What characteristics would the "perfect street tree" require?
- Appropriate size
Just like the fable; not too big, not too small, but just the right size. Too big would probably mean a large trunk and large roots. Too small is just out of scale with the streetscape. In some situations, tree canopies could block signage for commercial buildings.
- Appropriate form
Some like them rounded, some like them columnar. What it can't be is "droopy", i.e. the branches cannot curve downward. Hopefully, it would naturally be branching upward and give a lot of trunk height.
- Adaptable to pruning
Some trees don't like being cut. Maintenance crews will come along and make sure people, cars and trucks can pass under the lowest branch(es). Utility crews will come along and top branches right at the power and/or phone lines.
- Climate specific
Trees, like people, need to thrive and not merely survive. Here on the coast of California, that means the choices of trees should probably come from one of the five Mediterannean climates of the world (California, Northern Mediterannean, Chilean coast, Australia and South Africa). Natives often don't work well as street trees (because most natives like well-drained soils).
- Drought tolerant
Irrigation (if there is any) will probably not work or not work well in a few years after planting. Irrigation will get run over, may never get adjustment or simply may not have been put in well. Don't count on it for your street tree to thrive (see above).
- Available without custom growing
Nurseries simply should have them in stock. Size is not that critical and a University of California study showed that smaller stock will catch up with larger stock in a relatively short time - tell that to your City Council !!
- Minimal maintenance
America is short on the budget for maintenance, let's face it. Park budgets may contain provisions for new parks, but all too often do not contain money for new employees to maintain the urban forest. Choose the right tree, and plant it well and that will help.
- Smog tolerant
It's only getting worse, rarely better. Some trees choke or develop diseases easier in dense urban settings.
- Tolerance of compacted soils
Good planting requires all trees have loose soil for their roots. Too often the ground has been compacted and then we can't figure out why the trees don't survive well. Trees grow in soil, never in concrete. Don't scrape the side of the tree planting area. Avoid heavy machinery in the areas that are to recieve plants.
- Well- behaved roots
Trees need water - one of the few basic requirements. Roots seek out water. If the water is under the sidewalk, that's probably where the roots will head. Don't pick a tree that requires large amounts of water to survive and provide watering tubes with bubblers for each tree.
- Medium growth time
Maybe it's okay to have your grandchildren appreciate the large street trees you selected, but most officials don't think that way. On the other hand, if the tree grows like a weed, it IS a weed.
- Insect and disease resistant
Keep up with what is attacking trees in your area. I check with the local Agricultural Extension agent.
- Lack of fruit drop
People can slip on tree fruit and big leaves. Cars can be stained due to fruit juices. This really limits the trees you can use, but cuts down on the complaints.
GOOD LUCK !
STAYING ALIVE: STREET TREE SURVIVAL IN THE INNER-CITY
FRED SKLAR and RICHARD G. AMES
California State University, Hayward
A study was conducted from 1978 to late 1984 of the survival of inner-city street trees planted by the Oakland, California urban forestry program.
The tree survival rate of approximately 60 to 70 percent from the trees planted in neighborhood parkways through urban forestry sponsored block parties contrasted sharply with less than one percent survival of trees planted earlier by the Model Cities program without community participation or ceremonial plantings.
Explanations for the high tree survival were tested using participant observation, interviews with residents, and a small panel. Tree survival was not found to be related to the explanations provided by urban forestry ideology; namely, that residents had been educated to hold both instrumental and expressive values toward trees. Rather, the explanation appeared to be the function of an unintended solution to the issue of parkway ownership. The tree planting process which included species selection meetings and tree planting ceremonies tended to define the parkway trees as a resident's property, thus decreasing the significance of the parkway-property barrier.