By Ron Allen, UC Master Gardener, Mariposa County
MARIPOSA — With warm, dry months ahead, many folks around Mariposa County with oak trees by their house or on their landscape might be asking how best to ensure the health of those trees. The short and happy answer is that you do not need to do too much. But you should also not make certain basic mistakes. Here are some guidelines for summer oak tree care.
Do not disturb the ground within 6 feet of the trunk; the roots within this area are particularly sensitive. Keep all other plants, even in pots, more than 6 feet from the tree. Oaks have surface root systems that extend out about 30% farther than the tree’s dripline. In this critical root zone (CRZ), don’t add surface fill, adjust the grade, or allow surface structures (such as retaining walls) that trap 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 the sensitive surface roots, avoid driving or parking vehicles upon the CRZ.
It is a mistake to water established oak trees in the summer; this promotes the growth of fungi that can hurt or kill the tree. The blue oak, in particular, has the habit of dropping its leaves early during very dry periods. This is a survival mechanism for times of drought, and the tree should not be watered.
If possible, postpone pruning until cold weather. It is OK to prune dead and dying branches as needed, but limit any pruning of healthy branches to those at most one inch in diameter. Never prune more than 20 percent of the oak canopy at one time.
A good cut leaves the branch collar intact (left), but a poor cut (right) removes some bark from the trunk on the left and leaves a stub of branch wood to the right.
Don’t clutter the ground under the tree with other plants. It is OK to apply a thin layer of coarse mulch, such as oak or pine chips, an inch or two deep beneath the tree. This keeps the roots cool, suppresses competing weeds, and replicates the protective layer of oak leaf mulch that the trees establish in nature.
Follow these practices and your household oak trees will do well during hot months. Trees out on the landscape can generally be left alone. They have been there a long time, and without any intervention, they will be there a lot longer.
UC Master Gardeners of Mariposa County are located at 5009 Fairgrounds Rd., Mariposa. For more gardening and event information, visit our website at http://cemariposa.ucanr.edu/Master_Gardener and Facebook page (UC Master Gardeners of Mariposa County).
UC Master Gardeners staff a helpline serving Mariposa County, including Greeley Hill, Coulterville, and Lake Don Pedro. Please contact us at 209-966-7078 or via e-mail at firstname.lastname@example.org.
Listen to us on the radio at KRYZ 98.5 FM on Wednesdays at 2 p.m and Saturdays at 5 p.m.