Street Tree Planting Procedures
Trees Approved with Reservations
- Albizia julibrissin. (silk tree, mimosa) Vulnerable to fatal canker attacks in Seattle.
- Betula alba. (white birch, weeping white birch) Regular aphid infestations - probably will not kill the tree, but sticky "honeydew" drips and makes a mess. Do not plant where people park their cars. Note: Many trees get aphids, some varieties of birch are more heavily attacked. River birch and Himalayan birch are somewhat less susceptible to aphids, but infestations will still occur.
- Catalpa. Brittle wood. Lots of debris from the tree over the course of a growing season. Should have a very wide planting strip to accommodate ultimate size.
- Crataegus oxyacantha a.k.a. C. laevigata. C. monogyna (Including Paul's Scarlet and several named varieties) Has aphid problem. Some cultivars are especially susceptible to the black spot fungus, which may defoliate the tree by July. C. monogyna is classified as a class C noxious weed by King County.
- Gleditsia triacanthos. (honey locust) Thorny - select only named cultivars to avoid thorns, such as "Shademaster", "Skyline" or "Imperial". Plant in wide planting strips. Pod midge gall can be a problem with all varieties.
- Juglans nigra, J. regia. (black walnut, English walnut) Messy fruit. J. nigra roots are alleopathic (toxic to the growth of some other plants).
- Liquidambar styraciflua. (sweetgum) Wood is somewhat brittle. They need an especially wide planting strip, as they can develop a robust root system.
- Platanus spp. (London plane, sycamore) Trees become quite large, so adequate planting space should be provided. They are susceptible to anthracnose diseases. Some improved varieties of London planetree have shown resistance to anthracnose in this area.
- Quercus paulustris. (pin oak) Lower limbs keep growing downward, and require lots of pruning when used as street trees. 'Crownright' is a variety that should be used to avoid this problem.
- Sophora japonica. (pagoda tree) In Seattle, it is highly susceptible to canker attack, which is often fatal.
- Ulmus americana. U. pumila (American elm, Siberian elm). American elm is highly vulnerable to Dutch elm disease. This disease is expected to kill the elms in this area. Newer disease resistant varieties may be approved for street planting. Siberian elms have brittle wood, and are prone to storm damage.
- Conifer trees are very desirable, but not generally recommended for street planting. The lower limbs can cause visibility/safety problems at driveways, alleys, intersections, signs, and signals. They may be approved for street planting if the site is deemed appropriate. The planting of Conifers is encouraged on appropriate private property sites.
Trees Prohibited for Street Planting in Seattle
- Acer negundo, Acer saccarinum, Acer macrophyllum. (boxelder, silver maple, and big leaf maple) Fast growing trees which have a tendency to have weak wood. Big Leaf Maple grows too large for most planting strips.
- Ailanthus altissima. (tree of heaven) King County classifies this tree as a Class C noxious weed. Roots are invasive, brittle wood, suckers freely, (produces new trees off of the root system, which may create a maintenance problem in the yard).
- Alnus rubra. (red alder) Brittle wood. Favorite of tent caterpillars.
- Malus. Fruiting apples. Fruit on walks.
- Prunus. Fruiting cherries/plums. Fruit on walks.
- Pyrus. Fruiting pears. Fruit on walks.
- Ficus carica. Fruiting Fig. Fruit on walks – aggressive root system.
- Populus spp. (Poplars) Tops are brittle and break up easily in storms.
- Robinia pseudoacacia. (black locust) Thorny, brittle.
- Salix spp. (willows, including weeping) Roots are particularly hard on sewers.
For more information, call the City Arborist's office at 206-684-TREE (8733).