Some Seattle trees have had it with these dry summers

It’s been a hot, dry summer in Seattle. This July was so warm it almost broke the record — you know, the one set way back in 2015.

Year after year of especially dry summers is killing some of Seattle’s trees. But it’s been harder on some trees than others.

Ray Larson is curator of living collections for the University of Washington Botanic Gardens, which runs the Washington Park Arboretum jointly with the city of Seattle. Larson drove a golf cart around the park on a recent Thursday, checking on the most vulnerable trees.

First on his list: birches, known for their beautiful paper-white bark, once beloved as a Seattle street tree. But the grove of birches Larson was driving to was anything but beautiful. "You can see how terrible they look," he said.

These trees thrived at this spot in the arboretum for decades. But now their tops are bare and brown. An insect, the notorious bronze birch borer, is killing them. It typically only attacks stressed trees, Larson said. The insect left them alone until recent hot and dry summers left these trees weak and vulnerable.

Because of the added stress, Larson believes it's time to stop planting birches as street trees in Seattle. They can still survive in the city near a stream, or where someone gives them extra care. Regular Seattleites can also help a stressed tree, such as a birch, Larson advised, by weeding around the base and giving it a thick mulch and a weekly drink of water.

Washington’s state tree, the Western hemlock, is also having a hard time adapting to climate change in Seattle's urban forests. Temperatures tend to be higher in cities.

Larson took the golf cart over a narrow bridge and past a row of hemlocks around 80 feet tall. Several look good, he said. But he one on the end appeared a sickly yellow. “One by one, these have started to decline in the last five or six years," Larson said. He had known this was coming. Last year, the tree put out a huge number of cones. Stressed by drought, many trees put out large quantities of seed on their penultimate year. When the end is near, they know it's time to invest quickly in the next generation.

Arboretum staff can’t save all the hemlocks. They can't save all the birches. They only have so much money for water and labor. Arboretum staff are facing some hard decisions about which ones to help along.

Here's one way people can help: Keep an eye on street trees. One of the most visible places where you can see trees dying and make a difference is on Seattle's streets.

On some arterials, large numbers of street trees have shriveled up and died. We don't know how extensive the problem is, though. The city only recently started collecting data on the trees it's planted, according to Seattle’s urban forestry manager, Darren Morgan.

Morgan said street trees that prefer shade, like dogwoods and katsuras, are faring the worst. Trees with waxy leaves tend to do better in heat. The Seattle Department of Transportation keeps a carefully curated list of trees that can survive drought. Morgan suggested crepe myrtles.

Larson, from the arboretum, suggested magnolias. SDOT's Morgan said the city waters new street trees consistently for three years. But the city relies on property owners to water them if needed after that. That means four- or five-year-old trees are at risk, as homeowners sometimes assume the city will keep those trees watered.

This year, even older, established trees are showing stress.