Treating Drought-Stressed California Oaks

While California’s oak trees are well adapted to survive the state’s drought, the right approach to supplemental irrigation is critical to prevent pests from taking advantage of drought-stressed oaks.

Oaks will reallocate resources to conserve water to sustain basic physiological functions. But that can lower the trees’ defenses against disease and insects, including borer colonization.

Trees showing foliage discoloration, leaf loss, and burned leaves should receive supplemental irrigation to mimic the rain associated with winter storms by watering later into the spring and again earlier in the fall. The following are basic tips for supplement irrigation:

  • The ground beyond the tree’s dripline and outside the canopy should be slowly soaked for several days every few weeks to saturate to the root zone. Wetting and drying should occur until the tree’s crown recovers and appears healthy.
  • Watering may occur into June or July, then slow or cease in August through September. Watering can resume in late October or November. If trees continue to appear stressed, an arborist can determine if other factors are affecting the tree.
  • A 12-inch-high, firmly-packed soil berm should be built 2 to 3 feet out from the trunk to keep the ground dry immediately around the trunk. Moist, warm soil increases the risk of root, crown, and collar rots caused by fungal pathogens.

For more information contact Michael Huff, Dudek’s urban forestry practice manager, at, or Robert Mackie, a Habitat Restoration Sciences landscape supervisor, at