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Water Levels and Soil Volume: How Climate and Rain Conditions Influence Recommended Requirements

In a recent post exploring the amount of soil it takes to grow a large tree, I challenged the notion that 1,000 cubic feet is a one-size-fits-all axiom. Ultimately, this rule of thumb allows only for the growth of a medium tree — and the more soil volume you include, the greater your chances of growing a big, healthy tree.

Likewise, it’s important to understand other factors when determining soil volume levels — particularly the tree’s water use, which varies widely depending on the field capacity of the soil, the evapotranspiration rates of the tree itself, and the leaf area index. When considering soil volume, you should always first consider the climate: dryer and hotter locations require much more soil than wetter, cooler regions. Further, landscape architects must also recognize the latest drought and rainfall patterns, as climate change continues to create larger and longer droughts.

Obviously, a tree planted in the Northeast has different soil requirements than one planted in the Southwest. But not all comparisons will be so simple. Take, for example, the Amazon Denny Streetscape project in Seattle versus Project 180 in Oklahoma City. While the rainfall levels are fairly similar — Seattle receives 40-45” per year while Oklahoma City gets 30-35” — the susceptibility to drought in Oklahoma is more pronounced: they only experience 72 annual rainy days versus Washington’s 156.

Simply put, Oklahoma City trees require more soil. The continual wetting of the trees in Seattle has obvious implications for tree soil. And less frequent and larger storms in Oklahoma means that much of the rain flushes away in a torrent and is not taken up by the trees. See below for the tree volume statistics for each project:

 

Seattle (Amazon Denny)

-187 cubic feet of soil on the tree openings

-340 cubic feet of soil volume per tree

-21 trees

Oklahoma City (Project 180)

-600 cubic feet of soil on the tree openings

-1,110 cubic feet of soil volume per tree

-568 trees

Project 180, Photos from 2014 Walker and California St. Oklahoma City.

Project 180, Photos from 2014 Walker and California St. Oklahoma City.

Because of the consistent wetting of the soils in the Northwest, the plant performance will progress very well until the canopy eventually matches the volume of the soil installed. The Seattle project is a green canopy dream: a dozen trees were planted among hundreds of Silva Cells in the heart of a newly renovated Amazon campus. The street trees border the campus buildings and act as a barrier between car and bike lanes. In addition, their shade extends into the sidewalk, protecting pedestrians from extreme heat days. And all of this was achieved with less soil volume than that used in Oklahoma City, thanks to the favorable water conditions of the Washington region.

7th Avenue between Blanchard and Westlake. Photo was taken 2021, Amazon Denny Streetscape.

7th Avenue between Blanchard and Westlake. Photo was taken 2021, Amazon Denny Streetscape.

7th Avenue between Blanchard and Westlake. Photo was taken 2021, Amazon Denny Streetscape.

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