raised garden beds
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!
A key element of our watering system, fed by our rainwater tanks, are the “water spikes” used in the beds themselves. I was sure we’d written about these when we first put them in, 3+ years ago, but I can’t find a post for them. So here it goes…
These spikes are traditionally used with grey water systems, as they have two pairs of drainage holes 10+ cms down (the regulation depth for grey water usage). According to our plumber, one spike feeds about a circle about 1m in diameter. To be on the safe side, we’ve got three for each raised bed. (We’ve also strategically used them throughout our new native garden at the back.)
The big advantage of this watering approach is that practically no water is lost to evaporation, as it all goes sub-surface. It also encourages plant roots to seek downwards, strengthening them again dry and hot days. It’s robust against blockages, as any sediment simply falls to the bottom of the water spike.
We had a whole bag of spikes supplied as part of our water tanks, already made up. But you can also make them yourselves, as follows:
- orange water spike (our plumber called it a ‘carrot’, for obvious reasons)
- 25mm end cap (standard item for 25mm polypipe watering systems)
- 3.5mm hole drilled in the top of the cap
- standard 4mm elbow inserted (heat the cap in hot water first, and use pliers)
This creates a ‘watering unit’, when then connects to standard 4mm polypipe (used for drip feeds, etc).
I’ve seen the ‘carrots’ available per piece in Bunnings, along with the other required bits. You can also get them in bulk at Reece.
Combined with a timer system and a network of 19mm polypipe, this is a great set-and-forget watering system for all our plants. We love it, and would recommend it to others.
When establishing our raised garden beds, I had thought this would proof us against issues in the soil, including the impact of neighbouring trees. How wrong we were. The camphor laurel has proven to be very vigorous, and when I turned over the soil in the bottom two beds, I found feeder roots coming up through the bottom of the bed. No wonder the plants weren’t growing properly!
No choice but to take drastic action (short of cutting down the noxious tree itself).
The starting point was to fully dig out the garden bed, which was no small task, as there’s 1.5 tonnes of soil by a previous calculation.
Having dug the bed down the level of the original gravel, I laid down a layer of bark mulch. This is something that I saw on Gardening Australia, where the nitrogen draw-down of the mulch rotting creates a low nitrogen layer. This should help to discourage or deflect the tree roots. (This is the experimental bit, I hope it works!)
Multiple overlapping sheets of weed mat should then form an impervious barrier to the roots, at least to some degree.
With the gardening in mind, I’d scavenged some old carpet from the side of the road. This was laid in to protect the weed mat against garden tools, and to provide an additional layer of proofing against roots. (And yes, I’m aware of the debate about whether carpet should be used or not for these purposes; in the end I decided that more was better than less when it came to roots.)
Getting 1.5 tonnes of soil back into the bed was my final workout. Having read that roots breaking down release toxins, I carefully removed the camphor laurel roots when refilling the bed, which slowed the whole process down.
Hopefully that’s all fixed now, and I can get back to growing vegetables!
I’m a great fan of “no dig” gardening, where garden beds are raised and then filled with layers of organic materials and soil. If nothing else, our earth at the front has turned out to be about 30% old building materials (tiles, slate, whole bricks, pipe, old iron).
There are many ways of creating a raised bed: railway sleepers, wood frames, bricks, hay bales. I decided to go for corrugated iron tanks, for two reasons: they were easy to install, and looked atractive.
Tankworks sells a wide range of garden beds, built to order.
I ordered two, 2m long, 1m wide, 1m deep, the same colourbond colour as our roof.
Some weeks later they arrived, and was I surprised. They were huge! 1m deep turned out to be totally impractical, so with the help of a friend with an angle grinder, they were cut in half.
The photo above is the first of the tanks installed at the top of the front garden (the sunniest spot). Still waiting for soil and cow manure to be delivered…
(And yes, there was still some serious mattock work to dig the trenches, generating a good pile of building rubble in the process. Still, a good break from sitting behind a PC all week!)