Care and Maintenance of Oak Trees: Summer Practices

By Ron Allen, UC Master Gardener, Mariposa County

MARIPOSA – With warm, dry months ahead, many people around Mariposa County with oak trees near their homes or in their landscape might be wondering how best to ensure the health of those trees. The short and happy answer is that you don’t need to overdo it. But you shouldn’t make some basic mistakes either. Here are some guidelines for caring for summer oaks.

Do not touch the ground within 6 feet of the trunk; the roots of this area are particularly sensitive. Keep all other plants, even in pots, more than 6 feet from the tree. Oak trees have surface root systems that extend about 30% further than the tree’s drip line. In this critical root zone (CRZ), do not add surface fill, adjust grade, or allow surface structures (such as retaining walls) that retain water. A few low-water native California plants are OK in the CRZ, but keep them six feet from the trunk. Because it compresses the soil and harms sensitive surface roots, avoid driving or parking vehicles on the CRZ.

It is a mistake to water established oak trees in the summer; this promotes the growth of fungi that can injure or kill the tree. Blue oak, in particular, has a habit of dropping its leaves early during very dry periods. It is a survival mechanism in times of drought, and the tree should not be watered.

If possible, delay pruning until it is cold. It is acceptable to prune dead and dying branches as needed, but limit pruning of healthy branches to those no more than one inch in diameter. Never prune more than 20% of the oak canopy at a time.

A good cut leaves the branch collar intact (left), but a bad cut (right) removes some bark from the trunk on the left and leaves a piece of branch wood on the right.

Do not clutter the ground under the tree with other plants. It is acceptable to apply a thin layer of coarse mulch, such as oak or pine shavings, an inch or two deep under the tree. This keeps the roots cool, suppresses competing weeds, and replicates the protective layer of oak leaf mulch that trees establish in the wild.

Follow these practices and your domestic oak trees will do well during the warmer months. Trees on the landscape can usually be left alone. They have been there for a long time, and without any intervention, they will stay there much longer.

UC Mariposa County Master Gardeners is located at 5009 Fairgrounds Rd., Mariposa. For more gardening information and events, visit our website at and Facebook page (UC Master Gardeners of Mariposa County).

UC Master Gardeners has a hotline serving Mariposa County, including Greeley Hill, Coulterville, and Lake Don Pedro. Please contact us at 209-966-7078 or by email at [email protected]

Listen to us on the radio on KRYZ 98.5 FM on Wednesdays at 2 p.m. and Saturdays at 5 p.m.