The benefits of street trees are well documented. Beyond their obvious aesthetic contribution to our urban spaces, trees filter air pollution and slow stormwater runoff. Trees provide shelter and food for wildlife and shade for humans. Studies have found that tree-lined streets improve human health by lowering blood pressure, and street trees decrease traffic speeds, increase foot traffic in business districts, and raise property values for both homeowners and businesses.
But it’s hard to be a street tree in the city. Power lines loom overhead, and compacted soil lies beneath your feet. Repairs and upgrades to utilities damage root systems. Many young street trees struggle from being under-watered and over-pruned. Passing trucks break off branches on mature trees, and dogs stop by every few minutes to read and answer their “pee mail”. The stress of street life and overplanting of favored species leads to increased susceptibility to insects and diseases. And of course, the worst pests of all are humans, who damage both the canopy and roots by ignoring development and maintenance guidelines and polluting both the air and soil.
Fortunately, there are many things Seattle homeowners can do to plant new street trees and to protect the trees we currently have.
What’s a Street Tree?
Per the City of Seattle’s Street Tree Manual, all trees growing within ‘public places’ are considered to be street trees, and ‘public places’ include “public right of way and the space above or beneath its surface, whether or not open or improved, including streets, avenues, ways, boulevards, drives, places, alleys, sidewalks, planting strips, squares, triangles, and plazas that are not privately owned.” Trees planted on land owned by the Parks Department are not considered street trees.
Whose Tree Is It, Anyway?
The City owns both the right of way and the tree. Somewhat paradoxically, responsibility for maintenance of the tree lies with the homeowner.
Maintenance? What Maintenance?
Happily, a well-planted street tree requires little maintenance beyond an occasional summer soaking, so put those pruning shears away. The only maintenance a homeowner is required to perform is to maintain the required clearances above the sidewalk (8’) and the parking/travel lane of the street (14’). Pruning a street tree is heavily regulated, and a permit is required to remove more than 15% of the canopy or any single branch wider than 2 inches in caliper. Removing or damaging a tree in the right of way can result in some rather hefty, and in my opinion, well-deserved fines. While citizens are legally allowed to obtain permits and perform significant pruning, considering the potential legal and safety ramifications, major pruning of street trees is a job best left to a city-approved professional.
It’s just as important to protect the root system as the canopy, so don’t forget that it is illegal to park in the planting strip. Storage of heavy items under tree drip lines is also prohibited. Even the temporary storage of items like construction materials piled beneath the drip line can compact the soil and suffocate roots to a sufficient degree as to cause a slow decline and eventual demise. Arborists refer to this as “Contractor’s Blight”.
Planting Procedure
· Site selection
As the old saying goes, “When planting a tree, look up, look down, and look all around.” Trees need head room and shoulder room, so make sure your planting strip is at least 4 feet wide. (Most species will need a wider planting strip.) If you live on the side of the street that is blessed with utility lines, you will want to select a species that stays under 25 feet tall at maturity. Avoid conflicts with other utilities by observing the following standards:
§ 3 ½ feet back from the face of the curb
§ 5 feet from all underground utility lines (Call 811 to locate lines)
§ 10 feet from power poles
§ 7 ½ feet from driveways (10 feet recommended)
§ 20 feet from street lights and other existing trees
§ 30 feet from street intersections (to avoid driver sightline issues)
· Species selection
The next step is to select your tree, which is, of course, the fun part. Choosing the largest tree possible for your site will create the biggest bang for your civic buck in terms of ecological benefits. Your choice must be approved by the City (visit Seattle.gov’s Street Trees page for more information). Certain species are not allowed such as fruit trees (fallen fruit creates slipping hazards), trees with aggressive root systems (Bigleaf maple, cottonwoods, poplars, willows), and trees with brittle branches (alder, black locust). A handy SDOT Street Tree Selector Tool is available online to help you make your choice.
· Apply for a Free Tree Planting Permit
Contact SDOT at (206) 684-TREE, email Seattle.Trees@Seattle.gov or visit SDOT's website for information on obtaining and completing a free Tree Planting Permit. You may be able to obtain help with the permit and perhaps even a free tree by working with the folks at Trees for Neighborhoods.
· Call Before You Dig!
As mentioned, you are required by law to call 811 before you dig to locate any underground utilities. The number is easy to remember, because if you don’t use it, you may end up calling 911 instead to report a broken gas, power, or water line. Mark your proposed planting location in white paint and then an SDOT arborist will schedule an inspection prior to permit approval. It is not necessary for you to be present at the inspection. Note that the fine folks at Call-Before-You-Dig do not locate sewer lines. You can use this online tool to locate your side sewer line; always plant at least 5 feet away.
· And now we wait…
The SDOT arborist will evaluate your proposed site based on proximity to utilities, structures, and existing trees. Despite the city’s strong desire to increase our tree canopy, not every site is suitable for a tree. If your permit is denied the City will inform you of their decision and work with you to find a more suitable location, if possible.
· Success! It’s Planting Time!
After your permit is approved, it’s at long last time to plant your tree! If you are physically challenged, need help transporting your tree, or just need help navigating the permitting process, assistance with installation and logistics may be available if you participate in the Trees for Neighborhoods program mentioned earlier. If you plan on tackling the job by yourself, loads of information about selection, planting, and maintenance is available from the city of Seattle, and here is Swansons helpful guide: How to Plant Trees & Shrubs.
A Few Favorites
The list of Approved Street Trees is lengthy and can be bewildering to new gardeners. Here are a few suggestions to help narrow the list:
Small Trees
Photo: Crepe Myrtle, Greg Butler
photo: Houzz.com
Vine Maple (Acer circinatum)
Minimum planting strip width: 5’ (wider is better)
Height at Maturity: 25’
My love of our native vine maple is well-documented in a previous blog post; it’s a tough, hardy little tree that does well in almost any garden situation. On the street, it’s best planted in well-amended soil on a partially shaded site and may struggle in full sun. Outstanding fall color, seeds support a variety of wildlife. Relatively narrow in youth, it tends to fatten up a bit as it ages (as do we all).
photo: Gondahara, Wikimedia Commons
Japanese Snowbell (Styrax japonica)
Minimum planting strip width: 5’
Height at Maturity: 25’
Similar to Vine Maple in form, some varieties of this shrubby little tree sport sweetly fragrant bell-shaped white flowers in spring, followed by butter-yellow fall foliage. Interesting zig-zag winter twig structure captures raindrops and holds them like a necklace made of translucent pearls. Same conditions as Vine Maple.
photo: Wikimedia Commons, Fanghong
Crepe Myrtle (Lagerstroemia indica)
Minimum planting strip width: 4’
Height at Maturity: 20-25’ (largest varieties)
Generally a bit more upright in form than the previous two suggestions, this Southeast Asia native is a much better choice for full sun situations. Can be reluctant to flower in our cool climate, but the reflected heat of urban pavement will often produce a fantastic display of white, pink, red, or mauve flowers in late summer/early fall. Brilliant autumn color in tones of yellow, orange, and red, and the fallen leaves have the common courtesy to immediately crumple down to next-to-nothing, so fall cleanup is minimal. Gorgeous exfoliating bark on older trees make this a true-four season specimen. Drought tolerant, but still appreciates an occasional deep soaking. Can be late to leaf out, so be patient.
photo: Great Plant Picks
Crabapple (Malus ‘Adirondack’, Malus sargentii 'Tina' & 'Golden Raindrops')
Minimum planting strip width: 5’
Height at maturity: 20’
A great choice for a narrow planting strip, the mature width on this species is a mere 10’. Criminally underplanted, the newer, disease-resistant crabapples make an excellent substitution for the ubiquitous, pestilence-ridden flowering cherries that dominate our streetscape. Brilliant crimson buds eventually open to pure white flowers, followed by long-lasting half-inch berries in shades of orange and red. Fruit is greedily gobbled by birds. Fall color is described as “not terribly significant”, but hey, if some of the leaves didn’t turn brown in the fall, the Mamas and the Papas would have had to change the lyrics to “California Dreamin.” Very drought tolerant when established.
Medium Trees
Photo: Eddie's White Wonder Dogwood, nurserytrees.com
photo: Wikimedia Commons, Rüdiger Wölk
Dove Tree (Davidia involucrata)
Minimum planting strip width: 5’
Height at maturity: 30’
Opinions on this tree are all over the map, ranging from “Wow, that’s amazing” to “Dude, who toilet–papered your tree?” Of course, to the true aficionado, the weirder the better, so Davidia is popular among collectors, plant nerds, and hortisexuals, and little-known elsewhere. 19th century plant explorers drove themselves to exhaustion attempting to find a specimen from which to gather seed, and the tree caused a sensation when it finally flowered in Europe. Bizarre 7” long creamy white bracts (not flowers) that are said to resemble handkerchiefs or doves flutter on the tree when the wind blows in late May and early June. Variable fall color with pastel shades of red and orange. Cinnamon colored bark extends season of interest. Best in well drained moist soils with a bit of shade and shelter from wind. Slow to flower when grown from seed, the cultivar ‘Sonoma’ begins flowering as soon as its second year.
photo: Great Plant Picks, Richie Steffen
Eddie’s White Wonder Dogwood (Cornus ‘Eddie’s White Wonder’)
Minimum planting strip width: 5’
Height at maturity: 30’
This hybrid of our native dogwood (Cornus nuttalii) and the east coast’s native dogwood (Cornus florida) shows strong resistance to the anthracnose fungus that attacks the summer foliage of its two parents. Like the Dove Tree, the “flowers” are actually bracts that surround the tiny flowers. Relatively narrow form makes it a good choice for skinny sites. Full sun to light shade, reasonably drought tolerant when established. Nice red fall color, intricate branch structure and mottled bark provide winter interest. Showy red fruit, reputed to edible, but you probably won’t get to try it because the birds will beat you to it. Deservedly popular.
photo: Wikimedia Commons, Dinesh Valke
DeGroot’s Littleleaf Linden (Tillia cordata ‘DeGroot’)
Minimum planting strip width: 5’
Height at maturity: 30’
Known as basswood or lime tree on the continent, but sorry, you won’t be making Key Lime Pie from this one, nor will you be whipping up a skillet of pan-fried bass with lemon-garlic sauce. While completely lacking in culinary uses, Tilias are tough, dependable street trees, and very tolerant of urban conditions. When I say dependable, keep in mind that there are records showing Tilia being used by humans dating back to around 760 A.D. In late June and early July the tree is covered in pale yellow flowers that pack a magnificent fragrance. An irresistible source of nectar for pollinators, Tilias hum like a power plant when in bloom with the buzz of busy bees. Fall color is pale yellow. Average garden water during dry periods, especially when young. Scandinavians avoid the tree after dark as it is thought to be a home for elves and fairies, so steer clear of Tilias after sundown in Ballard.
Large Trees
Photo: Ginko Biloba, Greg Butler
photo: Greg Butler
Red Oak (Quercus rubra)
Minimum planting strip width: 8’
Height at maturity: 60’+
Stout, stately and statuesque, Quercus rubra is classic street tree, but at 75’ (or more) in height and 45’ in width, it admittedly requires a lot of space. Native to the eastern U.S., from the Mississippi River to the Atlantic. Deep rooted and trouble free, red oaks are drought tolerant in our area when established, requiring only occasional summer watering to look their best. Fall color is a magnificent combination of orange and red. Easily grown in full sun to part shade in any well-drained soil. Acorns provide sustenance for a variety of birds.
photo: Wikimedia Commons, Dr. Charles Nelson
Copper Beech (Fagus sylvatica)
Minimum planting strip width: 6’
Height at maturity: 50’
Another massive tree that requires a large space. Native to central and southern Europe, brought to the U.S. in the mid-1700’s. Tolerant of any free-draining soil and appreciative of a bit of summer water. Rich, deep purple foliage casts deep shade and takes on fiery hues of red and yellow in the fall. Long lived. Edible nuts that require considerable effort to shell. One of Seattle City Arborist Nolan Rundquist’s personal favorites.
photo: Greg Butler
‘Green Vase’ Japanese Zelkova (Zelkova serrata ‘Green Vase’)
Minimum planting strip width: 6’
Height at maturity: 45’
Often recommended as a replacement for Elms, which have been devastated by Dutch Elm Disease. Although Zelkova is in the same family as Elm, it is highly resistant to DED. Vase shaped with a tidy, layered branch structure. Exfoliating bark on older trees adds winter interest. Tolerates urban conditions well. ‘Green Vase’ is a uniform selection and a perfect choice for tree lined avenues. Excellent fall color.
photo: ucmpberkely,edu
Ginkgo (Ginkgo biloba)
Minimum planting strip width: 6’
Height at maturity: 50’
A true survivor, Ginkgo’s fossil records date back 65 million years. Extremely long lived, specimens have been observed at over a thousand years old. The tree’s posture in youth is that of a gangly, awkward teenager, but it matures into a real show-stopper. Distinctive bi-lobed foliage turns a glowing butter yellow in the fall and really brightens up a dreary Seattle autumn day. Average garden water when young, becoming more tolerant to drought with age. The species is dioecious, having both male and female trees. Look for a male clone such as ‘Autumn Gold’ or ‘Maygar’, as the female trees bear nuts surrounded by a rather foul smelling pulp. Extremely tolerant of urban conditions, several Ginkgos survived the bombing of Hiroshima in 1945. Let’s hope your Ginkgo isn’t faced with that sort of challenge!
Now, go plant some trees!