Oak Trees

Why are oak trees protected by law?
In Los Angeles County, any native oak tree over 8 inches in diameter 4.5 feet above grade is protected by law and cannot be cut down without a permit. Los Angeles was one of the first counties in the state to enact an Oak Tree Protection Ordinance in 1982. The idea was to recognize the unique role oaks play in the ecosystem. Oaks are a keystone species, functioning much like the hub of wheel. Over 5,000 species of insects, 58 species of reptiles and amphibians, 105 species of mammals and over 150 species of birds rely on oaks for some of their life cycle. If the oaks are gone, so are these associated species. In addition, oaks are held dear by many people, admired for their beauty, strength and longevity.

What is my oak worth?
Well, that all depends on its size, condition and location. The International Society of Arboriculture has a standard for establishing the value of a tree, and oaks by virtue of their role in the ecosystem, rank fairly highly. To determine the value of an individual tree requires the assistance of a trained arborist.

Another way to think about the value of an oak is based on the benefits it provides. A study done by American Forests in 1985 found that each mature tree provided $275 worth of real economic benefits each year. These benefits are avoided costs for stormwater runoff control, groundwater infiltration, temperature moderation, air pollution reduction, and carbon sequestration. A mature tree located on the west side of a house significantly reduces summer air conditioning costs.

Finally, in the Santa Monica Mountains, homes having mature oaks typically sell for up to 30% more than homes without them.

How come there are lots of acorns some years, and few others?
The variability of acorn production has been a puzzle for many years. Walter Koenig, of the Hastings UC Reserve has spent years trying to figure it out. After looking at many variables like amount of rainfall, size of tree, etc., he thinks that the average temperature during the month of April is the key.

Why can't I irrigate under my oak tree?
Oaks have evolved to live in the Mediterranean climate of the region, with wet winters and dry summers. When water is introduced into the root zone of the tree during the warm summer months, the root pathogens living in the soil grow vigorously and attack the oak roots fiercely. The tree is always fighting off these pathogens, but summer irrigation gives the rots the upper hand, and the tree begins a long, slow decline. Often it takes many years for the effects of the root rots (mostly Armellaria sp. and Phythophthora sp.) to become severe enough to kill the tree. Unfortunately, there is no simple way to stop the root rot once it is firmly established. Remedies that encourage root regeneration and increase the overall vigor of the tree are needed. The best strategy is to avoid placing the tree at risk.

When is the best time to prune my oak tree?
During the rainy season before the new shoots emerge in the spring is the optimal time for pruning your tree. During the hot summer months, the tree is under tremendous water stress, and pruning at that time can be damaging. When it is really hot, the interior leaves do most of the work photosynthesizing, while the exterior leaves exposed to direct sun shut down for parts of the day in order to reduce the amount of water lost to transpiration. Deadwood can be pruned at any time of year.

The rule of thumb for pruning is to remove no more than 15% of the living leaves. Less is definitely more when it comes to pruning. The leaves produce the food for the tree. Removing large portions of the canopy at once is a severe stress on the tree, forcing it to use stored energy reserves to replace the lost foliage. The tree will often sprout twigs from large branches (call sucker sprouts, epicormic growth) in an effort to generate as much leaf surface as possible. If a tree has been severely pruned, then it might be necessary to leave these sprouts to grow for awhile until the outer canopy recovers. 

The best pruning job is so subtle that you can't even tell the tree was cut! Unfortunately, examples of bad pruning are much more common than good examples, and having trees trimmed is expensive. Resist the temptation to get your money's worth by trimming too much at once! It really pays to do a little at a time.

What can I plant under oak trees?
The key to planting under oaks without killing them is to use species that have the same water requirements, i.e., little to no summer irrigation. This means grass is OUT! The best ground cover with the canopy of the oaks is mulch, the fallen leaves and twigs from the tree. These decompose and release nutrients back into the soil, moderate the soil temperature, reduce stormwater runoff and support a complex soil foodweb that is beneficial to the tree. Competing plants with different irrigation needs destroys this system, and root pathogens infect the tree.

A good strategy for obtaining seasonal color under the trees is to use pots that are watered either by drip or by hand. These pots can hold all the thirsty colorful plants like azaleas, begonias, impatiens, camellias, etc. that are a problem for the tree when planted directly in the soil.

What causes the brown patches of dead twigs in oak trees?
Called "flagging" by foresters and arborists, these are the tell tail signs of twig girdlers at work. Often thought of as natures native oak pruners, these insects bore tunnels into the twigs to lay their eggs. This disrupts the movement of water and nutrients to the twig, which dies back. These are native insects and oaks have evolved to live with them. While they look a bit bedraggled, they do not kill a normal, healthy oak.