By Canopy Team on October 2, 2017
We’re making it a priority to share the most important tree care information as it relates to our oaks. They are adapted to our climate with hot dry summers and cool wet winters, yet the average residential yard does not match these conditions.
We can keep oaks thriving longer if we follow a few critical tree care instructions.
“Every lot or homesite which boasts an oak tree is worth more than one without trees. Property owners need to be educated to…realize that it pays to care for the health of their trees as they would for their own well-being.”
– Fredrick Law Olmstead, landscape architect of Central Park and Stanford University, Palo Alto Times, June 22, 1922
The area within 10 feet of the trunk of a native oak should remain undisturbed and clear of any vegetation. Competing plants such as lawns and ivy can trap moisture around the root crown, creating the perfect environment for fungus.
Ideally all competing plants within the drip line should be removed. However, you may plant drought-tolerant species that don’t require summer water, at least ten feet from the base of the tree. Laying mulch around the trunk (but leaving the root crown exposed) can help keep ivy and other competing plants from returning.
Mature native oaks—Valley Oak, Coast Live Oak, and Blue Oak—do not require and do not tolerate irrigation in the dry months. Ideally no irrigation should be applied during the dry summer months to the area extending from the base of the trunk out to the tree’s drip line.
If irrigation is applied near their trunk during the dry season, the Oak Root Fungus (Armillaria) will grow due to the combination of warmth and moisture. Do not water or allow water to collect around the root flare and do not allow sprinklers to spray on the trunk.
For recently planted and young oaks up to age five, visit our Tree Watering Guidelines page for tips on watering your tree.
The California Oaks Foundation explains that mature oaks need little or no fertilization. The leaves that fall from the oak should be allowed to remain under the tree, where they eventually break down and supply nutrients. However, if those leaves are removed, “light fertilization may be appropriate in landscaped situations to replace nutrients” that would have been provided by the leaf litter. Note that fertilization should only be done if the growth of the tree is poor and only in late winter or early spring.
Any necessary pruning should be performed by an ISA-certified arborist during the winter dormant period for deciduous species or during July and August for evergreen species.
According to the California Oaks Foundation, “when an oak tree shows yellowing leaves, one thinks it lacks nutrients. Generally, this is not the case. More likely, the tree is suffering from root or crown fungus.”
Contact an ISA-certified arborist for an inspection. An arborist should also be consulted if you see signs of decay in the wood, or clusters of mushrooms growing from or next to the trunk.
California Oaks Foundation’s care of California’s native oaks