Alien trees can’t be dismissed out of hand

Alan Buff, technical support and horticultural specialist for the City of Johannesburg, recently spoke at the Benoni Horticultural Society, at Four Seasons Nursery.

He focused his talk on the history of an argument surrounding alien trees in South Africa.

“Some trees, although they present a problem now, were planted in SA for good reasons,” Buff said.

“Blue Gums were planted for the mines, as the wood was ideal for use as mine props, but now they’ve become a problem.”

Buff said in Johannesburg, 80 per cent of the trees planted must be indigenous, but not always when it comes to sidewalks.

“You have to think of underground service lines, overhead power lines, traffic in the road and pedestrian space with sidewalks,” he said.

“We can’t plant most indigenous trees, because they either branch out too much, obstructing sidewalk space, have thorns, or don’t root deep enough and are then unstable in storms.

“Indigenous trees also don’t often prune well, whereas pruning is necessary for sidewalk trees.

“On the other hand, you also can’t plant any alien tree, such as evergreens. These trees pick up too much of the dust and traffic pollution, then die faster.”

Buff said it is important to eliminate invasive alien trees and bring back more indigenous species, but a sudden change after 100 years of growth, will have serious consequences.

“Our canopy, birdlife, oxygen production and biodiversity will all change for the worse if we suddenly switch over, it must happen gradually,” he added.

Buff suggested the different metropolitan municipalities should run a competition to see which city has more trees.

Using input from residents on social media this would serve as a census to count the trees in the metros.

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