Drought stress is becoming an increasing concern for Seattle’s urban forest, affecting trees in our parks, along our streets, and in our own backyards. As our changing climate brings higher average temperatures and less predictable rainfall, urban trees are facing greater challenges. Understanding how to identify and mitigate drought stress is important for preserving the health and vitality of our city’s trees.

By taking proactive steps, homeowners, businesses, and city officials can work together to protect our trees, ensuring a healthy, green canopy for years to come. Here are some practical tips for keeping our citywide tree canopy thriving during the dry season.

Key Takeaways

• Drought stress is increasingly threatening Seattle’s urban forest as climate change leads to higher temperatures and less predictable rainfall.
• Seattle’s urban forest is essential for improving air quality, reducing electricity use and our carbon footprint, managing stormwater, and providing habitats for local wildlife.
• Signs of drought stress in trees include leaf browning, stunted growth, and increased pest activity, and can ultimately lead to higher tree mortality rates if not addressed.
• Effective tree care during drought includes deep watering, using soaker hoses, proper mulching, and avoiding heavy pruning and fertilization.
• Seattle residents, business owners, and city officials can work together to not only maintain the health and beauty of our urban forest, but to expand it for future generations to enjoy.

Understanding Seattle’s Urban Forest

Drought stress isn’t just a threat to individual trees, or even compromised trees. The entire urban forest is at risk as summers get drier in the Pacific Northwest.

Seattle’s urban forest is a comprehensive network of trees and green spaces that spans our entire city. It encompasses:

• Trees in public parks and green spaces
• Street trees lining our roads and sidewalks
• Trees on private residential and commercial properties

This diverse collection of trees plays an important role in our city’s ecosystem by:

• Improving air quality by filtering pollutants and releasing oxygen in our bustling city
• Reducing the urban heat island effect, lowering temperatures within the city, reducing energy consumption, and lowering our collective carbon footprint
• Managing stormwater runoff, limiting flooding and soil erosion, while also keeping our waterways cleaner
• Providing habitats and food sources for birds, insects, and other wildlife, promoting a rich and diverse ecosystem within the city
• Enhancing the aesthetic appeal, walkability, and bikeability of our neighborhoods for residents and visitors alike

Seattle takes pride in its impressive tree canopy, which covers approximately 20% of the city. This places our city among the top-ranking urban areas worldwide for tree density.

But unless homeowners, businesses owners, and city officials take the threat of drought seriously, much of this tree canopy is at risk of weakening or dying altogether.

The Impact of Drought Stress on Seattle’s Urban Forest

Drought stress poses a significant and growing threat to Seattle’s urban forest. With changing climate patterns leading to more frequent and prolonged dry periods, our trees face increasing challenges. If left unaddressed, drought stress can have severe consequences:

• Increased susceptibility to pests and diseases: Drought-stressed trees are more vulnerable to pest infestations and fungal infections.
• Structural weaknesses: Lack of water can lead to brittle branches and poor root development, increasing the risk of falling limbs.
• Reduced growth and vitality: Drought-stressed trees may experience stunted growth, wilting, premature leaf drop, and a thinning canopy that fails to provide much needed shade.
• Higher tree mortality rates: Longer summer droughts have led to the death of countless mature trees in recent years. Although these trees can be replaced, it can take many years for young trees to match the canopy coverage of a larger, established tree.

What Causes Drought Stress in Seattle’s Trees?

Drought stress occurs when trees don’t get enough water to meet their physiological needs. In Seattle’s urban and suburban environments, this is caused by:

• Insufficient rainfall: Seattle is experiencing longer periods of drought in the summer.
• High temperatures: Temperatures are increasing in the PNW, leading to higher evaporation rates, and less water reaching tree roots.
• Poor soil conditions: Compacted or shallow soils limit the amount of water that can penetrate and reach tree roots.
• Limited root space: Urban infrastructure from roads, sidewalks, and structures restricts root growth and limits water uptake.

Signs of Drought Stress

Identifying drought stress early is crucial for maintaining tree health. Look for these signs during summer or other dry periods:

• Leaf browning or yellowing, wilting leaves, and/or premature leaf drop
• Stunted growth
• Sparse or thinning canopy
• Cracks in the bark or dead branches
• Increased pest activity on or around trees

Healthy trees that are receiving adequate water typically exhibit vibrant, green leaves or needles, consistent growth patterns, and a lush, full canopy, even in the heat of summer.

How to Care for Trees During Drought

Preventing drought stress is all about keeping trees hydrated during our increasingly dry summers here in Seattle. But it’s not just about turning the sprinklers on. Watering trees the right way, and helping them retain as much moisture as possible, will limit the effects of drought and allow your trees to thrive when the temperatures rise.

• Deep and infrequent watering: Water trees deeply to encourage the growth of deep root systems, which can access water stored deeper in the soil. The goal is to water less frequently but ensure the water reaches deep into the root zone, typically 12-18 inches down. Early morning or late evening watering reduces evaporation, allowing more water to reach the roots.
• Use soaker hoses: Soaker hoses slowly water the root zone of plants. They don’t spray, so very little water is wasted compared to traditional sprinklers. You can save water and money while also ensuring water reaches your tree roots.
• Mulching: Apply a 2–4-inch layer of organic mulch around the base of the tree, ideally extending to the drip line (or even beyond). Mulch helps retain soil moisture, suppresses weeds, and regulates soil temperature. Be sure to leave space around the trunk to prevent rot (don’t pile mulch up against the trunk).
• Avoid over-pruning: During drought, trees are already stressed. Heavy pruning can worsen their condition by removing vital foliage needed for photosynthesis. Focus only on removing dead, diseased, or damaged branches to prevent additional harm.
• Aerate soil: Compacted soil can limit water infiltration and root growth. Aerate the soil around the tree to improve water penetration and root access to oxygen. This can be done manually or with specialized equipment. Try to limit foot and vehicle traffic near trees to prevent further compaction.
• Avoid fertilization: During drought conditions, avoid fertilizing trees, as it can stimulate new growth that requires additional water to sustain. Focus on maintaining the tree’s existing health rather than promoting new growth (wait until fall to fertilize).

Check out this article for some additional summer tree care tips specific to the Seattle area.

Working Together to Protect Seattle’s Urban Forest from Drought Stress and Other Issues

Seattle is well-known around the country for its eco-friendly initiatives and policies. But protecting Seattle’s urban forest at large isn’t just a job for city officials. It’s a community effort that requires active participation from residents, businesses, and local organizations. Here are several other ways you can contribute to the health and preservation of our city’s green spaces to ensure a robust and thriving urban forest, even in the midst of ever-increasing drought:

1. Plant native trees: Participate in community tree-planting events or add native species to your property. These trees are well-adapted to local conditions, more resilient to environmental stressors like drought, and provide valuable habitat for wildlife, enhancing biodiversity and ecosystem resilience. One of the city’s goals is to start replacing aging, non-native deciduous trees with longer-living native conifers.
2. Collect and recycle rainwater: The city of Seattle recommends setting up rain barrels to capture roof runoff. This stored water can be used to irrigate trees and other plants during dry periods, reducing reliance on treated water supplies.
3. Volunteer for tree care initiatives: Join local groups or city-sponsored programs that focus on planting new trees, distributing free trees to homeowners, watering, mulching, and pruning public trees, and removing harmful, invasive trees and vines that displace native species.
4. Support local environmental organizations: Consider donating to or volunteering with organizations dedicated to preserving and expanding Seattle’s urban forest. Your contributions can help fund tree planting projects, public education initiatives, and advocacy efforts for stronger environmental protections.
5. Go green: By embracing eco-conscious lifestyle choices, you can do your part to mitigate climate change impacts that contribute to drought stress in our urban trees. This might include reducing energy consumption at home, driving less and walking/biking more, or supporting local, sustainable businesses—options that are all particularly easy to pursue here in Seattle

Additional Resources for Seattle’s Urban Forest

To deepen your appreciation and understanding of Seattle’s urban canopy, check out these resources from the city:

• Tree Walks: The city has mapped out numerous walking routes in a handful of Seattle neighborhoods, allowing you to discover and learn about the diverse tree species that make up our urban forest. These self-guided tours offer a unique perspective on the city’s green infrastructure.
• Heritage Trees: Seattle’s Heritage Tree Program recognizes and protects trees of exceptional size, age, or historical significance. Seek out these living landmarks to gain a sense of the city’s arboreal history.
• Urban Forest Management Plan: For a comprehensive overview of Seattle’s urban forest strategy, review the city’s Urban Forest Management Plan. You don’t need to work in public policy to educate yourself on the current state of our urban forest, the challenges it faces, and the actions needed to preserve and expand our tree canopy for future generations.

Protecting and Preserving Seattle’s Tree Canopy

Protecting Seattle’s urban forest from drought stress is ultimately a shared responsibility. By recognizing signs of stress early, implementing proper watering techniques, and investing in overall tree care, we can ensure our entire city remains a thriving, green oasis for generations to come.

Here at Seattle Tree Care, we have decades of experience assisting homeowners, business owners, and the City of Seattle in maintaining a robust urban forest. Our professional arborists offer expert guidance and services to ensure your trees receive the best care in any season. Call us today at 206-789-0534 to have your trees inspected, or use our online form to request a service now.