Species prohibited on public rights-of-way
Boise Community Forestry wants the City of Trees to have a healthy, safe and long lived "urban forest". Boise's residents want the same thing. One of the most important ways to assure this is to choose appropriate species to plant. The Tree Selection Guide is a good tool to use to find an appropriate tree to plant in the landscape or as a street tree. There are some trees, however, Community Forestry does not allow as street trees on public right-of-way. You also may want to consider some of the potential drawbacks of these trees before planting in your own yard. The following is a list of such trees. It is against city policy to plant a tree of these species on public right-of-way. This is not a complete list, so please contact Community Forestry at (208) 608-7700 or by Email before purchasing a tree for planting on public right-of-way. Please consult the online Tree Selection Guide for an extensive list of excellent trees approved for planting on public right-of-way.
Silver Maple (Acer saccharinum)
This Class III species has weak wood that is prone to extensive decay creating an extreme public hazard. The species may appear viable despite the hazardous condition. Silver maple also have shallow, aggressive roots that are more likely to damage sidewalk and curb than many other allowed species. Species grows quickly to 70+ feet tall and wide. Silver maple is more prone to storm damage than any species growing in Boise.
Red Maple (Acer rubrum)
A Class II species with shallow aggressive roots capable of ruining sidewalks and curbs. To this point, these trees have shown poor long term performance. You are not likely to find many healthy, mature red maples in Boise. New varieties of this species are frequently introduced to the market, some of which claim to improve on weaknesses of earlier varieties. It is too early to tell if these claims are justified in real world application, and it's very difficult to control planting of only those varieties. No varieties have yet claimed to improve on shallow rooting. Species grow to 40-60 feet tall and wide at maturity.
Red Maple, Freeman varieities (Acer x freemanii)
This species is a cross between Silver Maple and Red Maple, and shares many characteristics with each species. Varieties include Autumn Blaze, Armstrong, Celebration and others. Again, this species is a relatively new introduction, but shallow roots have already been noted on most real world plantings.
Boxelder or "Sensation Maple" (Acer negundo)
Though they are maples, the boxelder suffers from many of the same problems as the silver maple, having weak wood and issues with extensive decay. The Sensation Maple is a variety of Boxelder commonly sold in retail nurseries. They also tend to attract Boxelder bugs more than other maple species. The tree can appear to be quite healthy despite extreme hazardous conditions.
Paper Birch and European Birch (Betula papyrifera and Betula pendula)
These trees are not well suited to the low humidity or high summer temperatures in Boise. They are also extremely susceptible to Bronze Birch Borer (an insect) as a result of this stress. Instead, consider River Birch (Betula nigra), which has very attractive bark, great fall color and has performed very well into maturity in most areas of Boise. River birch is resistant to the heat and to Bronze Birch Borer.
Poplar (Populus species)
Weak wood and prone to extreme decay. Trunk is too large for street tree use on public right-of-way. Large, aggressive roots will damage sidewalks, curbs and driveways even at a young age. Roots will sprout new tree growth in yard. Species is quite short lived.
Quaking Aspen (Populus tremuloides)
Does not favor Boise's hot, dry summertime climate. It is very susceptible to borer insects and leaf spot diseases. A very short lived tree that propagates by sprouting up from roots into lawn and garden areas.
Willow (Salix species)
Weak wood prone to extensive decay. Tree can appear viable despite hazardous conditions. It has large, aggressive roots. The weeping habit of most varieties also makes it inappropriate for use along streets and sidewalks.
Elm (Ulmus species)
Certain elm species and varieties are actually encouraged as street trees ( Ulmus americana "Valley Forge" and "New Harmony", Ulmus carpinifolia "Prospector"). Other species such as Siberian elm (Ulmus pumila), English Elm (Ulmus procera) and some varieties of American Elm (Ulmus americana) are undesirable. The English Elm and certain varieties of American Elm will be killed by Dutch Elm Disease (DED). Some new varieties of American elm have been developed that are resistant to DED (such as Ulmus americana "Valley Forge" and "New Harmony"). Most elms have other pest problems apart from DED, such as Elm Leaf Beetle, scale and bark beetles. Do not plant a sprout that has come up in your yard, as it will not be resistant to DED. Contact Community Forestry for more information.
Black Locust (Robinia pseudoacacia)
Though they used to be quite popular, this species is now extremely susceptible to Black Locust Borer (an insect). Seeds are mildly toxic.
Honeylocust variety "Sunburst" (Gleditsia triacanthos "Sunburst")
This variety of the popular species has shown to be very prone to several leaf insects and canker diseases. It is also slow growing and low growing. Other varieties of Honeylocust have proven to be very successful in Boise (Shademaster, Skyline, Moraine).
Black Walnut (Juglans nigra)
The walnuts are not only a mess, but can also be a hazard. This tree is a favorite of aphids, which drop copious amounts of sticky "honeydew" that blankets everything under the tree. Recently, a disease or other climate problem (yet to be positively identified), has proven to be fatal for this species. Rapid decline and death of this species is seen everywhere in the Treasure Valley.
Tree of Heaven (Ailanthus altissima)
Clearly misnamed by some fool, this species is one of the most prolific producers of root and seed sprouts known. It is a great tree for the zoo, because not even a hungry herd of buffalo can eradicate it once it is established. Extremely rapid (rabid) growth to 60 feet tall or higher.
European Ash (Fraxinus excelsior)
This species is extremely susceptible to Ash borer (an insect). Also commonly declines and dies from root graft failure in middle age. Consider White Ash (Fraxinus americana) or Green Ash (Fraxinus pennsylvanica) instead.
Conifers in general
Pines, spruce, fir etc are very difficult to prune out of sidewalks and streets. Also a hazard for traffic, sign and pedestrian visibility.
Several other species including Mountain Ash (Sorbus), Sumac (Rhus), American Sycamore (Platanus occidentalis) are not allowed for many of the same reasons listed on other species.
Many trees sold on the open market are grafted cultivars which have characteristics that are considered desirable, such as a particular flower color or being seedless. The root portion of these trees does not have these desirable characteristics. Do not transplant sprouts that appear in your yard onto public right-of-way or allow sprouts growing in the right-of-way to continue to grow. It is likely a species or variety that is undesirable for many of the reasons listed above.
Please visit the Tree Selection Guide to discover many, many trees that are very desirable for right-of-way or any landscape planting!