Oaks, including Oregon white oaks/Garry oaks (Quercus garryana), are science’s superstar trees. They are often described as one of the best lifestyle trees on the planet for the number of species that live on, in, or interact with them. Figures differ, but more than 200 caterpillar species use oaks as a host plant. More than 345 animals, reptiles, birds, insects, plants, fungi including truffles, a mistletoe (Phoradendron villosum) that grows only on Oregon white oak, and even algae interact specifically with Oregon white oaks.
Doug Tallamy, professor of Entomology and Wildlife Ecology at the University of Delaware, is a fierce advocate for native plants of all kinds. His book, The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees, traces oaks month-by-month through the year. Oaks do a lot of growing underground while producing a rich environment for the creatures dependent upon them, he says, and aren’t overly noticed by humans. Tallamy lauds oaks as a powerful keystone species. These are “native plants critical to the food web and necessary for many species to complete their life cycle,” as noted by the National Wildlife Federation.
Constance A. Harrington, a US Forest Service researcher at the Pacific Northwest Research Station in Olympia, Washington, is an Oregon white oak expert. Her research has confirmed that white oak, a long-lived species, is not shade tolerant and needs sun and space. When free to grow, they can grow from 50–90 feet tall. Drought tolerant white oaks, she says, can live in either cool, humid or hot, dry conditions such as we have in the Rogue Valley (-30° to 116° F.). They have a deep taproot, roots flexible enough to grow around rocks and are wind resistant. The greater the branch spread that results in a large, wide crown, the more acorns will be produced, Harrington notes.
Many white oaks are old and established. The heartwood is hard to core with a tool that removes a wood sample to count the number of tree rings and age a tree. The height of a white oak or the density of its branches do not tell its age. In the Rogue Valley, it’s assumed that mature trees may be up to a couple of centuries old or more.
As the tree bark ages, it gets rougher and from a distance can look like lizard skin with chalky hieroglyphics! Oregon white oaks, as opposed to California black oaks (Quercus kelloggii) that are at the northern edge of their range here, tend to have whitish bark. Leaves are an easy way to determine white versus black oak: white oak leaves are lobed (rounded) where the leaf juts out; black oaks have pointed tips.
But why get excited? Traveling around the Rogue Valley, there are groups or swaths of oaks, often enough appearing mixed in with other trees or shading cows in a pasture. Why are Professor Tallamy in the East and Connie Harrington in the Pacific Northwest sounding a loud alarm?
It’s a crisis. The Oregon white oak/Garry oak has been a disappearing act since non-Indigenous settlement began in the Pacific Northwest more than 200 years ago. Estimates for the somewhat scraggly-limbed but lush, leafy trees are that less than 10% (6% in the Willamette Valley) of the Quercus garryana species survives two centuries on, mostly in sparse fragments on the landscape.
What happened? A Washington state guidance paper published in 2024 identifies “agricultural expansion, suburban and urban development, fire suppression, and conversion to more commercial Douglas fir” (that overgrow oaks with a higher canopy) as reasons for declining survival. Invasive species such as Scotch broom, Himalayan blackberry and English hawthorn and others have severely encroached on oak stands.
Documented Indigenous use included frequent, non-severe burning to improve oaks’ acorn crops and perhaps to gather prey animals for hunts. The Willamette Valley has 6% of the pre-settlement oaks and their successors left. The drier Rogue Valley, slower to remove oaks from traditional stands, has more remnants although no one is sure what percentage of Oregon white oak remains here. Most Oregon white oaks are on private property in this state.
Protection of existing oaks and encouragement of seedings are a dual strategy to save the species:
Existing Trees
Don’t remove a live oak tree (if possible). If a tree has to be removed in an open area and there’s a choice, try to preserve the oak. When you “release” an oak or small stand of oaks this way, the oaks have a chance to thrive.
Don’t overwater! Oregon white oaks are heat and drought adapted, expecting toasty temperatures when they have transitioned from spring growth to summertime acorn grow-out. Watering will depend somewhat on the oak’s location and your landscaping.
Oregon white oak roots extend beyond the drip line (the edge of the leaf canopy). Any machinery driven across the roots may damage or eventually kill the tree. So might damage to the trunk base by a mower or weed trimmer.
Harrington’s research indicates that the instinct to prune Oregon white oak so that no dead-looking or bare-seeming branches remain works against white oak survival. Sideways epicormic or “pin” branches rise from buds underneath the bark and directly help the oak build its crown.
Don’t worry if your Oregon white oaks produce wildly different numbers of acorns per year. A year of heavy production – filling the ground below and around the trees – is called a mast year. There are many theories why production varies, but it’s assumed that this survivor species of oak has a strategy.
Oregon white oak, a hardwood, tolerates lack of water, and its drought resistance means that it can withstand wildfire better than many conifers, for example. If its landscape is prepared for a wildfire, chances are that the oak will experience a less severe fire (as Indigenous peoples did by burning small areas quickly and less severely).
New Oregon White Oaks:
Many people struggle to get new Oregon white oaks to survive. The acorns are beloved as food by deer, squirrels, Steller’s jays, acorn woodpeckers, and any creature that can crunch open the acorn shell. Squirrels cache acorns in the fall to dig up later. Whether dug in by squirrels or finding ideal soil conditions to nestle in unmolested by critters, the “naturals,” – seedlings that grow without human intervention – are considered by foresters to be the hardiest and probably healthiest of white oak seedlings.
As gardeners, it can be fun and challenging to gather intact (no holes drilled through the shell) acorns and plant them. The long taproot establishes quickly so digging up seedlings, whether naturals or planted by you, may not work. Harrington’s advice is if you want to seed, do so in the fall when the acorns are ripe because oak plant roots get established in the fall. Plant the acorn deeply (4–6 inches) and/or use a metal mesh cage to reduce animal theft.
Soils should be appropriate (Oregon white oak grows mostly in clay soils in the Rogue Valley). Water seedlings but don’t overwater (as with any dry-loving native plants). Squirrels or birds dropping acorns may decide to replant for you! Lastly, oaks are slow growers. Planting an acorn now is a gift to your grandkids.
What are the round, yellow, red, sometimes green apple-sized things growing on or fallen under the oaks? These galls, also called oak apple galls, are caused by Cynipid gall wasp larvae that create the enlarged, swollen plant tissue. The expert consensus seems to be that this Oregon white oak gall wasp species is benign, another lifestyle-dependent species.
Oregon white oak is resilient, despite Tent caterpillars, stem and branch cankers, powdery mildew, anthracnose, and chronic, slow mortality from Armillaria root disease (that’s already in Rogue Valley soils).
So far, Quercus garryana is resistant to Sudden Oak Death (SOD).
The Mediterranean Oak Borer (Xyleborus monographus), detected in California and in Oregon in 2018, targets many types of oak, including Oregon white oak. Harrington notes that “it’s a borer, but larvae eat fungi the adults bring with them that causes a wilting disease which can kill branches and eventually whole trees.” Clues include a tiny bark entrance hole and pale boring dust at the tree base.
References
Nolan, M. P., and J. M. Azerrad. 2024. Management recommendations for Washington’s priority habitats: Best management practices for mitigating impacts to Oregon white oak priority habitat. Washington Department of Fish and Wildlife, Olympia, Washington. https://wdfw.wa.gov/publications/02465
Harrington, Constance A.; Devine, Warren D. 2006. A practical guide to oak release. Gen. Tech. Rep. PNW-GTR-666. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 24 p
Swiecki, Tedmund J.; Bernhardt, Elizabeth. 2006. A field guide to insects and diseases of California oaks. Gen. Tech Rep. PSW-GTR-197. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. 151 p.
Olympia, Washington, is an Oregon white oak expert. Her research has confirmed that white oak, a long-lived species, is not shade tolerant and needs sun and space. When free to grow, they can grow from 50–90 feet tall. Drought tolerant white oaks, she says, can live in either cool, humid or hot, dry conditions such as we have in the Rogue Valley (-30° to 116° F.). They have a deep taproot, roots flexible enough to grow around rocks and are wind resistant. The greater the branch spread that results in a large, wide crown, the more acorns will be produced, Harrington notes.
But why get excited? Traveling around the Rogue Valley, there are groups or swaths of oaks, often enough appearing mixed in with other trees or shading cows in a pasture. Why are Professor Tallamy in the East and Connie Harrington in the Pacific Northwest sounding a loud alarm?
across the roots may damage or eventually kill the tree. So might damage to the trunk base by a mower or weed trimmer.
oaks? These galls, also called oak apple galls, are caused by Cynipid gall wasp larvae that create the enlarged, swollen plant tissue. The expert consensus seems to be that this Oregon white oak gall wasp species is benign, another lifestyle-dependent species.
Oregon State University Extension
