- Author: Denise Godbout-Avant
(From old proverb)
I love to stroll under the shade of majestic Valley oak trees in the oak woodland riparian habitat along the Stanislaus River at Caswell State Park in Ripon. I visualize Lakisamni Yokut women (the indigenous people who lived in Stanislaus County area for millennia) as they gathered acorns.
Vital Roles
Early humans built their homes, created tools, built shelters and ships from oak wood. Oak galls were used to make dyes, writing ink, and tan leather. Today we use its strong wood to construct furniture, flooring, cabinets, and wine barrels. If you like truffles, thank oak trees, since truffles have a close relation with the roots of oak trees. Truffles are almost impossible to grow. Instead, truffle farmers plant oak trees, hoping to create favorable conditions conducive to the growth of truffles.
California Oaks
- Valley oak (Quercus lobata) – One of California's iconic species, it is the largest oak tree found here, living up to 300 years. The Valley oak grows where there is a water table within reach of the roots, often near creeks and rivers. They grow quickly, reaching 20 feet in 5 years and up to 60 feet in 20 years. A deciduous tree, it's distinguished by deeply lobed shiny green leaves and long, narrow acorns.
- Interior live oak (Quercus wislizenii) – An evergreen tree, growing up to 25-80 feet tall, it is found in hilly or mountainous areas, near creeks and streams, living up to 200 years. It can produce two types of leaves at the same time, one with a serrated edge and the other with a smooth edge, and produces small, thin acorns.
- Blue oak (Quercus douglasii) – A deciduous tree found in the hot, dry foothills, it grows to be 20-60 feet tall with blue-green leaves which vary in size and shape. With a lifespan of 200-500 years, their acorns are fat and stubby.
Oaks tolerate fire due to their thick, furrowed bark and tough leathery leaves. During wildfires, the larger oaks in areas cleared of fuel may scorch, but rarely burn completely. Damaged trees will resprout from the root crown.
Acorns
An oak tree can produce millions of acorns during its lifetime, but only one in 10,000 acorns grows up to be an oak tree. Acorns are highly nutritious, carbohydrate-rich, and were a diet staple of the Californian indigenous people. Mammals and birds who eat acorns include the Acorn woodpecker, Yellow-billed magpie, California ground squirrel, and Mule deer. However, acorns are toxic to dogs and horses.
The indigenous people called the California scrub jay the “gardener bird” because of its propensity for caching thousands of acorns and not eating all of them, which helped replenish and expand oak forests.
Oak Galls
A favorite oak gall of mine is “jumping galls,” the size of poppy seeds, round with a dot in the center. Some years large numbers of them drop and litter the ground and sidewalks. The galls “jump” each time the larva moves inside. You can see this in action in this YouTube video: https://www.youtube.com/watch?v=0VI7USm4J5I
Oak Challenges
Today, oak trees face many challenges. Disease, drought, and fire can all destroy oak seedlings. Young oaks are stepped on by grazing animals or run over by machinery. Full grown oaks are often damaged or killed when new homes, roads, stores, or businesses are built. Sudden Oak Death is a disease caused by the pathogen Phytophthora ramorum,discovered in Mill Valley in 1995. Causing a rapid color change from green leaves to brown in infected oaks, it has killed thousands of live oak, black oak, tanoak, and Shreve oak in 14 California counties. Climate change is also putting pressure on oak trees.
To maintain a forest or woodland, each oak tree needs to produce just one replacement tree in its lifetime. You can help regenerate California oak habitat by caring for an acorn seedling and protecting it from harm while it grows into a mighty oak.
All photos by Denise Godbout-Avant unless otherwise noted.
/h3>/h3>/h3>/h3>/h3>Advice for the Home Gardener from the
Help Desk of the UC Master Gardener Program
of Contra Costa County
Gardener's Request: We just moved into a 30-year-old house in the East Bay hills, and we expect to stay here a long time. We have numerous oaks and Bay trees on the property. We've heard from neighbors that we should get those trees reviewed by a trained arborist familiar with Sudden Oak Death (SOD), e.g. identification, disposal, and potential danger to other trees in your neighborhood. Can you please recommend sources?
MGCC Help Desk Response: Thank you for contacting the UC Master Gardener Program Help Desk with your questions about your oak tree and how to identify sudden oak death (SOD). The links below include information on identification, disposal, and potential danger to other trees in your neighborhood.
Accurate disease diagnosis can be difficult because the symptoms caused by SOD (Phytophthora ramorum) are very similar to those caused by other fungi, insects, or adverse environmental conditions. The only way to confirm a P. ramorum infection is to take a sample and analyze the affected plant tissue in a certified laboratory. Several of the links include guidance on laboratories.
External symptoms of SOD canker development can include the bleeding of a thick, sticky sap. It oozes out of the bark, not from a crack or hole, typically smells like the inside of a wine barrel and is a deep burgundy but can vary in color from nearly black to an amber-orange. This brochure, A Homeowner's Guide to Sudden Oak Death, has a good image of the sap on a coast live oak:
http://www.suddenoakdeath.org/wp-content/uploads/2010/08/Homeowners-Guide-to-Sudden-Oak-Death.pdf
California bay trees are the primary carrier of SOD fungus. You should see symptoms of infection on the leaves if they are infected. SOD shows as leaf spots and usually brown tips surrounded by a yellow halo: https://nature.berkeley.edu/garbelotto/downloads/sod_diagnostic_report_final.pdf
The recommendation is to remove bays within 30' of susceptible oaks. http://nature.berkeley.edu/garbelottowp/?p=1063.
Here are some local contacts with local links for more information and possible tests of your trees:
The SOD Blitz Project informs and educates the community about Sudden Oak Death, gets locals involved in detecting the disease, and produce detailed local maps of disease distribution. The map can then be used to identify those areas where the infestation may be mild enough to justify proactive management:
http://nature.berkeley.edu/garbelottowp/?page_id=148.
California Oak Mortality Task Force for information and how to find arborists they have trained to identify and sample for SOD infection:http://www.suddenoakdeath.org/
Contra Costa County
Department of Agriculture
2380 Bisso Lane
Concord, CA 94520
Phone: 925-608-6600
And, finally, here is the link to the UC Pest Note with even more detailed information about SOD:
http://ipm.ignore.edu/PMG/PESTNOTES/pn74151.html
Best of luck to you!
Regards,
Help Desk of the UC Master Gardener Program of Contra Costa County (SLH)
Note: apologies... an earlier posted version required a minor editorial change for clarity.
Notes: Contra Costa MG's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/. MGCC Blogs can be found at http://ccmg.edu/HortCoCo/ You can also subscribe to the Biog.
- Author: Dan Macon
In the space of several days in early June, I received phone calls from two foothill cattle producers about an unusual number of dead and dying blue oaks on their annual rangelands. The first rancher's observations were limited to his home place; the second rancher was noticing the blue oaks dying on leased grazing land from Auburn to Nevada City. In mid June, I visited one of these operations and noted several things:
- Some of the trees that the rancher said had leafed out normally in spring appeared to be entirely dead and devoid of leaves.
- Several trees appeared to be dying from the top down or on individual branches. Many of the leaves on these trees also appeared to be scorched.
- These trees did not appear to have any lesions on their trunks - no wounds or noticeable fungal growth.
Several weeks later, I published my summer newsletter and included a short blurb asking readers to contact me if they were noticing anything unusual in their blue oaks. Within an hour of sending the newsletter electronically, I had emails from several landowners noting similar conditions. The issue, it seems, is more widespread than just a couple of random trees!
While I'm no expert on the diseases of blue oaks (or any other tree, for that matter), I'm fortunate to have colleagues within the University of California who are! I contacted Dr. Matteo Garbelotto, a Cooperative Extension Specialist in Forest Pathology at UC Berkeley. Dr. Garbelotto has studied a variety of tree diseases, and he immediately suggested collecting samples from some of our foothill trees to try to figure out what is happening.
This week, Dr. Doug Schmidt from Dr. Garbelotto's Forest Pathology and Mycology Lab joined me in collecting samples. We collected leaves with evidence of scorching, soil samples from the base of infected trees, and tissue samples from the trunks at eight sites from Placer to Yuba County. The lab will test these samples over the coming weeks to try to isolate the pathogen(s) or other factors that may be causing blue oaks to die. We hope to have some preliminary answers in about six weeks.
In the meantime, you can help us understand the extent of the problem. Take note of any recently dead or currently dying blue oaks on your property. Take photos of the entire tree, a close up of the leaves, and any other unusual features. And complete our Blue Oak Mortality survey to help us build a database of impacted areas.
Contact me at dmacon@ucanr.edu if you have questions!
Oak trees typically grow in woodlands, in conditions ranging from shallow soils with little moisture to the deep, fertile soils of the alluvial valley floors. These trees are as diverse in appearance as the conditions they grow in, ranging from 15 to150 feet tall. They differ in shape, color, leaf texture and type of acorns, and whether they are deciduous, evergreen to semi-evergreen.
If you are preparing to plant an oak tree (or an acorn), you need to know the soil type, sun exposure and available water at the potential planting site. The valley oak (Quercuslobata) flourishes in the deep, fertile, well-drained soil typical of floodplains and valley floors. The scrub oak (Q. berberidifolia), black oak (Q. kelloggii) and coast live oak (Q. agrifolia) all thrive in Mediterranean-type climates characterized by mild, wet winters and hot, dry summers. These species want no summer water. If you are looking to plant an oak tree in a dry habitat, on shallow, rocky, infertile soil, your best choices would be blue oak(Q. douglasii), canyon live oak (Q. chrysolepis), Oregon oak (Q. garryana), interior oak (Q. wislizenii), or the humble leather oak (Q. durata).
There are many reasons to keep an existing oak tree.They increase your property value, prevent soil erosion, provide shade and shelter for wildlife and are beautiful to boot.
To keep an oak tree healthy, take care to protect the sensitive root system. Roots can extend beyond the tree's drip line by at least one-third of that distance. Do not dig, grade, trench, compact, asphalt, concrete or pave around your oak tree. These activities may fatally damage the root, trunk or crown.
Changing the grade, trenching, mounding or creating a swale may also lead to changing the natural drainage. If a mature oak gets more water than it is accustomed to, it will decline.
Keep thirsty plants and plantings out of the root zone. Irrigation from agriculture plays a big role in the decline of the valley oak. Choose compatible plants to site under and around your oak.
Many California native plants cohabitate happily with oaks. Here are some great resources for suitable plants:
http://www.napavalleycnps.org/
http://www.californiaoaks.org/ExtAssets/CompatiblePlantsUnder&AroundOaks.pdf
http://www.mostlynatives.com/plant-characteristics/under-oaks
http://www.laspilitas.com/groups/oaks/Planting_under_oak_tree.html
The only fertilizer an established oak needs is its own organic leaf litter for mulch. Keep leaf litter in place, just not up against the trunk. Mulching conserves water, helps moderate ground temperature, improves soil structure and increases organic matter.
If you must prune an oak tree, follow the Tree Care Industry Association guidelines, which you can find online. Pruning improperly can lead to wounds that attract insects and promote decay. Poor pruning may also stimulate rapid shoot growth that increases the potential for powdery mildew. It will also compromise the natural beauty of the tree and may make the tree structurally unstable. For the best results, consult a local arborist.
Many organisms have symbiotic relationships with the oak tree. Lichens flourish on oak trees where there is sufficient moisture, taking nothing from the oak. Many creatures find the acorns a carbohydrate-rich and fatty addition to their diet. Acorns are appreciated by deer, squirrels, woodpeckers, livestock and boar. Oak trees also shelter many nesting birds, arboreal salamanders, invertebrates and insects.
The most complicated relationship the oak tree has is with mycorrhizal fungi. These fungi live in and on the roots and extend way beyond the root system. The oak provides carbohydrates (food) for the fungi and brings up deep water that the fungi can't access. In return, the fungi break down minerals and nutrients and make them available to the oak. In addition, they produce chemicals that inhibit bacteria, thus protecting the tree from disease. With some attention, we humans can also be among the organisms that live symbiotically with the oak tree.
Tree Walk: Join the U.C. Master Gardeners of Napa County on Monday, October 5, from 10 a.m. to 11 a.m., for aguided tree walk in Napa's Fuller Park. The walk is free but pre-registration is strongly recommended as space is limited to 20 people. The walk will repeat on Monday, November 2, from 10 a.m. to 11 a.m. Both walks start in Fuller Park at the corner of Oak and Jefferson Streets in Napa.Online registration
Workshop: U. C. Master Gardeners of Napa County will host a workshop on “California Oaks” on Saturday, October 10, from 9:30 a.m. to 11:30 a.m., at the Martha Walker Garden in Skyline Park, 2201 Imola Avenue, Napa. Stroll around Skyline Park and the Martha Walker Garden to view oaks in their native habitat. Discover what grows alongside and underneath oaks. Learn about planting under oak trees in your own garden, how to care for oaks and about Sudden Oak Death and other stresses on oaks. In case of rain, the workshop will move to the University of California Cooperative Extension office,1710Soscol Avenue, Napa.Online registration (credit card only)Mail-in registration (cash or check only)
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. U. C. Master Gardeners of Napa County ( http://ucanr.org/ucmgnapa/) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.
By Brent McGhie, Butte County Master Gardener, August 22, 2014
These stately trees provide Californians with iconic vistas and are prized for the shade and beauty they give landowners. Oaks also furnish wildlife with food and shelter and, not incidentally, increase property values. By keeping a few cultural practice guidelines in mind, landowners can become good stewards of their oaks and preserve these trees for generations to come.
The key to living with oaks is to disturb them as little as possible. This is especially true of the root crown (the base of the trunk) and the root zone. The root zone of an oak tree covers an area one-third again larger than the outermost edge of the tree's foliage. Strive to keep this area as natural as possible.
The fungal disease “oak root rot” (Armillaria mellea) has evolved with California oaks. It is parasitic on oaks and other plant species, but if oaks are growing naturally in our moist winter/dry summer environment, this fungus is kept in check. However, if oaks are provided with summer irrigation in their root zone, Armillaria can become an aggressive, deadly pathogen. Do not water any plants under oaks during the summer: if they need watering, they do not belong there. Instead, native plants that are adapted to our Mediterranean climate can be grown under oaks. If winter rains are unusually low, a supplemental watering can be provided in the early spring. Do this by watering deeply in the outer two-thirds of the root zone.
Mature oaks do not normally require pruning, exceptfor the removal of dead, weakened, diseased, or dangerous branches. However, to incorporate oaks into a fire-resistant landscape, branches should be pruned away from the ground and kept from touching or hanging over buildings. Additionally, if the landowner wishes, an arborist can be contracted to thin 10-20 percent of the leaf area from branches three to six inches in diameter. Such thinning allows increased light into the tree canopy and decreases branch weight and wind resistance. Light pruning can be done any time of the year, but heavy pruning should be accomplished during the winter dormant season for deciduous oaks and in July or August for the evergreen interior live oak.
Under natural conditions, decomposing leaf litter provides oaks with an adequate supply of recycled nutrients, so oaks do not
normally require fertilizing. If the leaves beneath an oak have been raked up, supplemental feeding is appropriate. If they do require fertilizing, nitrogen is the element most heavily used by oaks; it should be applied at the rate of two to four pounds of actual nitrogen per thousand square feet of area. Either nitrate or organic fertilizers can be used, but organic fertilizers are preferable because their nutrients are released more slowly and they improve soil structure as well.