Click here for a general overview of the factors that influence how deep tree roots grow. Or, continue reading below to dig in to the science a bit deeper.
By James Urban, FASLA
Here’s an objection we face a lot when working on Silva Cell projects:
“Tree roots don’t grow more than 24″ (65 cm) below the surface, so soil volumes deeper than that are not viable.”
Not so! Let’s get debunking.
First, I should say that I am paraphrasing from our Director of Science + Design, Peter MacDonagh and our partner, Jim Urban, FASLA. This is Peter and Jim’s bailiwick, not mine, and I’m pulling together various work that they’ve compiled here.
The graphic above (sorry for the poor quality!) shows the response of 5 different species of trees in 2 different soil types: shallow and deep O/A horizons. The key here is that O/A soils are macropore-rich and have oxygen and water moving freely through the soil column (O refers to Organic, A refers to the topsoil). The hatched box indicates drought stress that each tree is under, and the trees in the study were small caliper.
Deep Tree Roots:
The mollisols (deep >20 feet [6 meters] prairie soil — all O/A soil horizons) have 2 broadleaf tree root depths in columns #2 & #4: 11 feet (Black Walnut) & 15 feet [4.5 meters] deep (White Oak). Both the Walnut & Oak have the least drought stress. The loams in the Silva Cell are functionally equivalent to mollisols. So, a 4 foot [1.2 meter] deep rooting space in O/A in Silva Cells will easily be utilized by tree roots.
Shallow Tree Roots:
The entisols (shallow <3 feet [.9 meter] forest soils) have 3 tree root depths: 1.5 to 3.5 feet [0.4 to 1 meter] deep (Florida Dogwood, E Red Cedar, Sugar Maple). The E Red Cedar is an evergreen and not a typical choice for a street tree. The two broadleafs, the Dogwood and the Maple, are suffering significant drought stress.
The conclusion here is that tree roots follow the oxygen-rich macropores, whether they are in deep or shallow soils. The volume of this soil type determines above else how large the trees will grow.
Orjan Stahl has investigated roots and, while field investigating tree roots in Stockholm, has found over 300 instances of tree roots at 7 feet [2.1 meters] deep & over 170 instances of tree roots at 9 feet [2.7 meters] deep. Of the >500 trees investigated for root depth, roots at 7 and 9 feet [2.1 and 2.7 meters] deep were the most common depth. One tree had roots 23 feet [7 meters] deep. This is the largest root depth study to date, in terms of total numbers and variety of species. Based on this study, one can safely say that no trees will have any difficulty growing down 4 feet [1.2 meters], the approximate depth of a 3 Silva Cell-deep system, in oxygen rich soils.
Hinckley, T.M., R.O. Teskey, F. Duhme, and H. Richter. 1981. Temperate hardwood forests. In Water Deficits and Plant Growth. VI: Woody Plant communities. Ed. T.T. Kozlowski. Academic Press, New York, pp 154-197.
Photos via James Urban