Planting Oaks
By Denise Levine, U. C. Master Gardener
To conserve water, many Napa Valley gardeners select native plants for their gardens. California oaks have evolved root systems that take advantage of seasonal winter rains and survive our usual summer drought conditions.
Young oak trees will grow rapidly from seed in Napa Valley gardens and are more likely to adapt to some watering and other hazards of development than their older counterparts.
To start an oak tree, find the heftiest, healthiest acorns you can. Choose only freshly fallen acorns that are greenish and shiny and just beginning to turn brown. Acorns start to turn yellow when they get old and won't sprout as easily.
Throw away any acorns with holes and check a few by cutting them open to make sure they are firm with white meat inside. The acorns that you pick up in your area are from the oak species that will grow naturally there. Always remove those darling little caps they wear. They won't need them.
You can plant acorns directly in the ground. Find a site where your oak can grow undisturbed. If you have hardpan, break it up so the taproot can develop properly. Young seedlings develop a taproot rapidly. This root anchors the seedling and supplies it with nutrients and moisture during its first few summers. Although a seedling can survive without this taproot, the tree's drought tolerance will be greatly increased if its root system develops undisturbed. For a garden requiring little irrigation and maintenance, this is essential.
Plant soon after the acorns fall to take advantage of winter rains. Bury the acorns tips down one to three inches below the surface, placing six to eight in a 10- inch circle. Don't plant too close to your home or walkway, or where a mature tree might block a cherished view.
When the seedlings are two to six inches tall, select two or three of the strongest to develop and remove the others. Thin again the next year and leave only one tree to develop. Oak trees need lots of room and can grow two to three feet a year.
In the wild, only a small percentage of germinated acorns develop into trees. To improve this survival rate, irrigate the seedlings during their first two or three years. Seedlings in sandy or sandy-loam soil may need irrigation once a week during the first summer, once every two weeks during the second summer, and once a month in subsequent low-rainfall years. Check your soil moisture with a soil auger or shovel.
For rapid growth and deeper root development, make a basin around the tree that will hold about 10 gallons of water. The basin rim should be thick so it will not crack. After the taproot is well developed, you will not need to irrigate.
Fertilizing an oak seedling during its first year can burn tender roots, so don't do it. If you want to speed growth after the first year, sprinkle one of the commonly available lawn fertilizers on top of the soil in early spring. Be sure it does not contain any weed killers or herbicides. Water deeply so the fertilizer has a chance to penetrate the soil and reach the tree roots.
If you planted your oak in a turf or lawn area, keep a 30-inch ring of bare soil around the tree's trunk to protect from too much moisture and injury from garden mowers.
By nature, young oaks grow twiggy, and every twig takes energy. To promote fast vertical growth, shorten unwanted lower branches, leaving a strong scaffold of branches to grow with angles of attachment of 45 to 65 degrees. Retain as much leaf surface as possible to encourage your oak to grow as quickly as possible.
Young oak trees need to be protected from gophers, jackrabbits, deer, pets and other threats. Small temporary fences can protect your longterm project until the terminal or top bud is out of reach (about five or six feet tall).
Oak trees are a wonderful part of the Napa Valley landscape. While sudden oak death and oak root rot take their toll on some older trees, it is reassuring to remember that trees are a renewable resource. Plant an oak tree this year.