How Much Soil Do You Need to Grow a Big Tree?

It seems pretty universal that everyone likes trees. We find them pretty, and calming, and they make us feel healthy.

Space, pollutants and engineering requirements are all major factors that affect the growing conditions for urban trees. But other than picking a climate-appropriate tree, soil availability is one of the biggest elements in their success or failure. One thing that isn’t well-understood, especially on a policy level, is just how much soil it takes to actually grow a big tree in the built environment.

So, how much soil do you need to grow a big tree?

This is a question I get all the time from people who are designing streetscapes. Their efforts are a team building exercise: the public right of way is a shared space of public concern, so they have to deal with many constituencies.  Cars, buses, water, electric, gas and other utilities compete for space and dollars in the project. Trees as infrastructure is a strategy to set these essential plants on an equal dais as the other necessities in a project. So, the question of “How much soil does a tree need?”  must compete with the list of utilities and construction limitations including budget. Therefore significant downward pressure exists to give the tree the absolute minimum soil necessary to grow a big tree.

The start of soil volume science.

Many people turn to the adage that one needs 1000 ft3 of soil to grow a big tree, and they start there as a design goal. Then people try to pare it back. “Is 750 enough?”, “What about that big electric bank in the PROW? Can avoid it and get 400 ft3 in shared rooting volumes?” Specifiers get lost in the numbers and succumb to the pressures of competing for space within a streetscape. The numbers are abstract, and there is not a lot of difference in terms of plant response for 400 ft3 of soil and 600 ft3.

If you wanted to know what 1000 ft3 of soil per tree looked like in Silva Cells, this would be it. Soil took precedence over utilities I this streetscape, even though it it has every kind of conduit imaginable. This is at Julliard near Lincoln Center for the Performing Arts on east 65th street in Manhattan. Thanks to MNLA! photos 2008- 2018

So where did that adage or rule of thumb of 1000 ft3 come from?

In 1992, James Urban, FASLA published in the Journal of arboriculture in 1992 some data from Nina Bassuk at Cornell University, and others. Nina’s paper asserted that it takes 2 ft3 of every square foot of crown projection (for metric=.66M3/M2 of CP), and much more accurate the 1000 ft3 myth.  Jim highlighted a number on a range which was easily multipliable, and the myth was born. It was revolutionary at the time, and therefore the original chart has been copied hundreds of times in planting manuals and policy documents. Including the Landscape Architectural Graphic Standards which sets out the same 1000 ft3 number in the original chart. As it turns out, the rule of thumb (myth) only gives you a medium-ish size tree, nor does any standard volume talk of water use, a central tenet of the Bassuk paper. Since the early ’90s, further work has been done regarding soil volumes for trees, but it’s amazing how persistent numbers can be!

Nina Bassuk based her recommendation on water use, which varies widely depending on the field capacity of the soil, the evapotranspiration rates of the tree itself, the leaf area index, & Etc. The big take away for landscape architecture is that dryer and hotter locations need much more soil volume than wetter and cooler ones. An obvious comparison of Phoenix to New York is not really relevant because the plant material is so different (Mesquite versus London Plane), but according to the paper you would need over 4,000 ft3 feet of soil to grow a medium sized Red Maple in Phoenix, whereas in the NE the same 16” Acer Rubrum would only need about 1000 ft3 .

Hotter and Drier? You need more soil

Landscape Architects should keep in mind however that in dryer areas, more soil is warranted due to the hotter and drier clime. Take for example the project Amazon Denny Streetscape in Seattle (courtesy of Site Workshop Landscape Architecture, versus Project 180 in Oklahoma City courtesy of OJB Landscape Architecture. Obviously the trees of the latter need more soil due to the climate.


Another way to think about soil volume

So what is a better way to discern the volume of soil a tree needs?  It’s pretty much settled arboriculture that the same amount of wood is above ground as is below ground. This is odd because there are twice the number of shoots as roots, but this is because I believe we classify the tree above ground into separate categories whereas roots all fall into a single category for comparison’s sake. The morphology is different, but the weight is the same.

When you look at a Silva Cell site you can tell how big a tree is going to get by the extent of the cells. This West 8 project at the New World Symphony in Miami Beach gives you that impression of how large this live oak will grow before potentially becoming soil starved. Planted 2009, photo 2018.

What are the design implications of this horticultural fact? This means that if the designer wants an over arching canopy from tree to tree, they will have to link the trees with soil so that the canopy follows where the root can grow.  On the municipal scale, the implication of tree physiology is even greater, and largely ignored. Those cities with aggressive canopy goals of 40-50% coverage will have to create soil rooting volume of 1/2 their real estate area to reach the shading necessary to reduce urban heat island effect. It’s also the reason why I have never been a big fan of “million tree” efforts and the like.

BUT this ratio of 2 ft3 of every square foot of crown projection (for metric=.66M3/M2 of CP), is not one size fits all! For example, do you know that there is little research on columnar or fastigiate trees? The crown projection of these trees is smaller, so would they not get smaller soil volumes? That is not the case because trees with a narrow canopy eventually tend to spread like their wider cousins. Therefore I anticipate what the mature canopy of the fastigiate tree will be. As it grows it will become less columnar, and wider. So, I would treat trees with a narrow spread the same as I do their wider cousins. Assume the same volume you would give a spreading tree and a columnar one of the same genus.

As examples take these two projects one by  Scott Byron & Co., Inc.  at the corners of Wabash and East Erie street at Driehaus Capital Management LLC in Chicago, and the other at  Westminster Presbyterian Church in Minneapolis.

What about other plants?

Also there is little research on other kinds of woodies, like Vines, Hedges and even Palms, but we can make some educated guesses. Hedges are the most simar top trees, but they only have two sides. To calculate this soil volume, first determine the maximum desired size of the hedge. Like trees, we recommend find that you need about two cubic feet of soil for every square foot of hedge. Palms are not even trees per say. I usually recommend just a few cells per palm, since the actual canopy is so small. Larger palms would more soils, but just take the crown diameter and convert that area into square feet and then multiply by 2.5 (the assumed planting depth) to get to a defensible number. Soil volumes on vines are basically empirical, and starts by estimating the approximate coverage by visual estimate and assuming about 2-3 square foot of vine coverage for each cubic feet of soil.  That would suggest that one ½ square foot of wall coverage for each cubic foot of soil.

Mitigate Windthrow Potential of the Urban Forest

Researchers who visited post-hurricane sites found that many incidents of tree failure could have been prevented with appropriate design and management. A strong root system is one of the most critical factors that allow trees to withstand hurricane-force winds in urban landscapes. Limited rooting space presents a challenge to creating sustainable landscapes. Strategies for developing strong root systems on newly planted trees and preserving the roots of existing trees is essential.

For situations where the planting area is surrounded by paved surfaces, Dr. Edward Gilman (RET) and Traci Partin of the University of Florida Gainesville provides guidelines for the minimum amount of soil based on tree size at maturity. 300 ft3 for small trees; medium size trees are 1200ft3 , and the recommendation for large trees is 1800 ft3. You can see the relationship in this chart.

 

Dr Gilman points out there are two components to soil space: 1) the total soil volume needed to sustain a tree for a reasonable period, and 2) the open soil area needed immediately surrounding the trunk to accommodate trunk flare growth. Any tree opening less than 20×20 are a compromise.

With such a large recommended soil volume I thought it appropriate to feature this wonderful Kapok Tree (Ceiba Pentrandra) at the Lincoln road extension project in City of Miami Beach planted by Raymond Jungles, Inc.  It’s been in the ground for 12 years now, is quite large and has weathered hurricane beautifully.

Please do check out the UF IFAS website. There is much more information there as to how to design for a wind stable urban forest, not just soil volumes.

Use your head

Trees need an adequate volume of root, oxygen-rich soil to thrive, and minimum soil volume practices are a powerful tool for advocates to leverage better growing conditions for trees, especially in urban areas. 1,000 cubic feet (28 cubic meters) of soil per tree, is a good rule of thumb, but I believe the industry has progressed way beyond the paltry number. Let’s go for 2500 ft3, keep in mind that the size of the tree is the same above ground and below ground. Small ornamental trees with less volume are a legitimate design and desirable, but shade trees need more soil than you think.

Not all areas can provide this kind of soil volume for each tree. Budgets, site constraints, and more are all factors in the final decision. However, between tools like soil sharing, creating root corridors from tree pits to nearby soil volumes like lawns and parks, and engineered solutions like suspended pavement, we can get a whole lot closer to getting our city trees what they need.


 

*A previous version of this post contained our original adaptation of the James Urban data. 

 

 

 

 

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