axils of current year’s twigs.

The resulting acorns form long, conical shapes making them one of the longest fruits with warty knobs on the caps.

Acorns provide both food for wildlife and the means for regeneration of this disappearing species. 

While younger bark is thin, gray and has a checkered or fissured appearance, older bark is thicker, usually up to several inches thick and deeply fissured with flat- tened ridges making it rough to the touch. 

Bark and Oak Apples

One unusual characteristic of the valley oak is the appearance of conspicuous brown balls, as big as golf balls, often noticed among the foliage.

These are galls, popularly known as oakapples, that result from gall wasps depositing eggs at the base of vegetative buds along the stems. Stimulation of plant hormones cause the protective growth around the developing larvae.

Look around the base of these trees on a sunny day and you may find jumping galls. These tiny creatures use the same strategy as the Mexican jumping bean, namely to reach shelter from the sun, they jump until they land in shade.  

As America’s largest oak, this tree is excellent for shade and loved by birds and wildlife. Native Americans also used the acorns as food, and the tree as firewood and charcoal. Unique to California, this oak is also one of California’s most familiar icons. Ironically, the valley oak has also been more impacted by human activities than any other oak in the State and recent research indicates that the species is in demise over much of its range.

Protect & Enjoy

Protecting our native trees requires our active partnership and contribution to the common goal of resource management. Oak, sycamore, and bay trees hold a unique place in our local ecosystem and provide habitat for 81 species of resident and migratory birds.

Urbanization continues to present a challenge within the sustainable capability of the ecosystem. Open spaces, clean air and homes for wildlife are worth protecting, but will demand support for the native trees existence in the local landscape.

Trees are living resources and assure healthy living habitats for interconnected flora and fauna, and the people that live within these habitats. To ensure that our trees will survive and thrive, please protect them by following these suggestions:

• Do not injure the trunk with objects.
• Keep all machines and weed whips away from tree trunks.
• Practice conservative pruning.
• Do not over water.