In Search of the Best Street Tree
David Eran, Landscape Architect
Recently, everyone is talking and writing about the Urban Forest, its great significance for the city and its citizen's well-being. Many cities have already completed a comprehensive survey of its street trees, with emphasis on characteristics of species, location, age and condition. Such a survey, when based on digital data, is supposed to assist in managing and maintaining the urban forest, as well as serve a tool in planning the expansion of that “forest”.
Street trees compete, for their spatial needs, both above ground and underground. As our cities become more congested, this fight for the optimal conditions of street trees seems too often lost ahead of planting.
Add to this, that we face fast-growing awareness of the critical limitations of the traditional planting techniques, primarily the volume aspect of the growing substrate.
Global warming and more specifically the urban heat element, contribute increasing importance of the shade factor that street trees can provide.
With such a background, it is no wonder that the obvious question arises: which is the perfect street tree?
People will provide quite different answers as to their preferred characteristics in choosing a street tree.
This is a natural result of diverse backgrounds, tendencies and points of view.
If we want to make an objective judgement in the selecting process, we need to set some preference criteria. Accepting the natural diversity of answers to the question "what is a nice, beautiful, attractive etc. tree", I will attempt to reach the answer by considering only measurable criteria. I hope this article will help in setting such measurable criteria.
Please keep in mind that this article is based on Israel’s environment and experience, therefore be sure to make the necessary adjustments when trying to apply the ideas and/or the information to other places.
Ornamental trees in Israel and suitable street trees species
1. A recently compiled list of tree species, sub-species and varieties has reached the fantastic number of over 1,300! Of that number one has to write-off about 300 names, that had been included only in old lists and presently are not mentioned in any botanical garden, commercial nursery or private collection lists.
2. For ornamental uses, we should omit the rather large group of afforestation species, primarily conifers, Eucalyptus, Acacia species and their relatives.
3. The exceptional concise book (in Hebrew) about ornamental tree in Israel, by Nisim Pines, published in 2008, describes about 300 species, sub-species and varieties.
4. Counting all street trees species planted in Israel within the last 100 years, that list will contain over 120 species.
5. My present list, recommended for consideration, includes some species not yet used as street trees and excludes those species known to be either too large or have unaccepted nuisances. Thus, we start our filtering process with about 90 species and varieties.
1. Street terms
Roadway (= travel lane, vehicle lane, vehicular lane, drive lane)
Sidewalk (= pedestrian path, pavement)
Bike path (= bike lane, cycle lane)
Median (= median strip)
2. Criteria Terms
Nuisances: Includes juicy litter, medium or larger Oak acorns, dry twigs, dry fruit capsules or pods, up-heaving surface roots, limbs tend to break.
Decorative Values: This includes any of the following characteristics: showy blooming, fruit, showy fall color, remarkable trunk, special color of young growth.
Skeleton Shaping Convenience: Mostly any young tree will need some shaping, in order to nurture strong leader and select major limbs, correctly spaced and directed. In some species this will require a lot of time (summer and winter treatments) and for quite many years following planting.
Uniformity: While some species will present quite uniform size, shape, blooming volume and time, when propagated from seeds, other will show great variability.
Longevity: For a street tree, a short life span can be estimated at 30 years, whereas long life span will be over 70 years.
Tree Form (silhouette): When trees are planted in smaller intervals than their mature crown size, they will not show their typical form. This attribute is relevant only for trees planted in significantly larger intervals than their crown diameter.
B. Street Trees Criteria
To start with, see the following list of criteria, typically used by experts and people alike, in selecting street tree species.
The list, as it is presented here, does not appear in any order of importance or significance.
1. Shade Contributing
2. Tree Size- Height / Width
4. Decorative Values
5. Skeleton Shaping Convenience
6. Climate Compatibility
7. Soil Requirements
9. Insect and/or Disease Vulnerability
10. Water Requirements
12. Tree Form (silhouette)
13. Sun Exposure
14. Planned Tree Location Within the Street Section
Once we have a list of criteria, we can start the major task: sorting the criteria in order of importance.
Considering the fact that some criteria are crucial for proper development and survival of street tree, we have to accept and designate these factors as critical.
There are four critical criteria for survival of the trees:
Among these four criteria, the “climate factor” should be regarded as a rigid one and beyond our ability to change or manipulate.
Likewise is “sun exposure”, because the direction of the street and its buildings’ location and height are pre-determent conditions.
Bear in mind that almost all street trees species, recommended for Israel (excluding palms), require at least 6-7 hours of direct sun, for healthy growth. In regards to sun exposure, typical street situation will satisfy these requirements, unless very tall buildings are located adjacent to right-of-way lines.
The two other criteria, water and soil requirements:
In most cases we can adjust site situation or conditions to suit the trees’ needs.
Because of the uncertainty of the planting period, we should plan implementation of an irrigation system for a 5-8 years minimal period of supplementary summer water supply.
As to soil requirements, a typical street tree planting pit, with the added expansion volume of soil or similar material (like CU Soil, etc.) will reach at least 6-8 m³ and being “added soil”, or “soil substitute” we can control its properties.
Most trees’ species require “good drainage” for their planting site, so this is a mandatory criterion, which will be included in the soil/substrate requirements and therefore will be out of our selection process.
C. Criteria Discussion
Left with ten criteria, let us divide them into two groups:
1. Personal Preferences or Objective Criteria
1-1 Shade Contributing
1-2 Planned Street Tree Location Within the Street Section
2. Trees’ Characteristics:
2-1 Tree Size- Height / Width
2-3 Decorative Values
2-4 Skeleton Shaping Convenience
2-7 Tree Form (silhouette)
2-8 Insect and/or Disease Sensitivity
In a hot and dry country like Israel, trees’ shade in the city should be regarded as one of the major contributions of the urban forest. Trees reduce the urban heat by 3-5°C under their canopy and people (pedestrians and cyclers alike) feel much better while maneuvering under shade. Also, people prefer to park under shade, assuming parking next to the street curb is allowed.
To achieve maximum cooling affect, continuous shade is needed. Better still, to have the shaded strip cover additional areas such as sidewalks, bike lanes and even parts of cars/buses/truck lanes, if canopy height allows it.
In order to allocate a quantitative measure of the expected shade, we need to consider two factors: tree size in maturity and planting interval.
The first factor is a tree characteristic, whereas the second one is a planner decision. If for example, we select a 6.0 m. canopy diameter (when mature) species, we will not achieve continuous shade with 8.0 m. planting intervals.
Decreasing planting intervals is one way to achieve continuous shade, if we don’t mind the quite dense and crowding trunks appearance along the street.
Planting intervals consideration should include lighting post intervals, as a minimum of 3.0-4.0 m. is typically required between a lighting post and a tree trunk.
Tree shade varies in intensity, which is the result of the tree crown density. Ficus species have typically very dense, or “heavy” shade and Gleditzia’s shade is light. But for our purpose, of selecting a street tree for a specific site, we have ignored this difference.
Planned Street Tree Location Within the Street Section
There are a few situations in regard to the detailed location of street trees, within the right-of-way lines.
Right adjacent to roadway curb and it is a “no parking” zone. In such a case buses and trucks might move next to curb and a tree crown has to have a 4.5 m. clearance.
Be aware that shaping trees to stand freely, with a tall trunk will start in the nursery and requires continuous shaping, later on site. Some trees are simply not suitable for such forming. This is right especially for naturally small trees and shrubs shaped as trees.
1. Adjacent to a roadway curb is a parking lane. Assuming it will be legal to limit height of parking cars, we could overcome the mandatory 4.5 m. clearance, and use trees with a shorter trunk.
2. The trees will be planted in a strip between bike lane and sidewalk. This situation distances the street tree line from moving vehicles, thus allows the use of low crown species or low trained crown trees.
3. In median tree planting, the crown width is the limiting characteristic.
Typically, we have a narrow planting median of about 2.0 m. and a wide median of about 5.0 m.
Selecting trees, palms and/or shrubs for median strip is more of a landscaping issue to consider, as part of the overall street section scheme.
There is growing awareness of costly and dangerous maintenance of street trees, especially for the actions required to be done, while operating on the driveway or next to it. This leads us to be very careful in the selection process.
D. The Filtering Process
1. Step One – Regional Climate
Using a complete list, start by filtering the species suitable for the regional climate. This will filter out any tree that has less survival chances in the intended site.
2. Step Two – Trees’ Location
The next step is to determine the trees’ location in relation to vehicular traffic, parking lane, pedestrian and bike strips. This will eliminate species unsuitable for clearance requirements, tree skeleton shaping, and/or overall dimension at maturity.
3. Step Three – Shading Degree
In my opinion, the next filtering step relates to the shading degree required. If a continuous shaded strip is the goal, you will need to fix planting intervals. Not doing so, theoretically means that all species are suitable.
By adding a proposed planting interval, you will take off the list species with limited crown width to achieve a continuous shaded strip.
4. Step Four – Nuisances
In the effort to search for the perfect street tree, it is important to consider inherent nuisances in the species we examine. In “nuisance” we mean a negative attribute, primarily a feature that can cause risk of some sort to street users.
Noting several typical negative attributes:
- Juicy flowers or fruit
- Hard kernels, acorns, pods
- Shallow (surface) roots
- Exceptional wide trunk base
- Dropping dry twigs or seed capsules
- Attracting bees (though this attribute will be considered “positive” by some people.)
One can choose to avoid all or just part of them. Once you start the filtering process, you will notice that as you try to avoid all nuisances the list shrinks significantly.
5. Step Five – Decorative Values
Decorative values include showy blooming, fruit, seasonal leaf color or interesting trunk. Some of these positive values go hand in hand with their negative side, as the flowers, fruit or fall colored leaves drop to the sidewalk, bike lane and on parking cars.
Even though all these phenomena last only a few weeks each, they demonstrate the seasonal changes and enhance people’s awareness of nature annual cycle and hence their added value.
6. Step Six – Skeleton Shaping Convenience
Tree nurseries in Israel race to sell as fast as they can, meaning the tree is for sale immediately upon reaching the required trunk diameter, measured at 20 cm. height. Typically, at this stage, the trees lack a well-developed skeleton or are only at a very early stage of forming a good skeleton.
This implies that the task of full tree skeleton shaping should transfer to the municipal maintenance teams. Even when these teams include professional trees’ experts (which is quite rare!) the shaping task will take a good couple of years.
For young trees efficient shaping requires summer and winter treatments, using ladders or truck-mounted lifting baskets.
The differences in the amount of work needed to shape a street tree are of high importance. For example, in comparison of the two mostly recently planted street trees in Israel, Celtis australis and Jacaranda mimosifolia, it is clear that they have significant different labor requirements. The Celtis rarely has sharp-angled (“crouch type”) branches and therefore requires just some thinning-out of unwanted young branches. The Jacaranda, on the other side of this scale, is famously known for its narrow branching and necessitates continued efforts to achieve well-angled limbs.
7. Step Seven – Longevity, Uniformity, Tree Form (silhouette)
City's growing environment is generally detrimental to trees’ needs and therefore the longevity (life span) of street trees will be typically much shorter than of the same species planted in gardens, parks, along highways etc. If after other filtering steps one has still enough species on the list, this seventh criterion might help to narrow down the list.
The degree of uniformity of trees propagated from seeds is typically lower than trees vegetatively propagated. A species variety or clone must be propagated from cuttings or be grafted. If there is a choice, to choose the propagation method, it will be an advantage to prefer the vegetative way that will yield uniform trees.
Tree Form (silhouette): Though trees vary quite a lot in regard to their form, in the street situation these differences will barely be noticeable. The reason is that with relatively short planting intervals, trees’ crowns will meet along the row and thus the typical tree form will not develop.
For small trees, if selected, tree form can develop according to the tree nature. In such a case there is an option to use tree form preference.
E. Considering Top- Grafted Varieties?
There are two reasons for top-grafting ornamental trees:
1. The variety to be grafted naturally has a rather small ball-shape or weeping form.
2. The variety is very slow in forming the required trunk height.
The typical tissue strength, at the grafting point, is weaker than what is formed by bottom-grafting. Still, assuming the grafted variety will grow a small crown, there are good chances for reasonable life span. This requires professional and continuous maintenance, which is quite rare to find in many municipalities.
Also, expect growth of shoots from the stock. Such shoots should be removed when young. Failure to do so weakens the variety grafted and eventually can totally change the tree form and characteristics.
The expected crown diameter of trees in this group will be 3.5-5.0 m. Thus, in most cases they will be suitable for wide median planting, under utility lines and on extremely narrow sidewalks.
Depending on one's requisite criteria, the results may be a small or large number of species. The more we add demands, the shorter the list becomes.
Accepting that there is really no “perfect tree”, since nothing in nature can be titled “perfect”, it is our job to make compromises. In our extremely hot summer, we should be quite happy with even a short list of trees that will survive in the designated locality and will provide a continuous strip of shade.
Any additional positive attribute that might come along, with some species, should be regarded as a bonus!
On the same line of thought, accepting some negative attributes in our selection, might allow including species not too common, thereby enriching street sight, atmosphere and value.
G. Tree Lists
1.a Species not included
Analyzing the present existing street trees in Israel, one will easily notice the very limited number of mature (over 70 years old) species. Until the establishment of the state of Israel, the list of shade, as well as afforestation species was rather short. Selecting street tree species from this list had yielded not more than about 10—15 species.
Examining the suitability of these "Historian” species, we have to accept the notion that some do not conform anymore with present requirements. For example, Ficus nitida (=F. retusa) will not be accepted at any municipality nowadays, due to both juicy litter and extremely shallow roots.
1.b Species not included:
2. Recommended species for street trees selection: