“Nature does not complete things. She is chaotic. Man must finish and he does so by making a garden and building a wall.”

Robert Frost (1874-1963)

Garden lore: ponga retaining walls

Ponga retaining walls

The one truly New Zealand DIY retaining wall must be pongas – or tree ferns as they are known internationally. If you have a source of similar thickness ponga logs without having to raid native bush reserves, they can offer a surprisingly long-lasting retaining wall. These have been in place for well over 20 years, maybe closer to 30, on a vertical bank that is about 1.3 metres high. They are held in place by lengths of non-rusting fencing wire secured to waratah standards that have been driven into the bank behind.

We think a key to longevity may be the way the pongas are placed. If they are all upside down from the way they were growing, they are then dead and will eventually rot, though this happens very slowly. If they are placed the right way up, as some of these are, then they have the knack of springing back into growth from time to time. It means you have to stop them growing back into trees and trim off all the foliage every few years, but effectively you have a living wall.

The most common tree fern in NZ is Dicksonia squarrosa or wheki. It is more prized overseas and Britain’s Royal Horticultural Society has even given it an award of garden merit. As a retaining wall, it gives a natural look. In shaded areas, ferns and mosses will colonise over time.

First published in the Waikato Times and reprinted here with their permission.

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