Keeping trees firmly rooted: new soil mix tested

Grown in gravel and soil mix, they thrive and are harder to uproot
Esther Ng, Today Online 6 Mar 09;

Using gravel to plant trees is not usually an accepted practice.

But 20 raintrees were planted on a vacant piece of land in Jurong — with some having as much as 50 or 80 per cent of gravel thrown in with topsoil.

The purpose? To see if trees could be grown healthily in small urban spaces, even on narrow roadside verges.

For the past four years, the National Parks Board’s Centre of Urban Greenery and Ecology (CUGE) and Nanyang Technological University, have been experimenting with structural soil — a mix of coarse gravel and topsoil. So far, the results look promising.

“The trees are about the same size as trees grown in top soil,” said CUGE’s assistant director for research Tan Puay Yok.

“This is a positive result as structural soil is shown to be able to provide adequate water and nutrients for tree growth, as compared to normal topsoil,” he added.

Rain trees were used as they are the most commonly planted roadside trees in Singapore and they also grow fast.

The use of such soil mix also offers many benefits.

“Root space is increased, but also, the roots are forced to grow in between the gravel — in a more convoluted manner — this seems to make the tree stronger and harder to uproot, especially since Singapore gets a lot of heavy tropical rain,” said NParks streetscape director,Mr Simon Longman.

To ascertain this, the trees have been pulled down to determine the force required to uproot them. To date, all20 trees have been pulled down and a detailed study into their root systems will be carried out, said Dr Tan.

Other research projects in the works include planting “movable trees” in containers, and minimising the incidence of tree branches falling during strong winds or rain.