Tree root behaviour – in a nutshellThe spread of tree roots (or root mass) is often described as being roughly equal in size to the volume of the tree crown. That tree roots extend from the centre of the trunk out to the drip line of the crown at a depth of up to 2.5 metres is another common claim.

Recent research into how tree roots behave challenges these theories, particularly for urban growth environments. A number of factors have been found to influence root growth leading to a range of outcomes. These findings underline the importance of not making assumptions about the size, shape and location of tree roots and of undertaking careful testing before carrying out excavation work in the vicinity of existing trees.

Related: Top Products for Tree Root Management

Research shows that tree roots behave in pursuit of water, oxygen and nutrients to absorb from the soil, will grow upwards to the surface, laterally, vertically or a combination of directions and form multi-level root systems. Soil moisture (and seasonal variability) combined with compaction rates are often the most significant factors in determining root habit. Soil composition may also play a part with root systems tending to be more compact and fibrous in clay soils compared with sandy soils.

Roots may also thicken or a callus form where they come into contact with under-soil objects. Thickening can also be a response to prevailing forces on the tree such as wind or uneven weight of branches. The tree will build up reaction wood to counter balance such forces.

How do Street Tree Roots Behave?

The roots of street trees are especially vulnerable to soil compaction, restricting their ability to grow laterally and vertically. Indeed the frequent availability of water, oxygen and nutrients just below pavement level promotes vigorous and dense root growth in this confined area. Root growth is confined and leads to footpath heaving and other damage.

For all of these reasons it cannot be assumed that the majority of tree roots will be found within the drip line of the crown, or evenly spaced around the tree trunk. Further, while tree roots are generally in the upper layer of the soil in natural environments, in urban environments they can be deep or shallow depending on soil conditions.

Thorough investigations must always be carried out before excavation work is started in the vicinity of existing trees. Such investigations should include one or more test holes. A decision can then be made on the most suitable excavation method for the area.

In the long run, this makes the work more cost-effective while minimising damage to trees and the nearby built environment.