Self Wicking Garden Beds

In the past several years we have been trying to grow vegetables in our back garden in Melbourne. To this end we have created two vegetable plots measuring about 1.8 metres by 4 metres. We also have several cubic metres of premium garden soil carted in.

The regular vegetable plot

You can see the netting we put around the beds to keep the possums out.

The first few harvests were bountiful, we had tomatoes, cucumbers, beans, peas, herbs, potatoes and so forth. However after a couple of years, things stopped growing even though we religiously added compost and manure (and the occasional fertilizer feed).

The reason that we found out after we dug around a bit was that there was a bloody big tree (Camphor Laurel) that had sent feeder roots throughout the vegetable plot sucking it dry of both nutrients and water :(

Since we don't want to take down the tree, we had to look at other options - such as putting root barriers, frequent removal of tree roots or re-siting the vegetable plot.

The option that we've finally settled on to is self wicking garden beds.

We  built the following as a test bed ..

Prototype Self Wicking Raised Bed.

It consists of 2 rectangular frames one on each other. Each frame measures 1.2m x 2.4m x 200mm high, giving a raised bed height of 400mm in total.

The frames are alkaline copper quaternary (ACQ) treated pine which is safer than the traditional CCA treated pine which contains arsenic.

The Reservoir Layer

We lined the bottom frame with the following layers (from bottom up)

• Non woven poly weed matting (1.8m wide x 2.8m long)
• 200um poly water proof polyfilm (1.8m wide x 2.8m long)
• Non woven poly weed matting ( 1.8, wide x 2.8m long)

The layers are stapled to the sides of the frames after ensuring that they are resting comfortably & loosely against the ground. I also tried to level the ground as much as possible.

I also drilled an overflow hole about 70mm above ground into the side and fitted a outlet through the polyfilm & weed matting.

Once the initial 3 layers are down, I then installed a 50 mm slotted-agricultural drainage pipe connected to a 50 mm PVC downpipe. This is to help circulate the water across the reservoir. I then covered the pipes with scoria to a depth of 100mm. The scoria acts as a wicking material for the water in reservoir to transfer to the soil layer.

The final layers.

To separate the soil from the reservoir, we covered the scoria with another 2 layers of non woven poly weed matting (1.8m wide by 2.8 m long) stapled to the frame again. This hopefully will prevent soil from entering into the reservoir and interfere with the water flow. The use of the non woven poly weed matting also helps with the wicking effect (I hope).

We then filled the rest of the frame with garden soil, coconut coir, compost and some well rotted manure all mixed well. Then we covered the exposed soil with lucerne mulch.

I then used 15mm poly pipe to create the netting frame. This is relatively cheap and easy. I fasted them using saddle clamps to the inside of the frames.

The next steps

Once completed we have transplanted garlic chives, and transplanted some really sad looking asparagus into the bed. Now about 3 weeks later, these vegetables are doing extremely well.

I am going to build another 4 more of these types of beds but smaller (1.2m x 1.2m) and will provide step by step (well almost) pictures of the construction process.

Construction of a Self Wicking Garden Bed

Materials

• 8 pieces 1200 mm x 200 mm x 50 mm ACQ treated pine sleepers per frame
• a quantity of 100 mm treated pine self tapping screws
• 4 pieces of flat brackets (or whatever you call them)
• 1 piece of slightly larger than 1200 mm x 1200 mm square colorbond sheet metal
• a quantity of 15 mm button head self tapping screws
• 1 piece of builder's poly film (200um thick) measuring 1800 mm x 1800 mm
• 3 pieces of non woven polyester weed mat or frost mat 1800 mm x 1800 mm

The following are a sequence of shots showing how I built the raised bed..

The old vegetable plot

Digging out and levelling the base.

I needed to dig it out a bit so that the raised bed is not so high (on orders from she who must be obeyed). I then had to level it before I install the frames.

Frames installed with a colorbond base.

I attached the sleepers together with treated pine screws at each end directly to each other.

I then attached a colorbond sheet using button head screws and laid that color-side down. 

I then placed the frames on top of each other.

Securing the frames together

Laying the builders polyfilm for the reservoir.

Installing the overflow outlet

External view of the overflow pipe.

I secured the frames with each other using an l-shape bracket as seem. One bracket at each corner.

This seems sufficient to prevent the frames from shifting. Once soil is added, the additional weight will prevent any movement.

The next step (because I have a flat metal bottom), I directly laid the waterproof polyfilm