How to manage tree roots in cities and homes

Trees add significant value to our homes, neighbourhoods and cities. The cliche ‘leafy suburbs’ underscores the fact that it is the amount of greenery, more than anything else, that makes the difference between Nairobi’s affluent neighbourhoods and the less glamorous areas.

Research has shown that trees enhance the image and attractiveness of developments and improves a developer’s reputation and profits. Hence our approach to development should give priority to preserving existing site trees. Cutting down mature trees should be a last resort rather than the norm.

Alternatives to cutting

Of all the problems that trees pose to any development, their roots usually present the greatest challenges. The roots of certain trees can penetrate and compromise pipelines and sewer systems. They can lift sidewalks and driveways as well as topple walls and even entire structures.

Fortunately, there are many options to dealing with tree root problems without having to cut down the entire tree.

Root barriers

Root barriers are plastic or steel membranes that physically stop roots from growing beyond the barrier line. They work very well, especially when installed proactively when the tree is still young and the roots don’t have to be pruned.

There is quite a bit of work involved in their installation, however. You have to dig a trench 90 to 120 centimetres deep along the sewer line, driveway or wall that you don’t wish the roots to get to. If the tree’s trunk is close to the trench, though, any roots you cut in the process can compromise the stability or health of the tree. Consult a professional before cutting any roots to make sure there will not be a problem.

Chemical Control

Certain chemicals can be used to kill off roots that are growing beyond their bounds. Copper sulphate crystals, for instance, are very effective. The copper creates a “poison” zone within the soil around the structure that is to be protected from the evasive roots. As with all chemicals, however, caution is urged whenever this option is considered.

Air Gaps

Another effective means of controlling tree root growth is creating a “dry” zone between the tree with invasive roots and the structure to be protected. The zone excavated and then filled with materials that dry quickly, should have extremely large pores filled with air, have poor water holding capacity, and are impermeable to root penetration.

Baiting

Roots naturally grow towards areas with well moist and healthy soils. Baiting is providing these ideal growth conditions in some other area, rather than next to the infrastructure. Sometimes, this process may not be completely practical, but under special in-ground containerisation it becomes workable.

For practical purposes, an area with open soil is identified away from the infrastructure to be protected. The roots in that area are provided with near optimum resources and additional organic matter. The net result is a much higher survival and growth rate in that part of the root system as opposed to roots near the infrastructure.

-The writer is a landscape architect.