When trees grow, the roots become bigger too!
Written by: Werner Hendriks
When was the last time you went walking or cycling? Have you been annoyed too by the cracked sidewalks and pavers heaved out of place? Or even worse you have fallen over it yourself. Cities spend lot of money year after year to repair the pavement, just to prevent damage claims of residents.
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There is much speculation about what was first; the tree root or the crack in the pavement.
As a tree lover, I would argue that the crack was there earlier and the civil engineers are to blame for the problems. But the truth of the matter is unfortunately more complex. It is an interaction between various components, design (both civil and tree engineering), execution of the work and local conditions. With proper execution of both design and realisation, pavement lifting problems causes by tree root growth are preventable.
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Unfortunately, in public spaces, compromises often have to be made due to budgets or circumstances. And the set of expectations between responsible departments are different. Many studies have been done on the relationship between tree roots and pavement damage. Many of these studies relate to a particular location and or tree species. But after evaluating these studies, one can conclude that that pavement damage is mainly determined by two main facets.
– Bigger roots cause more damage. (Roots get bigger and bigger every year)
– Tree roots growing high in the profile (closest to the pavement) will cause most problems. Most damage and dangerous situations are therefore caused by the big roots growing just underneath the pavement.
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Some tree species develop bigger roots on average than other tree species. A correct selection of tree species is therefore important. The depth where the roots are growing is determined by the growing conditions under the paving. If you ensure better growing conditions at deeper layers, the risk of tree roots damaging the pavement is reduced to a minimum. But remember, if the root mass increases, regardless of the tree species, it will push away the soil around it, and paving resting directly above the roots will also be pushed up as a result.
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Research also shows that tree roots in gravel based structural soil grow more branched. This means less bigger roots, but as soon as the roots become larger than the spaces between the gravel (rocks)………… . Even in structural soil pavement lifting caused by tree root growth is eventually unavoidable.