It’s been awhile since I posted a new entry, but I’ve been thinking about this issue for years. So-called “street trees”, or those trees that are generally in the right-of-way of a municipality, have many challenges. Some issues are easily remedied or avoided; others are the result of the harsh environment of living in a city, with pavement over much of a tree’s root system. Poor soils, little space for roots to expand into, high temperatures exacerbated by paving, and the urban “heat island effect” are but a few of the foregone issues that street trees must endure. This post addresses one of the most difficult problems facing many street trees and the cities that are responsible for them – the planting of trees into sidewalk cutouts that are simply too small, such as this gem:
There are two factors that must be considered in this discussion. First is the age-old one of putting the “right tree in the right place”. The second is closely related: even if you have the right tree in the right place, it still needs an adequate amount of space to accommodate its potential fully mature root crown or root flair. If a tree’s root crown, the location where it enters the ground, is going to grow (yes, it is!), it needs to have adequate room so that it can grow to its full mature size without being constrained by the edges of the sidewalk, street, curb, gutter, and/or steel grates. I will go into more detail about the right tree in the right place in a future blog post. Here I want to discuss the second issue: planting trees in a location along the street where the future growth of the tree’s trunk and root crown will not outgrow the “cutout” provided in the sidewalk.
The issues are as follows:
1. Planting decisions– The tree will grow to a certain diameter at its base and will require that much clearance ultimately. Trees that will grow to a large diameter need a large enough cutout when they are planted. (The cutout is the planter size where the tree is planted, usually surrounded by concrete on 3 or 4 sides). There are two obvious ways to address this.
• If the sidewalk cutout is small and cannot be enlarged to accommodate a tree that will grow to be large, then a tree that won’t become so large should be planted.
• If a larger tree is desired, then the cutout should be enlarged so that it can accommodate the potential fully mature size of the tree.
This coast live oak (Quercus agrifolia) was too large for its original cutout, and there doesn’t seem to be any room for cutout enlargement. The result is clear and not desirable:
2. Maintenance decisions– Trees are often planted in busy pedestrian areas where, because the soil level in the planter is typically lower than the surrounding sidewalk, there is a fear that pedestrians may step into a cutout and trip. There are many ways that cities address this, some better than others. I’ve listed a few below, with their potential problems.
• Iron grates are often used because they are attractive, last forever, and can be enlarged as the tree grows. The problem is that they often are not enlarged before the tree grows into them, and they end up becoming imbedded in the tree’s root crown. Once this happens, it is damaging to the tree to remove the iron grate. It is also damaging to the tree to not remove the grate. Eventually, the tree may become girdled and die as a result of the imbedded grate. This is a fully predictable outcome and, in my experience, few cities have sufficient maintenance staff, organization, or budget to monitor their trees and adjust the grates as tree growth requires. This crape myrtle (Lagerstroemia indica) is overgrowing the unmaintained iron grate at its base:
• Another practice is the use of bricks, asphalt, or some other means of temporary paving inside the cutout. While this is potentially less damaging than an iron grate once the tree begins to grow into the paving (it’s less likely to girdle the tree and kill it), it can still do considerable damage to a tree’s root crown, once it begins to become imbedded. Otherwise, the issues are the same as above. Mainly, a requirement for maintenance crews to enlarge the opening to accommodate the tree’s growth, and the lack of sufficient resources to provide such care. This sycamore (Platanus racemosa) is breaking the temporary paving inside its cutout:
And this one is breaking the temporary pavement and lifting the surrounding sidewalk. This infrastructure damage could have been avoided:
Solutions to this array of potential problems for street trees include:
• If the cutout is small, and cannot be enlarged, planting a tree that will not encroach on the surrounding concrete once it reaches its maximum potential size.
• If the cutout can be enlarged, then it should be enlarged (preferably prior to planting) to accommodate a larger growing tree.
• Trees that are in existing cutouts, and which have the potential to outgrow them, should be monitored. When the tree begins to encroach on the concrete, the tree should be removed, or the cutout enlarged.
• If budget constraints prevent the proper monitoring and modification of restrictions to future growth (such as enlarging the opening in steel grates or paving) then these restrictions should be avoided.
• One solution to the dilemma of preventing a trip hazard around trees in city cutouts is to surround the tree with decomposed granite up to the level of the sidewalk.