Tree roots are a constant challenge for inner-city vegetable gardeners, particularly when they’re from camphor laurel trees. Over the years, I’ve taken various steps to protect the beds, including replacing some of the existing raised beds with entirely enclosed tanks. Because of the cost and effort, I only re-engineered the lower beds that were closer to the trees, hoping that this would be enough.
Sadly this was not the case. My top bed, closest to the road, had been steadily dropping in productivity, so I had a poke around. And sure enough, the bed was filled with tree roots, coming up from underneath.
So I decided to rework this as a wicking bed. Deep Green Permaculture has by far the best description of how wicking beds work, including a description of both advantages and disadvantages (something I haven’t seen anyone else cover).
The starting point was to dig out all the soil from the bed, tacking it down to a flat surface free of rocks or other sharp objects.
And this is just a fraction of the roots that I dug out of the bed, which wound themselves around the entire perimeter of the bed, reaching almost up to surface level. No wonder the bed was struggling!
The bed is lined with pond liner, which I obtained from Clark Rubber. (I noted that some instructions suggested using much cheaper builders plastic, but the Deep Green Permaculture notes strongly discourage this, as the thinner plastic just doesn’t last.)
A thin layer of scoria (rough volcanic rock) then goes down underneath the ag-pipe. (I obtained the scoria from BC Sands, and had it delivered in a 1-tonne bag.)
A 20cm deep layer of scoria was then laid down, covered by a layer of geo-textile. I also drilled an overflow valve at the top of the scoria, with a tank outlet screwed in.
I then re-filled the garden bed, adding a lot of home-made compost, fertiliser and trace elements.
All up, about a day’s labour was required, but the new bed is now back to being highly productive. I have enough left-over scoria to do another bed, which I’ll do when the season ends.
In April 2011, I wrote about our efforts to fight off tree roots that were invading our garden beds. We took the job pretty seriously I think, digging up the beds, laying down bark, weed matting and old carpet.
It didn’t work. Oh, it went fine for a while, and we rested easy. After a year though, the bottom three garden beds became less and less productive, and we knew we’d lost when we started digging up feeder roots from the trees just under the surface of the beds.
Doh! (For those that commented on the original post, I’m not too proud to say: you were right.)
Rebuilding our garden beds
This time around, I decided to put in what I hope will be a permanent solution: garden beds with solid bases.
I ordered them to match the current beds in size and colour, but this time with a solid base (basically the bottom half of a water tanks).
We considered creating wicking garden beds, which are an elegant solution. Unfortunately they don’t really match up with our existing watering system, which is designed to provide a small amount of water frequently (via water spikes).
So we decided to create “giant self-draining pots” instead. This is how we did it…
The serious labour at the beginning of the job was to dig out the 1.5 tonnes of soil out of the bed, and to clear away the pebbles around the base. That also surfaced the source of the problems: the multiple large tree roots that had invaded the bed via a small gap in the corner.
I then cut two holes in the side of the tank with a hole saw, for drainage.
These holes were designed to accept a 25mm threaded tank outlet, as follows:
The tank outlets are above the ground level, and will ultimately sit in the pebble bed above a layer of weed matting. I’ll also be connecting the outlets to lengths of 19mm irrigation pipe, drawing the water away, and making it even harder for roots to invade the bed.
With that done, and the bed leveled and in place, the next consideration is ensuring good drainage within the garden bed.
While I could’ve used gravel, it’s heavy to use, and will tend to clog up over time. Instead, I used 30mm Atlantis Flo-Cell, which lays down as sheets across the base of the beds.
Two layers were sufficient to get above the tank outlets, and to ensure good drainage.
A small amount of gravel was used to plug the gaps, and a layer of geotextile was then laid across it all.
After filling the bed back up with soil, the result looks no different than any of the other beds. But hopefully it will be proof against roots!
Is this a lot of work? Yes, oh yes. At the end of the day, though, there’s no point in having raised beds if they’re not actually growing food.
I’ve still got two more beds to do the same way. So any suggestions on improvements or modifications welcomed!