We are blessed with many native oak trees in El Dorado County. They provide shade for our homes, wood to heat our homes and food and homes for wildlife.
The most common species are blue oaks, valley oaks, live oaks, black oaks and canyon oaks.
Blue oaks are very common; they are a medium size deciduous tree with blue-gray-green leaves and pale gray, finely textured bark. They are very drought and fire tolerant, and they don’t like summer water.
Valley oaks used to cover much of the Central Valley, and they are the largest American oak; they are found only in California, usually below 1,500 feet and prefer deep, bottom-land soil.
They like more water than other oaks and are deciduous. The leaves are bright green and deeply lobed, while the bark is dark brown and coarsely textured. The trunks may be 6 to 8 feet in diameter.
The black oak is usually at higher elevations. It has deeply lobed, pointy, bright green leaves 4 to 6 inches long. It is deciduous.
The interior live oak grows like a weed in our county. It is evergreen with variable leaf shapes — smooth edged or prickly. The leaves are dark green and shiny on both sides. It is not fire tolerant, and it grows on very poor, rocky soil. It sometimes has a shrubby form, and trees may have multiple trunks.
The canyon oak is evergreen and looks much like an interior live oak. Leaves are smooth-edged or prickly. They are dark, shiny green on top, but the bottom is dull, grayish-green. The acorn cup has a golden, fuzzy covering.
Botanists realized valley oaks were declining by 1909 because of clearing for agriculture and housing. Early sprouting, non-native annuals compete for water and are known to stunt valley oak and blue oak seedlings.
Cattle trample and eat seedlings. Fires seem to help establishment of both blue oak and valley oak saplings by removing competing grasses and brush. The scarcity of both oak saplings was noticed about 100 years ago, but nobody really knows why there are so few.
There are currently many oak restoration projects. You can see one in progress on the east side of Silva Valley Road in El Dorado Hills.
There are lots of ways that humans affect oaks. Summer water and trenching, paving and grading are harmful.
Blue oaks are especially sensitive to summer water because it causes fungal infection of the roots. Don’t plant a lawn under a blue oak, and try not to trench or change the grade under the drip line of any oak.
If you want to prune an oak tree, be careful. Pruning out dead wood is good. If you must cut off a live branch, make your cut smooth and about ½ inch from the trunk.
Don’t allow the branch to tear off and leave a gaping wound in the trunk. A small branch should be cut close to the larger branch from which it grows. New bark will not cover the pruning wound if you leave a long stub, and fungal infections will invade the trunk of the tree through the wound.
For more information see the “USDA Field Guide to Insects and Diseases of California Oaks,” hastingsreserve.org and “Oaks of California,” Bruce M. Pavlik, et al.
Join Master Gardeners this Saturday, Jan. 21, to learn more about maintaining and protecting these wonderful trees. This free three-hour class will be held at the Veterans Memorial Building at 130 Placerville Drive in Placerville, and will start at 9 a.m.
Master Gardeners are available to answer home gardening questions Tuesday through Friday, 9 a.m. to noon, by calling 530-621-5512. Walk-ins are welcome. The office is located at 311 Fair Lane in Placerville.
For more information about public education classes and activities visit ucanr.org/sites/EDC_Master_Gardeners/.