Constructing garden Wicking Beds –May / June 2009

"The wicking worm bed is a highly productive growing system which not only produces more food from limited water, but also recycles waste organic material to provide plant nutrient and capture carbon. The essence is to form an underground reservoir of water or pond contained by a waterproof container or liner below the surface of the soil. Plants are productive because they have a continuous supply of water and nutrients." Colin Austin

We have extreme water restrictions here in Melbourne. All garden watering must be done by hand-held hose, and only on two mornings of the week, so this method of gardening offers a great solution. Water doesn't evaporate in the bottom of the wicking bed–it 'wicks' up to the roots, and, the top soil will stay soft, under the mulch. Now, we'll only need to water once a week in summer, and less during the rest of the year.

The inventor of this system, Colin Austin has clips on YouTube, showing how to do this in small plastic containers, for use in small spaces:
http://www.youtube.com/watch?v=80-dkv1Au-8&feature=channel_page
And here is his comprehensive and highly informative website: http://www.waterright.com.au
Here is another good website, with easy to follow instructions.
http://www.easygrowvegetables.com/
NOTE: I wrote to Colin Austin to ask for advice on the best level for water and drainage, and received a terrific reply, which I've posted (with permission) at the end.

We built our wicking beds using a combination of suggested methods, as follows:
The sleepers, soil, mushroom compost, and rocks were delivered to the front of the house, and carried by the two of us to the north garden, at the back of the house! Still, even with the hard labour, we get great satisfaction from working in the garden–and, like all passionate gardeners, we take deep pleasure in nurturing, 'knowing', eating, and sharing our abundant harvests with our excellent neighbours.

Materials needed for two 5x1 meter beds (16 feet long), one of which was terraced, so needed more sleepers:
23 red gum sleepers, each 3.5 meters (8 feet) long and 75 Ml (3 inches) thick. Red gum timber (or similar) is much stronger than treated pine, which will poison the soil and will rot on the ground anyway. The price is almost the same.
11 meter (36 feet) long (comes in roles 20 x 4 meter wide). We used black 200 um plastic sheeting, and we now think it just isn't strong enough. Next time we'll look for the orange industrial strength plastic, which would be much less sensitive to pointy things cutting through it, and since it won't be visible above the ground the colour won't matter. HINT: we found a hole in one role of plastic, and sealed it with pool lining glue.)
10 meters (33 feet) 70% shade cloth (you can cut and overlap pieces.)
10 meters agi-pipe (33 feet) (perforated agricultural tubing) + plus pvc pipe (one length cut in half for the two beds) w/ elbows for attaching to agi-pipe.
NOTE: The garden bed gets watered from the top via this pvc pipe, creating an underground stream which waters the plants from below, hence 'wicking'.

Soil, compost, and rocks (A square meter = approximately one metric ton):
1/2 sq. meter 7mm or 14 mm screening rocks (not river rocks)
1 sq. meter of organic soil
1 sq. meter mushroom compost (combined with our own organic vegetable compost)
Small bags of Blood & Bone, lime added to the soil from the old beds.

We plan on adding worms to the wicking beds next season, when the soil is fully wet.
NOTE: We purchased double this amount of soil and compost to spread around the other in-ground garden beds, which we plan to convert also.

Step 1. We removed the soil from our existing 5x3 meter garden bed and then leveled and terraced the ground for the two new beds, which must be completely level for even water distribution. We put a layer of soft-sifted soil on the bottom for cushioning, and created a waterproof bed with the 200um plastic sheeting (make sure no sharp objects can cut into the plastic sheeting).

Step 3. We covered the bottom of the bed to just above the pipe with the screening rocks.

Step 4. We connected PVC pipe to the agi-pipe, then we covered the rocks with the shade cloth, to separate the soil from the rocks and pipe.

Step 5. Then we sifted the old garden soil into the bed, removing stones, seeds, weeds and roots. (The above photo was taken from inside the house). This was backbreaking, tedious work,–bending, and reaching out with a heavy tray of soil, then shaking it to sift it out, so we put new soil in the second bed. Easy!

Step 6. We did it! We sifted the old garden soil, and added a generous mixture of our own organic compost, mushroom compost, lime, and blood & bone, –leaving a space at the top for mulch.

Step 7. We filled the top with a layer of organic sugarcane mulch, and drilled drainage holes. (see letter, below, from Colin Austin re. options for placement of drainage holes.) When it was finished, we added the water through the pvc pipe - testing how long it took to fill the bed to the drainage hole, and measured the water level in the pvc pipe, so we can know in future, by looking into the pvc pipe, when the water is low.

Step 8. Next, after a couple of days break, (to heal our sore bodies and attend to work in the studio) we built the second (terraced) bed on top of the old garden bed soil, (no more digging for us! :), using the remaining screening rocks to make a gravel pathway between. We waited until the waxing moon to plant above ground winter veggies, using a combination of seedlings started in our glass house, below, and seedlings from the local nursery.

Email message from Colin Austin, the inventer of Wicking Beds, June 2009 (I've highlighted the essential information in red.)

Hi Maireid,

Well we have great debates here on this issue of the best depth for the water and soil. Peter (of easygrowvegetables.com) and my neighbour and I spend many hours over a bottle of red wine aurguing this point. If you do not like my answer write to Peter (pva36152@bigpond.net.au) and you will get a different view.

What we do agree on is that water will only wick up some 300 mm. In my early experiments I had my holes much higher than Peter but I watered until it reaches the holes then I don't water again until all the water has gone. This means that the roots are only in saturated water for a short time. This seems to work fine if the beds are not regularly flooded by rain. We may get a couple of weeks when it rains every day and submersing the roots for that length of time is definitely not good.

Peter had his holes much lower to avoid getting the roots saturated.

Now we are making our beds deeper with about 300mm of soil above the high water line. Using the shade cloth this avoids any problem with water logging.

What we are debating now is how deep we can make the water reservoir. Out last lot of beds were 500mm overall giving 200mm for the water resrvoir and 300mm for the soil bed. This is working fine.

I think that we could make the water reservoir a little deeper still. This cuts down the time between watering. On the latest bed I tried 300m and 300mm. Before, when we had to dig the trench, making it deeper was hard work. Now that we're using raised beds digging is not so much of a problem. Because I am old the last bed I made I dug out all the top soil (about 300mm) then made the raised bed a further 300mm which was quite quick to make.

I have only just put the seeds in so do not know how it will work but I am optimistic. You can see picture on my other web site www.wickingbed.com

I will let you know how it goes.

Sorry there is not a simple answer. It also depends on what you are growing. I guess if you are after a rule of thumb make the soil layer equal to the natural root depth of the plant (for veggies about 300mm) and then decide the depth of the reservoir on how long you want between irrigations, but you will be lucky to get any benefit above 300mm.

Colin