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Caring for Your Oaks

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.

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.
Are there Oak Pests and Diseases I should watch for?

Yes! Current threats to Southern California oak trees include the invasive insects gold-spotted oak borer and the polyphagous shot hole borer. Watch this video to learn more and find out how to identify these pests.

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.
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.

Why should I prune?
  • To reduce fuel available to fire
  • To limit size of the plant (Best to plant the right tree in the right spot so that this is minimal)
  • To enhance plant health
  • To enhance plant structure (Remove crowded, crossed branches, weak branch crotch angles, establish main leader)
  • To remove suckers or water sprouts (But why are they present? Possibly due to previous poor pruning or disease. Check first for problems)
  • To improve or maintain flowering and fruiting
  • For safety-branches overhanging walkways, obstruct vision, low hanging
  • To retrain tree that has been poorly pruned previously
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 shutdown 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.
What should I prune?
  • Deadwood
  • Branches that cross each other
  • Broken or damaged branches
  • Branches rubbing against other trees or structures (The Fire Dept. requires a 10′ clearance from all structures, especially chimneys!)
  • Diseased branches (However, be sure to clean your clippers in solution of 1 gallon water with 3 tablespoons Clorox between cuts so as not to spread disease.)

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 a while until the outer canopy recovers.

How should I prune?

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.

  • For large branches, especially those that cannot be reached from the ground, it is usually best to call a professional
  • For smaller, accessible branches, use sharp tools and make cuts at the nodes
  •  In Los Angeles County, a permit is required to prune living limbs larger than 2″ diameter from native California Oak trees. Permits can be obtained from LA CO. Regional Planning (213-974-6411). The LA County Foresters may waive fees in some situations, i.e. if tree presents a hazard. Call them when in doubt to see if you qualify. (818-222-1108)
For more information, local arborist Gary Knowlton has some great videos on YouTube found at http://treecarepruningandplanting.com.
How do I choose a professional to prune my oaks?

Trees may add as much as 20% to the value of your property. They live a long time if properly cared for. Most trees require minimum pruning or other maintenance when done correctly. They enhance your environment by moderating temperatures, preventing run-off and erosion, providing wildlife habitat and aesthetic enjoyment to your surroundings.

Remember, you get what you pay for! Experienced tree care professionals cost more than a gardener with a chainsaw because they know more!

Be sure to:

  • Ask for evidence of contractors license, certificate of insurance and workers compensation (CA law requires that arborists doing tree work worth more than $350.00 have a contractors license and insurance)
  • Look for arborist certification and training (International Society of Arboriculture or National Arborist Association membership)
  • Clarify in writing what specific work will be done, how cleanup will be dealt with and the fee for services

Beware of:

  • Climbing with spikes-this is only done when a tree is to be removed
  • Topping-this can kill your tree, it can create a hazard by encouraging weakly attached shoots, and it looks terrible!
  • Poorly equipped personnel-safety while climbing trees is critical! Pros have proper saddles and ropes, a person on the ground, and they wear appropriate protective clothing when handling chainsaws or chemicals.