Healthy trees and healthy soils are the foundation of a drought-resilient urban forest
In the Willamette Valley, August is typically the hottest and driest time of year. For Lake Oswego residents, helping trees and shrubs through the dry season begins with the soil beneath them. Healthy, organic-rich soil acts like a sponge, absorbing and holding rainwater so it remains available to plant roots during dry periods. Urban soils, however, are often compacted, disturbed, or lacking organic matter. Leaving fallen leaves beneath trees and shrubs, adding compost, reducing soil compaction, and applying a 2–3-inch layer of wood-chip mulch can help rebuild healthy soil, conserve moisture, and reduce evaporation. Native plants can also contribute to a more resilient landscape because they are adapted to local conditions.
How to Help Trees and Shrubs During Drought
- Water deeply and infrequently. When supplemental watering is needed, water deeply about 1–2 times per week rather than watering lightly every day. Focus water on the root zone using a soaker hose or drip irrigation. Note: This general recommendation does not apply to established Oregon white oaks. See “Protecting Lake Oswego’s Heritage Oaks During Drought” below.
- Water at the right time. Water before 9 a.m. or after 6 p.m. to reduce evaporation. Early mornings are generally ideal.
- Use the Weekly Watering Number. Lake Oswego residents can sign up for the Regional Water Providers Consortium’s weekly watering guidance, which provides recommendations for the greater Portland metro area based on current weather conditions.
- Adjust watering based on the plant. Young trees need regular, deep watering as they become established. Established trees generally need less frequent watering, but some native species—including Oregon white oak—are adapted to dry summers and should not receive supplemental summer irrigation once established. Established trees generally need less frequent watering, while established shrubs and perennials typically require less water than lawns. Lawns can also be allowed to go dormant and turn brown during summer; they will generally recover when fall rains return.
- Water slowly to prevent runoff. Many Willamette Valley soils, particularly clay-heavy soils, absorb water slowly. If water begins to run off, pause and allow it to soak in before watering again. Irrigation timers can also be set for shorter, repeated cycles.
- Mulch the root zone. Apply 2–3 inches of wood chips or other appropriate mulch to help cool the soil and retain moisture. Keep mulch away from direct contact with the trunk.
- Protect the soil. Avoid unnecessary foot and vehicle traffic beneath tree canopies, where compaction can damage roots and reduce the soil’s ability to absorb water.
- Watch for drought stress. Wilting, curled or scorched leaves, early leaf drop, sparse canopies, and dying branches can all indicate that a plant is under stress.
Applying 2–3 inches of wood-chip mulch helps retain soil moisture, moderate soil temperatures, and protect tree roots. Slow, deep watering helps moisture reach the root zone while reducing runoff and evaporation.
Watering is only one part of creating a drought-resilient landscape. Healthy soil is a long-term investment that helps capture and store rainfall, while thoughtful watering helps plants access that water when they need it most. Together, these practices can help Lake Oswego’s trees and shrubs better withstand hot, dry summers.
Protecting Lake Oswego’s Heritage Oaks During Drought
For a mature, established Oregon white oak, the recommendation is simple: do not provide supplemental summer irrigation. Allow the tree to experience the naturally dry conditions of our summers.
Newly planted Oregon white oak seedlings are the exception.
Young seedlings need supplemental water while they establish their root systems and should receive regular, deep watering during the dry season.
Maintaining healthy soil and protecting the tree’s root zone are among the best ways to help an oak withstand dry conditions. Protect the area beneath the canopy from soil compaction and disturbance, and use an appropriate layer of mulch to help protect the soil and roots. Do not compensate for summer drought by irrigating the tree.
Avoid compacting the soil beneath the canopy or disturbing the roots, and monitor older trees for signs of stress such as dieback or thinning foliage.
Protecting mature oaks is an investment in Lake Oswego’s urban forest—these trees provide shade, wildlife habitat, carbon storage, and a living connection to the region’s natural history.
Fire and Drought Resilience
Drought resilience also connects to wildfire preparedness. As vegetation dries during summer, maintaining a landscape with less dead and highly flammable material can help reduce the potential for fire to spread toward homes. Lake Oswego’s wildfire prevention guidance recommends creating defensible space, removing dead plants and debris from roofs, gutters, decks, and yards, keeping grass and weeds short, pruning branches away from roofs and chimneys, and creating strategic separation between vegetation and structures where needed to reduce the continuity of flammable fuels.
This does not mean removing healthy trees and shrubs or replacing a diverse, layered landscape with grass, gravel, or bare soil. Instead, focus on maintaining healthy vegetation, removing dead and highly flammable material, and creating appropriate separation between vegetation and structures. Grass and gravel can be used sparingly and strategically, particularly in areas immediately surrounding homes and other vulnerable structures, while the broader landscape can remain densely planted with healthy trees, shrubs, and groundcover. A Firewise landscape does not mean removing all trees and shrubs; it means maintaining healthy, well-spaced vegetation and reducing the accumulation of dry, dead, and highly flammable fuels. For more information about creating defensible space, reducing wildfire risk, and preparing your Lake Oswego property for wildfire, visit the City of Lake Oswego’s Wildfire Risk Prevention
By improving soil health, watering thoughtfully, choosing plants suited to local conditions, protecting tree roots, and reducing wildfire fuels, Lake Oswego residents can help create landscapes that are healthier, more water-efficient, and more resilient to the challenges of a changing climate.
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