Tanoak is an evergreen hardwood tree or shrub with thick, leathery, oblong leaves 4–10 cm long that have pointed tips and parallel, evenly spaced side veins running from the leaf’s central vein to its edge and often ending in a pointy tooth at the margin. New leaves are covered with reddish-brown hairs that turn whitish as they mature. Older leaves are a smooth green on top, and a lightly fuzzy gray-green below.

The light reddish-brown bark develops deep fissures as the tree ages. Flowering catkins bloom in late spring to summer. The acorns are from 2.5–5cm long and grow singly or in clusters. Tanoak acorns have a prickly cap, rather than the scaly caps of the true oaks. 

Stewardship Considerations 

Tanoak is highly susceptible to Sudden Oak Death (SOD, caused by the pathogen Phytophora ramorum). The disease can infect and kill all sizes and ages of tanoak—seedlings, saplings, and mature trees. SOD infects trunks, branches, twigs, leaves, and leaf petioles (the slender stems that support and attach the leaves). Tanoaks can be infected without showing cankers or bleeding symptoms, making diagnosis difficult. SOD-infected soil can be transported on tools, tires, or shoes. Pruning during summer decreases the risk of transmission. Follow guidelines for preventing SOD. Do not broadcast chips, especially in areas where oaks are present. Clean equipment before and after working with tanoak.

Tanoak sprouts vigorously after burning or cutting. Most sprouts arise from lignotubers beneath the ground.

Bottom left: Tanoak with Sudden Oak Death (P. ramorum) infection. External bleeding symptoms on a tanoak trunk are not always visible in tanoak SOD infection.

Bottom center: Tanoak acorns and flowering catkins.

Bottom right: New and mature tanoak leaves.

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Learn more about tanoaks from the California Native Plant Society