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Previous topics:
Planting Large Trees
Street Trees
Tree Topping
Dutch Elm Disease
Fruit tree pests
Silva cells
Stabilizing slopes with trees
Memorial trees
Trees under powerlines
Maintaining trees in planting strips
Hiring an arborist

Planting Large Trees

Q. Why should I plant a large tree?

A. Healthy, mature trees provide many benefits to people and the environment. To maximize those benefits, plant a tree with the largest mature size that fits your space.

Large trees often become treasured neighborhood assets.  Research has shown that they provide higher quality habitat for birds and other wildlife, and they have larger root systems to help stabilize hillsides and prevent erosion. Large trees do more to buffer weather conditions, providing shade on hot days to reduce the need for air conditioning, and blocking winter winds to help save on heating bills.

Large evergreen trees, especially conifers, are even better. Because large conifers grow so tall in our region, they produce a larger volume of leaf area on a smaller footprint – using the same amount of yard space but working harder to take in carbon dioxide, produce oxygen, and filter out air pollutants. And because evergreens hold their needles and leaves through the winter, our rainy season, they reduce stormwater runoff to a much greater extent than do small deciduous trees.

Before choosing a tree, make sure you have enough space for it to reach its mature size. If you have a big enough area, larger trees will provide the most benefits.

Street Trees

Q: Since conditions are so challenging for street tree, why bother planting them at all?

A: The challenges involved in street tree planting are numerous: limited soil volume, flooding or severely dry conditions in sidewalk tree pits, heat absorbed by surrounding pavement and building materials, soil compaction from walking and driving in the root zone, and injury caused by people or vehicles. So why bother planting them? The fact is, the benefits of street trees are greatest in dense urban areas surrounded by concrete. Benefits of trees are psychological, aesthetic, economic and environmental, but in this post I will focus on one environmental benefit: mitigation of urban heat island effect. Heat absorbed by pavement and building materials contributes to the “urban heat island effect,” which means that cities maintain warmer temperatures than surrounding areas. Warmer temperatures contribute to the formation of ozone and other air pollutants. Trees provide shade, which reduces sunlight hitting the concrete and thereby diminishes the heat island effect. Evapotranspiration, the process through which water vapor evaporates from leaves, also cools urban areas. In short, the toughest environments for trees are the ones that need them most.

Tree Topping

Q: Can I top or prune a City-owned tree to improve my view?

A: The City does not allow the topping of trees for private views. Topping is a process that will cause long-term harm to the trees, increase work later when the tree re-sprouts, and will lead to hazardous situations with the tree as it decays at the topping point. The City may work with adajacent residents to prune trees correctly if possible; however it is not always possible to accomodate views through pruning. Residents who wish to see trees pruned for views can request a tree trimming permit from the department managing the trees in question. Click here for information on pruning trees in parks, and here for street trees.

Dutch Elm Disease

Q: What is Dutch elm disease and how do I know if it’s affecting my trees?

A victim of Dutch Elm Disease.

A: Dutch elm disease (DED) is a fungus that can kill mature elm trees. The first symptom of DED is a sudden wilting or drooping of leaves, often in a single branch or limb of the tree. The wilting leaves are grey-green in color, but within days or weeks turn light brown. The fungus will continue to move through the tree’s vascular system and will eventually kill the tree. Dutch elm disease is spread from tree to tree by the elm bark beetle, through natural root grafts and through careless human activities such as transporting infested firewood. The longer an infected tree is left standing, the more likely it is that the disease will be transmitted to healthy trees.

Trees that are in the right-of-way and are believed to be infected with DED are inspected by the Seattle Department of Transportation. Elm trees in parks are managed by the Seattle Parks and Recreation Department. If you suspect a tree in your yard is infested, you can have it tested by WSU Extension. Early detection by local citizens is the best tool Seattle has for controlling DED.

Fruit Tree Pests

Q: What can I do to protect my fruit tree from pests?

A: Two of the most serious pests infesting fruit tree in Seattle are apple maggot and codling most.

Codling moth: Adult moths are speckled grey-brown. Larva make entry holes on the side of the fruit. Traps are not useful for codling moths.

Apple maggot: Adult flies are ¼ inch long and have a black abdomen with white bands. The larva make tunnels through the fruit. Apple maggot traps can be effective, and should be installed in spring and kept on the tree until the fruit is harvested.

Damage to fruit by both codling moth and apple maggot can be controlled by bagging fruit when the fruit is young and before it becomes infected.

For more information on controlling these pests in Seattle, visit the City Fruit webpage.

Silva Cells

Q: What are Silva Cells and what's going on at 4th Ave and Virginia Street downtown?

Installation of Silva Cells at 4th and Virginia.

A: The City of Seattle Department of Transportation has taken a new direction in the effort to achieve the central Urban Forestry Management Plan goal to regain lost canopy cover lost to development. Under the direction of SDOT and product distributor DeepRoot, the Escala project at 4th Avenue and Virginia Street in downtown Seattle has undertaken the installation of Silva Cell, an integrated tree, soil and stormwater system designed by internationally renowned Landscape Architect, James Urban.

This first installation in the City of Seattle along the 4th Avenue frontage of this prominent project, required as a condition associated with an SDOT permit to allow tree removal, maximizes the value of limited space in the right of way. The system utilizes the same space to provide soil conditions optimally designed for the growth of street trees while providing full support for standard sidewalk construction. In addition to adding points for any project seeking LEED Green Building certification, the Silva Cell system is currently under evaluation by the Washington State Department of Ecology as a means to meet stormwater management requirements. The future looks "bright green" given applicability to a wide range of private development and public projects in the urban environment where trees and paved surfaces must coexist in a sustainable manner for the long term.

Hiring an Arborist

Q: What is a certified arborist? What services do they provide, and how can I find one to hire?

A: Certified arborists are trained tree professionals certified by the International Society of Arboriculture. Certified arborists must pass an extensive test and have at least three years of professional experience as well as ongoing education. Not all those who advertise as being tree care professionals are certified arborists – make sure you hire one who is.

Arborists can prune trees of all sizes, inspect and treat trees for pests and diseases, and safely remove dead or unsafe trees. Arborists can also plant trees, provide emergency tree care, remove stumps, and address issues such as fertilization. More information on the services an arborist can provide can be found here.

Certified arborists in the Seattle area can be found through the International Society of Arboriculture. More information on how to hire an arborist can be found here.

One caution: a quality arborist will never recommend the “topping” of a tree. This is the practice of cutting off the central leader, or main trunk, of the tree at any height. This is an extremely hazardous practice that could make your tree less safe, more costly to maintain, and eventually kill the tree.

Stabilizing Slopes with Trees

Q: I have a terraced backyard on a very steep slope The slope is covered in English ivy and blackberry. I'd like to take them out and replace them with native or less invasive species, but I'm concerned about destabilizing the soil. Can you give some advice about timing and the rate of replacement?

I'd like to plant some trees, too. Which trees have roots that will stabilize soil and grow well on slopes? I would like them to be tall enough to provide some privacy as well.

A: Removing ivy and blackberry from your yard is a great idea. When removing the ivy and blackberry it is important to remove as much of the root system as possible; for ivy it’s the running roots and for blackberry it’s the large subterranean root balls. This will minimize the amount of plants that will re-sprout.

The best time of the year for invasive removal and planting is right now. The soil is soft and the roots are easier to remove. This is the best time to plant as the dormant plants have a chance to get established before our long dry summers. A mixture of trees and shrubs work best for slope stabilization. Their roots stabilize the soil at different depths.

The Green Seattle Partnership does forest restoration in Seattle parklands throughout the city. They have many sites that deal with invasive removal on steep slopes. They are a great source and variety of hands-on innovative ideas for slope restoration.

Memorial Trees

Q: The mother of a close friend of mine recently passed away. Are there any public spaces were we could plant a commemorative tree or dedicate a tree in memory of my friend's mother?

A: Memorial trees in public spaces are a very special way to celebrate life and honor the memory of a loved one. There are two programs that can help you plant trees in public spaces.

The Seattle Parks Foundation has a tree donation program that works with individuals to either plant or sponsor a tree in memory of a loved one in a city park.

The Green Seattle Partnership has many opportunities to plant trees in Seattles forested parklands as part of a massive urban forest restoration effort. There are restoration sites throughout the city that will be planting native trees and shrubs throughout the fall/winter planting season. Though the Green Seattle Partnership does not have a memorial tree program, this is a way to plant a tree that will benefit the urban forest.

Trees Under Powerlines

Q:Years ago someone planted 2 cedar trees on the strip between the sidewalk and curb in front of my house. These trees are huge and are under power lines. The city at some point topped the trees and continues to cut and butcher them every year. It is a shame but I think these trees are better to be cut down. Is the city able to help me take care of this?

A: Our City Light experts had this to say:

It sounds like the trees are too big for their location and have to be trimmed to maintain a safe clearance around the power lines. Seattle City Light maintains a 10 foot clearance around the high voltage electrical lines by having electrically qualified tree trimmers safely prune the trees ( ).

Long term, it makes more sense to replace trees like this with smaller trees that don't require such extensive pruning to protect electrical safety and reliability. Seattle City Light will replace trees when we are working in a neighborhood so you may have to wait until we come back. When work is upcoming, we leave door hangers for residents whose trees are going to be pruned. Use the contact numbers on the door hangers to discuss opportunities for potential removal and replacement of your trees through the Urban Tree Replacement program ( ).

Planting the right tree in the right place is key to keeping Seattle green.

Maintaing Trees in Planting Strips

Q: Who maintains the trees on my planting strip?

A: All planting strip trees are regulated by the Seattle Department of Transportation (SDOT), but only a percentage are maintained by SDOT. If a tree was planted by a property owner, or has grown naturally, the property owner is responsible to maintain the tree. If you are unsure who may have planted your tree, you can call (206) 684-TREE (8733) to obtain information about maintenance responsibility.

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