At first glance, I've always thought retaining walls made with ponga just looked temporary. But they can actually last a very long time indeed. I wouldn't build a major structural wall out of them, but a small garden edging job can be completed easily and quickly using the trunk of our national emblem.
It's easy to think of the ponga log as a fashion dinosaur from the 1970s but I think that really just comes down to how they are used. And anyway, what's wrong with dinosaurs? or the 1970's for that matter?
Aesthetically the ponga log is a real nod to nature and works perfectly for a backyard vege bed. I've often used ponga trunks for soil stabilisation on steep sites where they blend in perfectly with mulches and hold the soil in place until planting can establish itself.
The classic of course is the ponga fence, a row of vertical ponga trunks trenched into the ground and wired together. Over time many of these can sprout to create a real living wall. Even with a small garden wall it is possible to get fronds sprouting out of the end of your wall. And then there are the myriad trees and shrubs you might find sprouting forth once the log takes on a bit of moisture: the ponga log is quite a good nursery.
Many of the plant species in the bush will have seeded into your ponga log. Rata vine makes a nice job of covering your logs, putting on a good show in spring.
Installation is a relatively easy affair. I've used a leftover fence post cut into three and set in concrete, then drilled a couple of holes through the posts, threaded some wire which I've looped around the trunk and tightened it at the back of the post. The ponga logs are available in 1.8m or 2.4m lengths. Because the end which grows closest to ground is thicker than the foliage end, I've top and tailed the logs to keep the wall relatively level.
Run a string line beyond the two ends of the retaining wall so the pegs holding the line don't get in the way. My string line is for the posts, not the front of the wall. The ponga logs will sit on the front of the posts meaning the face of the wall will sit approximately 150mm out from my string line.
Clear the ground in the position of the wall. I usually dig back approximately 300mm from the string line to give me plenty of room to work in.
Dig three holes per 2.4m ponga log. My holes are as deep (450mm) as my wall is high (450mm).
Mix concrete and place a 50mm blob at the bottom of each hole then set the post to the string line make sure it is level both ways. If you use instant concrete, put a paver in the bottom of the hole instead of 50mm of concrete.
Drill two holes 50mm apart for each log on each post then run a piece of wire through the bottom hole.
Put the ponga in place then thread the wire for the top hole and tighten by pulling the wire with a pair of grips, then twist using a screwdriver. Be careful not to over-tighten the wire as it could snap.
Repeat for the second layer and remember to top and tail the logs so the thin end sits on the thick end.