How Does the Silva Cell Affect Street Tree Value?

By L. Peter MacDonagh, RLA, ISA, LEED-AP

This post has a lot of statistics. If you are tired this may not be the blog post to read. Reading it when you are well rested will probably help.

The May 2010 issue of Landscape Architecture Magazine has a terrific article in it, “What is a Street Tree Worth?” This study supplies us with a very valuable piece of quantitative research. The details also allow us to extrapolate significant information about the cost of the Silva Cell relative to the value-added of a nice street tree. But I’m getting ahead of myself.

Here is a summary of some of the salient details of the research from this study:

– The 2,608 houses were sampled from residential neighborhoods with free standing, single family homes on the East side of Portland, Oregon. The lot/property parcel configuration in E. Portlandmakes it clear which neighbor “owns” a street tree.
– The sample size of 2,608 houses is robust.
– The median house price of $259,000 U.S. is very sound. In Portland, this would mean a house of about 3 bedrooms/2 baths/1 car garage in a good neighborhood. (Median: 50% of houses are higher than this dollar value/50% of houses are lower than this dollar value). The median house price of $259,000 is high for the South & Southeast United States, but it is low for the East and the West Coast of United States/Canada. For the Midwest, this is about right for Chicago, but high for most other Midwestern cities.
– The street trees in the study were very well evaluated. They were qualitatively evaluated on the ground, and the average canopy area was calculated with GIS (Geographic Information System).

The research shows that each house “owned” (in front of their house) 0.558 street trees. Street trees sat somewhat between 2 houses. The tree value added to median sale price of $259,000 per house was $8,870, or 3.4%. The street trees that generated the $8,870 value had an average canopy diameter of 20 feet (6.1 m) and a canopy area of 312 square feet (29 square meters). While the median time on the market of 71 days is quick because this study was done the housing boom of 2006/2007, houses with trees in front still sold 1.7 days faster that those without (about 2.5% faster than the median).

Now, let’s put this in context of what we’re doing at DeepRoot using an example.

The $8,870 added value per house of a street tree in this Portland study is more than the average cost of installation per street tree based on a recent project called the Marq2 in Minneapolis.

The Marq2 was not a residential project, but bear with me. If the average street trees in this Portland study had been planted with Silva Cells, the net real estate value of these street trees would cover cost of the Silva Cell installation once the tree reaches a 20 feet (6.1 meters) canopy diameter and a 312 square feet (29 square meters) canopy area. We know that to reach this size, the average street trees from the study will need access to 624 cubic feet (17.6 cubic meters) of soil, which would require around 62 Silva Cell frames per street tree.

The average Marq2 street tree has access to 670 cubic feet (18.9 cubic meters) of soil, including the 5 foot by 5 foot tree opening. We can safely predict that the average tree canopy area of the Marq2 trees will be 335 square feet (31.1 square meters).

This means that for the Marq2 installation, the real estate appreciation of the street trees alonecompletely covers the cost of the Silva Cell installation. This does not factor in the significant added benefits of on-site stormwater management, which is significant. Remember that the City ofMinneapolis opted to spend around $1.5 million on the installation of the Silva Cell rather than $7.5 million on a revamped sewer system to meet their stormwater goals. You don’t need to be too alert to understand how good that math is.

Neat stuff!

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