20 July 2022

There are numerous publications on the root plate dimensions of windthrown trees, and while published data are plentiful, studies of mature tree root systems that have not been uprooted are few. Research has successfully communicated the understanding that tree roots have a basically horizontal orientation, to the point that even many tree professionals now believe that deep rooted trees are a myth.

A tree root can grow to a very great depth
In the paper, “On The Maximum Extent of Tree Roots,” dd 1991, E.L. Stone and P.J. Kalicz summarised previous root depth studies of 49 genera and 211 species growing in a wide variety of soil types. They found numerous examples of trees reported to be growing roots to over 10 meters, and one report of a tree that grew roots to a depth of 53 meters as roots in rock fissures can grow really deep. Clearly, a tree’s ability to grow deep roots is not a significantly limiting factor in designing urban tree planting solutions.

How deep tree roots grow is not determined by tree species
All tree species have their own natural habitat in which they thrive better than other species. But this does not imply it is the best and only suitable location for this specific tree specie. Every tree specie has its own genetic characteristics. This results in the root systems of some trees being deeper than others under the same conditions. So far, the strongest hypothesis for explaining the ability of certain tree species to grow deeper is the “root growth opportunity” hypothesis, which states that the roots of all species can penetrate the soil with the same pressure and that tree species tolerant of wet soils can grow roots during wet periods when soil strength is low, while species less tolerant of wet soils cannot. Yes, there are tree species genetically able to grow deeper roots than other species, but these differences are nil compared to the difference in root depth under different soil conditions.

All trees can develop a deep root system if soil conditions allow
Whilst the genetic characteristics of trees play some part in the rooting pattern, soil conditions are of overriding importance. All tree roots are genetically capable of growing to depths of several meters, but depth is strongly influenced by soil conditions. The most typical limitations to tree rooting in urban areas are soil compaction and poor drainage. And in our country, the Netherlands, limitation by groundwater tables, sometimes only 50 cm beneath pavement. Since these conditions are quite common in urban areas, shallow rooted trees are often seen as “typical.” More and more urban trees are planted in a specially installed growing place solution, so-called urban tree planting systems, which allow root growth beneath paved areas.

How deep tree roots grow depends on three simple factors
Roots require three things: water, oxygen, and soil compaction levels low enough (or with void spaces sufficiently large enough) to allow root penetration. If all these conditions are met, roots can grow to great depths. Under ideal soil and moisture conditions, roots have been observed to grow to more than 10 meters deep. All three factors are to be assessed as a whole.

Soil bulk density
Roots are unable to penetrate far into soil horizons that are of high bulk density, i.e. over-compacted soils. Soil compaction arising from urban land development and use is a more pervasive cause of root restriction for landscape trees. Compaction occurs as soil is compressed, which degrades structure, diminishes porosity, and increases strength — the soil’s physical resistance to penetration.

Soil aeration
All tree roots need oxygen to respire, but some flood-tolerant species have strategies to help them cope with reduced levels. For most, when the oxygen falls below 10–15 % in a soil, root growth is inhibited and completely stops at 3–5%. Such conditions occur when airspaces in the soil are replaced by more soil (compaction), water or gases such as carbon dioxide.

Soil moisture
Waterlogged soils result in poor gas exchange which depletes the soil of oxygen and leads to anaerobic conditions and subsequent root death. Soils with permanently high water tables typically cause trees to develop very shallow, widespread rooting systems. Drought conditions also cause some trees to produce a shallow root system to maximise rainfall interception near the soil surface. If there is a deeper subsurface supply of water, roots may well exploit it, provided that the soil conditions at that depth are suitable for root penetration and respiration.

Knowing how to grow deep or only shallow roots, you can adjust your design to your needs. Or ask our technical design team to assist you.