Healthy roots, healthy trees

The various benefits of trees are well documented. Trees in urban situations in particular have environmental, ecological, economical and human benefits that result in many authourities and administrations valuing trees highly. 

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Credit: City of Melbourne

The roots of a tree provide stability and absorb oxygen, water and nutrients from the soil.

The roots of even quite large trees mostly exist in the top 20-30cm of the soil. Very few mature trees have significant tap roots.

Being so close to the surface means that tree roots are often affected by actions or activities that occur at ground level. In urban situations particularly, soil compaction can seriously inhibit the growth of a tree. Compacted soil reduces the pore space within the soil, resulting in a poor soil structure that limits the proper development roots.

Soil compaction within tree root zones can be remedied with expensive cultivation techniques – carefully conducted ripping, deep trenching and replacing the soil and air injection. Prevention of surface activities such as minimising foot traffic and vehicle/machinery activity is always the first step.

Even large, mature trees will benefit from a layer of suitable mulch. Suitable mulch may be organic or inorganic and should be course enough to allow air and water to enter the soil. An even mulch layer should be spread to the extent of the canopy of the tree – in suitable soils, tree roots can extend well beyond the canopy.

The root zone of a tree should not be covered with a further layer of soil. It is generally a mistake to add soil to make a garden bed under a tree. The extra layer of soil can ‘smother’ the tree’s roots.

It is usually acceptable however, to plant ‘understorey’ species directly into the soil alongside the roots of a tree. Many understorey plants can have beneficial effects on the tree.

Pultenaea (peas) and Acacia (wattles), are two families of Australian plants that have nitrogen-fixing properties – their roots contain nodules of bacteria that can convert nitrogen in the air within the soil, into valuable compounds that the tree roots can absorb. Good understorey planting will improve the fertility and structure of the soil.

Urban trees will often suffer from lack of water. Infrastructure such as footpaths and roads, buildings and drains, divert the run-off of storm water away from the tree root zone into a storm water pipe. Most urban trees will benefit from extra watering in dry periods. In some shires now, various techniques are being used to redirect surface water to the root zones of trees – constructing swales around treed areas, installing porous paving, etc.

In the urban landscape at least, perhaps the best thing that can be done for the health of a tree is to include the tree as a part of the plan from the beginning, not as a final addition.