What kind of trees should Spokane property owners plant?

Property owners considering contributing to Spokane’s urban tree canopy need to keep some things in mind to ensure their young trees will grow old enough to provide real benefits.

Drought tolerance, resistance to pests and disease, and an ability to thrive in urban environments are all important to consider, while some areas may need trees adapted to particularly moist or acidic soil. The size of the full-grown tree can also dictate where it makes sense to plant – an oak that can reach 100 feet might not be best placed under a power line, for instance.

The city of Spokane maintains an approved street tree list with dozens of options that consider all of these factors, as well as a species’ expected ability to survive in a changing climate. These range from the 15-foot-tall Fringe Tree, which gets its name from its silky, threadlike flowers, to the 120-foot-tall Tuliptree.

While the trees recommended by the city can be expected to do well in the environmental conditions noted in the city’s list, it’s also important to consider what trees are already common.

“It’s very good to have a diversity of species, because when diseases happen, they target a particular species,” said Spokane Urban Forester Katie Kosanke.

Plagues from pests or disease have devastated tree species in the past. Chestnut blight wiped out the American Chestnut, once one of the most widespread species in the country. Butternut canker proved catastrophic to native species of walnut. Dutch elm disease has been one of the most devastating shade tree diseases in U.S. history.

Today, one of the most closely watched threats to American trees are emerald ash borers, which threaten ash trees, a common tree in cities.

“With Dutch elm, many cities learned the hard way by losing all of their trees,” Kosanke said. “Many of those cities have been planting ash, but now there’s emerald ash borer, and it’s getting closer and closer to us, so we haven’t been planting that species for years in preparation.”

The International Society of Arboriculture recommends an urban forest have no more than 20% of its trees belong to any one genus and no more than 10% of any single species, the city notes in its approved street tree list. Current research is promoting an even more conservative limit of 10% in any single genus.

Among city-managed street trees, the most predominant species at over 18% were Norway maples as of 2013, with over 14,000 trees. Maples as a genus make up about 25% of the city’s street and park tree population. Ponderosa pines are the second-most abundant species, making up around 9% of all city-managed trees, while pines as a genus make up about 16%.

The city has identified 96 species that it manages, though more than half of the number of trees are made up of just nine species.

Some species aren’t recommended at all, regardless of their rarity in the community, Kosanke noted. For instance, cottonwoods and black locusts might not be appropriate outside of riparian environments, trees of heaven are aggressive growers and non-native, and willows can have aggressive roots that damage sidewalks while draping their branches too low for passing garbage trucks.