Gravel layer under permeable paving can reduce street tree root damage

Pavement damage from street tree roots can be minimised by incorporating a layer of gravel between soil and permeable paving, and this can also make pavement maintenance less risky to trees, Australian researchers suggest.

Adelaide - image: boostsamurai (CC BY-SA 2.0)
Adelaide - image: boostsamurai (CC BY-SA 2.0)

Permeable paving may already incorporate a gravel layer in its base for structural and hydrological purposes, the researchers from the universities of University of South Australia and Melbourne pointed out.

Conducted in the southern city of Adelaide over nearly five years, their experiment tested whether such a gravel base layer deterred shallow growth of Pyrus calleryana ‘Chanticleer’ roots.

It found that roots of larger diameter were fewer in number and grew deeper in the subsoil beneath permeable paving than beneath impermeable paving.

"The gravel functioned as a buffer between the footpath surface and roots in the subgrade, potentially minimising pavement damage and maintenance requirements," they concluded.

And they suggested: "A gravel base devoid of woody roots may also facilitate repair of the pavement surface, should it become necessary, while eliminating or reducing harm to trees."

But they noted: "No differences in tree height or trunk diameter were observed between trees grown in permeable paving and those grown in impermeable paving."

They also cautioned that the interaction between tree species, soil type, paving type, climate and ground water level is "complex" and merits further study.

The results are published in Urban Forestry & Urban Greening.

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