Winter 2025

Protecting Our Local Oaks

By David George

The native oak trees that grace our Contra Costa County gardens face constant threats from habitat loss, imported diseases, nasty pests, and the effects of climate change. These threats destroy thousands of local oaks each year. However, we can all play a vital role in protecting these magnificent trees from destruction and ensuring they thrive in our region for generations to come.

Loss of Natural Habitat

We are fortunate to have five local varieties of oak because of our varied Contra Costa microclimates. Each of these five varieties has distinct needs. The evergreen coast live oak (Quercus agrifolia) grows best in cooler coastal conditions such as west Contra Costa and the East Bay hills but also tolerates shadier inland locations. Massive valley oaks (Q. lobata) love warmer inland locations where summer heat fuels growth. Their lateral spreading roots seek reliable underground water sources, such as stream beds or shallow water tables in alluvial plains near rivers. Black oaks (Q. kelloggii) can be found on north-facing slopes in the East Bay hills, where soil retains water longer into summertime. Hot weather-loving blue oaks (Q. douglasii) and interior live oaks (Q. wislizeni) populate the slopes of Mt. Diablo and Central Valley foothills. Blue oaks have adapted to dry conditions by going dormant in mid-summer if soil dries out, while interior live oaks halt growth and retain water with a thick waxy coating on their leaves.

The availability of water is key to these oak varieties and is also one of their threats. They adapt to dry conditions but cannot tolerate wet soil which can cause root rot, especially in summer months. To protect them in your garden, keep irrigated water to a minimum under tree canopies (“drip lines”), remove excess soil or mulch from their root crowns, plant low-water-use landscaping under them, such as California natives and succulents, or leave the tree’s drip line area unlandscaped.

Diseases

Oak root rot is caused by a fungus (Armillaria mellea). The fungus grows into the tree’s root system when soil is excessively wet for extended periods. It is the most common cause of oak tree decline and death in our area and can spread to other plantings around infected oak roots. It disrupts the flow of water and nutrients through the roots. Water sparingly around oaks to prevent it.

Sudden oak death (SOD) is caused by an introduced non-native pathogen, Phytopthora ramorum. It has grabbed headlines by killing over a million oaks in large tracts throughout coastal central and northern California and has now spread to coastal Oregon also. Bay laurel trees, tanoaks, rhododendrons, and camellias are common host plants for P. ramorum (they become infected but survive), so remove those species near your oak tree and also avoid planting cultivars of these species near oaks. Don’t aid the pathogen’s spread by relocating or harvesting limbs for firewood from host trees within affected regions.

Mistletoe (Phoradendron flavescens) is a hemiparasitic plant that grows on a wide variety of host trees but primarily infects stressed oak trees in our local region. Hemiparasitic means it creates its own food through photosynthesis but obtains water and nutrients from the host tree. Over many years, this theft can cause an oak to lose vitality and eventually kill it. Mistletoe is toxic to humans, especially its berries. To combat mistletoe, make sure the oak has sufficient (but not too much) water and full sunshine. If you can reach it, periodically prune back mistletoe plants to the branch bark to slow its growth and reduce its damage, or remove the infected limb at the trunk.

Pests

Oaks are amazingly resilient against pests and in fact, support more beneficial creatures than are threatened by dangerous pests. They are called a “keystone” species because of this essential relationship with the ecosystem. Many common pests in our area such as aphids and oak moth caterpillars damage or kill oak leaves but do not kill whole trees. With time, oaks will produce a fresh crop of replacement leaves. But several pests can mortally wound oaks, and, if found, deserve professional treatment.

Oak bark and ambrosia beetles attack oaks and certain other broadleaf trees including California buckeyes and tanbark oaks. Like fir tree borers, the adults drill small holes in the bark to lay eggs in or near a tree’s vascular system. Larvae feed on vascular tissue and eventually cut off the flow of water and nutrients to the tree. Adult beetles (dark and about the size of rice grains) chew their way out through the bark and leave more holes. Call a professional if you see sap oozing from small holes in the bark as they can treat the tree and prevent the tree from dying.

The glass-winged sharpshooter is a large leafhopper that feeds on an oak tree’s fluids and nutrients. The pest by itself does not kill oaks, but it can introduce and spread a pathogenic bacterium called Xylella fastidiosa. X. fastidiosa can cause bacterial leaf scorch in oaks. If you notice leaves that look to be scorched by heat but without a heat wave, call a professional arborist to diagnose the pathogen.

Climate Change

Is our region becoming warmer and wetter over time? Many non-native pathogens and pests thrive in warm wet climates, which creates new challenges for local oaks. In the past, frost and freezing temperatures helped kill many disease spores and pests before they could cause real damage to oaks, while dry weather retarded the spread of wet soil pathogens. As these natural defenses diminish, time will tell whether our local oaks are resilient and adaptable enough to survive. The urgency to protect our local oaks has never been greater.

Follow these links for more information about native oak tree care and threats:

How to Manage Pests in Gardens and Landscapes: Oak
https://ipm.ucanr.edu/PMG/GARDEN/PLANTS/oak.html

Oak Tree Management and Pest Resources
https://ucanr.edu/sites/gsobinfo/resources/oak_tree_management_and_pest_resources/

Search results for "Oaks" from UCANR
https://anrcatalog.ucanr.edu/Items.aspx?search=oaks

California Oak Mortality Task Force
https://www.suddenoakdeath.org/about-california-oak-mortality-task-force/