The coast live oak is a beautiful tree native to California. To help our oak trees, we landscaped with California native plants and spread wood chips as mulch. A disease called Sudden Oak Death (SOD) has spread through the San Francisco Bay Area, and it kills coast live oaks. There’s a risk that SOD can be spread by bringing in wood chips, so we are exploring other mulch options.
The coast live oak, quercus agrifolia, is an evergreen tree growing to 10 to 25 meters tall. A native plant, the coast live oak has adapted to the long, dry summers of our Mediterranean climate. However, if oaks are watered throughout the summer, they can get crown and root rot, which can shorten their life, so we landscape with native plants that require only occasional summer water.
When we landscaped the yard five years ago, we spread wood chips as a mulch to suppress weeds, add nutrients, and reduce evaporation. The original wood chips have gotten much thinner and need renewal.
We attended a SOD talk and a treatment class by researchers at the University of California at Berkeley. We learned that the SOD pathogen, phytophthora ramorum, can be spread by wood chips and sawdust. The pathogen needs warmth and moisture to become active, but the pathogen can live in dry wood until favorable conditions arise. Generally, when a coast live oak is infected, it dies, so the consequence of an oak being infected by SOD is severe.
We get wood chips from arborists who prune and chip trees. Unless the arborist is pruning and chipping our trees, we don’t know the identify of the trees being chipped, or whether the trees are infected. In the Spring of 2011, volunteers sent in samples that were tested for the SOD pathogen, and the results were published. SOD is widespread in the mountains near us, but trees in the valley floor are generally not infected. In 2011 SOD spread rapidly due to a wet spring, and 97% of the trees tested in the mountains near us were infected.
“Experts predict as many as 90 percent of California’s live oaks and black oaks could die from the disease within 25 years.” With the severe consequence (death) if an oak is infected, we don’t want to risk infection by importing wood chips or host plants that might carry the SOD pathogen.
We are trying approaches to mulch the oak leaves. Coast live oaks are large and drop a lot of leaves each year. The leaves are thick and stiff, so they don’t not decompose easily. Dry leaves are brittle and will break when stepped on. We’ve tried stepping on leaves or crushing them with gloved hands. Oak leaves have sharp thorns on leaf edges. This manual approach is very slow. We’re considering purchasing an electric leaf mulcher to speed the process.
In summary, people with coast live oaks should consider risks of SOD when bringing in wood chips for mulch. Mulch is valuable, so we’re exploring approaches to mulch our oak leaves as an alternative to wood chips.