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UC Cooperative Extension Ventura County
669 County Square Dr. Suite 100
Ventura CA 93003
Phone: 805.645.1451
Fax: 805.645.1474

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Living with Oaks

There are indeed some special factors to keep in mind when homeowners have established oaks on their property or are considering incorporating oaks into an existing landscape.
There are twelve species of oak (Quercus sp.) that are found on the Central Coast. Some, the “live oaks,” are evergreen. There are also “valley” oaks that are deciduous. Both of these groups develop a tree form as they grow. Finally, there are also “scrub” oaks, which develop a shrub form in the landscape. Research has shown that within these groups there are ecological niche variations. What that means is even within a species, groups of trees/shrubs are specifically adapted to local environments. I mention this because if you buy an oak to place in your landscape, it is best if you obtain a plant that has been grown from a local seed source.
All species of oaks on the Central Coast have evolved in what is called a Mediterranean climate. That means they grow and develop best under a regime of cool and rainy winters and moderate temperature and dry summers. These facts are important because within the oak ecosystem there are a number of organisms that live with and on oaks that also require the Mediterranean climatic regime to maintain an appropriate relationship with the oaks.
There are fungi that can attack oaks. These fungi are especially sensitive to changes in the environment of the oak. The first and most important one is Armillaria mellea. This fungus causes a disease also known as “oak root rot,” “Armillaria root rot,” “Honey mushroom fungus,” and “Shoestring fungus.” This organism has evolved with California oaks. It is a true parasite, deriving its nutrition from oak trees and other species it attacks. If oaks are grown in a moist winter, dry summer environment, the oaks and the fungus live in balance and each survives. Unfortunately, often times when we place oaks in a landscape, we place them in an environment with lots of summer moisture (irrigation), or we convert their environment to a summer moisture regime. When this is done, the Armillaria fungus can become an aggressive, destructive pathogen. Many California oaks have been killed by this fungus in summer irrigated landscapes.
I have seen many landscapes that are designed around the existing California native oaks. How these landscapes are designed and irrigated is very important to the long-term preservation of the oak trees. Some people have done it right and others have done it very wrong. Death of the oak trees often occurs in the latter situation.