Native oaks don't like to be watered in the summer, yet you may have heard of an oak somewhere that has lived happily in a lawn for decades. That's what consulting arborist and horticulturist Deborah Ellis calls the George Burns syndrome: all his adult life he smoked heavily and drank excessively, but he lived to 100 years. How come? A genetic quirk.
More commonly, water in the dry season will weaken a native oak and make it susceptible to pests and diseases that may eventually be fatal. Other abuses include disturbing the root zone, covering roots with excess soil or rocks, removing the leaf litter, pruning excessively, fertilizing unnecessarily, or spraying for oak moths whether they need it or not.
In a talk to the Gardening with Natives group last month, Ellis discussed the dos and don'ts of oak care and dispelled some myths.
Myth: Oaks need to be thinned or cleaned every year or two.
Fact: Necessary and beneficial pruning, Ellis said, comprises removing broken, dead, diseased, and dying branches, reducing end weights, and fixing specific structural problems such as codominant stems. Oaks do not need to be thinned, and unnecessary pruning shortens their lives and makes them more susceptible to decay and failure. Ellis said the “lion-tail” pruning commonly done, where interior branches are removed and foliage is left only at the very tips of branches, is especially detrimental. To maintain healthy trees, keep as many low branches as possible. A low center of gravity, as well as weight concentrated near the main trunk, makes a safer tree.
Myth: Irrigating a young oak makes it tolerant to irrigation the rest of its life.
Fact: All oaks need some irrigation during their first two to three dry seasons, until the root systems are established. After that, irrigation is detrimental.
Myth: Valley oaks can be planted in lawns.
Fact: Valley oaks are as susceptible to excess water as coast live oaks.
Myth: Keeping any irrigation or disturbance outside the dripline of a mature oak is far enough.
Fact: It's a judgment call, and it depends on the individual tree, the species, and the macro- and microenvironment. Farther is better, Ellis said, and a 20- to 40-foot radius would be great. Another rule of thumb is to measure the tree's diameter at a height of about 4 feet, then triple the diameter. Avoid any root disturbance or irrigation within this distance (3 x diameter) in every direction to protect the buttress roots. Anything besides leaf litter (or, at first, up to 3 or 4 inches of mulch) counts as a disturbance – piles of construction materials, dog runs, driveways, garden paths, planting beds.
Myth: Oaks need special care.
Fact: Less is more, Ellis said. The simplest and cheapest solutions are the most effective. Leaf mulch from the tree is the best ground cover, fertilizer, and soil conditioner. It enhances root growth, encourages earthworms, improves the soil, and provides habitat for spiders and other beneficials.
© 2007 Tanya Kucak