What You Can Do?

About 80% of the City’s tree canopy is located on private property, with the remaining 20% on public lands such as parks, natural areas, and rights-of-way. In the coming year, the City will conduct a full tree canopy assessment, which will help determine the amount of ash trees citywide.

While the City is actively preparing to slow the spread of the emerald ash borer (EAB) and protect our urban forest, residents play a critical role as well.

Here are steps you can take to help:

Learn more about the EAB
Determine if you have an ash tree on your property
Work with an ISA-certified arborist
Properly care for your ash trees
Avoid moving firewood
Help detect and report EAB

Learn more about the EAB

City staff will be hosting workshops, attending community events such as Forestry Day at the Farmers’ Market, and presenting at neighborhood meetings. Find upcoming events at: www.lakeoswego.city/emerald-ash-borer

Determine if you have an ash tree on your property

Learn how to identify ash trees.

Work with an ISA-certified arborist

Property owners are responsible for maintaining trees on their property. An ISA Certified Arborist can assess your trees and help you decide whether to treat, remove, or replace them.

There are management options available:

Removal and Replacement
- Ash trees will eventually become infested and require removal if left untreated.
Consider removing trees in poor or declining condition early, as dead trees are more hazardous and costly to remove.
Lake Oswego City Council has approved updates to the City’s Tree Code. As part of this update, the City has created a new low-cost, over-the-counter tree removal permit for EAB-susceptible ash trees. The permit allows property owners to proactively remove ash trees before an infestation occurs or remove trees that become infested.
- Effective October 1, 2026, a new Dead, Dying and EAB-Susceptible Tree Removal Permit will be available that allows property owners to proactively remove ash trees. It is a simple, over-the-counter permit that allows removal of EAB-susceptible trees regardless of the tree’s condition or whether EAB infestation has been confirmed.
- Positive identification of an ash tree is required through either a certified arborist or photo verification.
- The City of Lake Oswego requires tree removal permits for trees 6 inches or larger in diameter at breast height (DBH). The permit costs $26 per application, regardless of the number of ash trees being removed.
- Replanting requirements depend on where the tree is located. Replanting is required when an ash tree is removed from the public right-of-way, from within a protected Sensitive Lands Overlay, or when the tree was required as part of an approved landscape plan.
If you would like to request removal of a tree 6 inches in diameter or larger, please visit the City's tree website - www.lakeoswego.city/trees - for applications, or contact Planning staff at [email protected] or 503-635-0290 for assistance.
Pruning and removal of ash trees can only take place in the fall and winter months when adult EAB are dormant.

Insecticide Treatment
- Treatment can be up to 95% effective when applied by a licensed professional.
- Treatments are most effective when started before significant decline (no more than 20% canopy dieback).
- Reapplication is recommended every 1–3 years.
- Spring (after leaf emergence to avoid pollinator impact or soil leaching) and fall are ideal treatment times.
- Trees over 6 inches in diameter (measured at 4.5 feet above ground) respond best.
- Applications should be performed by trained, licensed professionals to ensure safety and effectiveness.

Do Nothing

This option may be appropriate for trees in low-risk areas away from infrastructure, housing and other structures, and high-use spaces.

Properly care for your ash trees

Healthy trees are more resilient to pests.

Water during dry months and apply mulch to retain soil moisture. Visit the Lake Oswego Tree Care and Maintenance Archives for a Spring 2023 article on “Supplemental Watering and Mulch to Prevent Drought Stress”
Do not prune or remove ash trees from April 1 to September 30 - this is when the adult beetles are active. A pruning and removal moratorium is in effect for this time period. Chemical treatment of ash trees are not included in the moratorium. Chemical treatments are most effective in the spring and summer and should be conducted after flowering is complete. .
Conduct pruning and removal of ash trees between October 1 through March 31 when adult beetles aren’t active.
When pruning is necessary, hire an ISA Certified Arborist.

Avoid moving firewood

Transporting firewood is a primary way EAB spreads.

Use firewood sourced within 10 miles of where it will be burned.
Learn more at www.dontmovefirewood.org.

Help detect and report EAB

Learn to identify ash trees and EAB.
Report suspected infestations to the Oregon Invasive Species Online Hotline.

Working together, we can help protect Lake Oswego’s tree canopy and slow the spread of Emerald Ash Borer.