Are Small Trees Always Appropriate As Street Trees?
by Michael Kuhns, Extension Forestry Specialist
Most people, including urban forestry professionals, assume that small trees are appropriate for use as street trees. Many people even believe that small trees are the BEST trees for such settings. Is this true?
In most street tree planting situations if a big tree is not appropriate then small trees also are not appropriate. How can this be? It seems like if a space is too small for a big tree then a small tree should be just right. Actually, though, if a parking strip (the strip between the sidewalk and curb) is too narrow for the trunk and roots of a big tree, it's also too narrow for the CROWN of a small tree. Small trees planted in narrow parking strips usually end up with crowns that go out over the street and sidewalk, yet the crown doesn't ever grow up high enough to get out of the way of traffic, parked cars, and pedestrians.
Large trees would actually be preferable in many narrow parking strips were it not for potential curb and sidewalk damage. Such damage can be reduced by proper compaction of sidewalk and street base material at planting time. Some curb and sidewalk damage is unavoidable, though, if large trees are planted in small strips.Many experts recommend a minimum 8 foot wide planting strip between sidewalk and curb before a large tree can be planted as a street tree. I think a strip nearly that wide is necessary for most small trees too, for the reasons stated above. A small tree planted in a wide or narrow parking strip causes more visibility problems than a large tree, even if it's not physically obstructing cars or pedestrian traffic.
So what can you do if you want street trees? For large trees, either plan for wide (8 foot minimum) planting strips, or use narrower strips and be sure to thoroughly compact sidewalk and street base material and plan on some eventual damage from root and trunk growth. Even then use the widest and longest strip possible. Select small trees whose crowns will grow to be no wider than the planting strip. Be sure you get good information on growth rates and tree sizes C Bradford pear was once touted as a small tree suitable for many tight areas, but I have seen Bradford pears with crowns 40 feet high and 30 feet wide and trunks a foot or more across.
Another option for small or large trees is to move the sidewalk very close to the street and plant trees on the other side, away from the street. This still gets trees near the street but eliminates the constraints of a narrow parking strip. Trees still need to be spaced back from the sidewalk several feet, but large trees can be easily accommodated. There is a drawback in that towns and cities often do not own or control property behind the sidewalk, so planning for a pleasing street treescape can be difficult.
This article won't end with a list of appropriate small (or large) street trees. Any tree can work as long as there's room for its roots, trunk, and crown. Good planning and planting will ensure we have the street trees we want for the future, whether small or large.