July 29, 2021 – By Ron Allen (UC Master Gardener, Mariposa County) - Right now we are in the midst of extreme, widespread drought. One telltale indication is that oak trees in Mariposa County are dropping their leaves. What should one do?
It’s a good time to review proper oak tree care during dry spells. You don’t need to do much, but you should avoid basic mistakes. Here are some guidelines.
Avoid disturbing the ground within 6 feet of the trunk; the roots here are particularly sensitive. Keep all other plants, even potted, more than 6 feet away.
Oak surface root systems extend about 30% beyond the dripline. In this critical root zone (CRZ), don’t add surface fill, adjust grade, or allow surface structures (retaining walls) that trap water. Some low-water native California plants are OK, if located away from the trunk. Don’t drive vehicles onto the CRZ; this compresses the soil and hurts the roots.
It is generally a mistake to water established oak trees during dry spells; this promotes fungi growth that can hurt or kill a tree. The blue oak, in particular, drops its leaves early during drought. The tree conserves its water resources with this survival mechanism.
Some studies have shown that native California oaks can benefit from summer watering. But this must be done selectively, carefully, and infrequently for a prized oak tree. Beginning 10 feet from the trunk, water slowly and deeply (to about 3 feet) the rest of the CRZ. Don’t water again for at least a month. Do this only two or three times during the summer.
If possible, postpone oak pruning until cold weather. Prune dead and dying branches as needed, but don’t prune healthy branches over one inch in diameter. Never prune more than 20% of the canopy at one time.
A good cut leaves the branch collar intact (above), but a poor cut (below) removes
some bark from the trunk on the left and leaves a stub of branch wood to the right.
Apply a thin layer of coarse mulch--oak or pine chips--an inch or two deep, beneath your oak trees. This keeps the roots cool, suppresses competing weeds, and replicates the protective leaf mulch layer that oak trees establish in nature.
Follow these practices and your household oak trees will do well during hot, dry 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.
For assistance, contact our Helpline at (209) 966-7078 or at firstname.lastname@example.org. We are currently unable to take samples or meet with you in person but welcome pictures.
Updated: The U.C. Master Gardener Helpline is staffed; Tuesdays from 9:00 A.M. - 12:00 P.M. and Thursdays from 2:00 P.M. - 5:00 P.M.
Clients may bring samples to the Agricultural Extension Office located at the Mariposa Fairgrounds, but the Master Gardener office is not open to the public. We will not be doing home visits this year due to UCANR restrictions.
Serving Mariposa County, including Greeley Hill, Coulterville and Lake Don Pedro
Please contact the helpline, or leave a message by phone at: (209) 966-7078
By email (send photos and questions for researched answers) to: email@example.com
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Master Gardener Office Location:
UC Cooperative Extension Office,
5009 Fairgrounds Road
Mariposa, CA 95338
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