Wicking garden beds (revised 22/07/2010) © 2010 Steve Williams 1

Wicking beds I was introduced to wicking beds by a feature in Costa’s Garden Odyssey on SBS. Intrigued, I started to read up on the subject and became increasingly impressed with the potential of this system in water-scarce southern Australia. I have just built my first wicking bed on a sloping site at the bottom of my driveway. It has been a ‘learning by doing’ experience, so I thought I would write a short ‘how-to’ article to share what I have found out so far.

What is a ‘wicking bed’ and how does it work? Wicking beds are raised garden beds that have a water reservoir below the plant roots. The water is drawn upward into the root zone by capillary action. This means that there is no need for overhead watering and a lot less water is lost through evaporation. Wicking beds are a very water-wise and low maintenance way of growing vegetables. They can be installed over existing garden or lawn or over concrete or paved areas. The water reservoir is created by lining the bottom of the bed with pond liner. Into this reservoir goes sand, gravel or woodchips (depending on the design). A PVC storm water pipe enables the gardener to fill the reservoir easily with a hose or watering can. The pipe also serves as an inspection pipe to check when the reservoir is empty. An overflow pipe sticks out the side of the bed about 20cm from the bottom and stops the bed from overfilling – you don’t want to end up with a raised bog garden! The bed won’t work properly if the reservoir or the soil above it is too deep. Trials have found out that the reservoir should be 20–30cm deep, with 30cm of soil on top for the plants to grow in (plus space on top of that for mulch). That makes it perfect for most annual vegetables. The growing medium should be free draining, e.g. a light, sandy loam with some organic matter worked in. Wicking beds should not be heavily fertilised as nutrients tend to stay in the bed rather than being flushed out. A worm farm or bio-box provides a steady supply of nutrients and soil micro-organisms.

The Drysdale Harvest Basket garden at SpringDale Drysdale Harvest Basket in collaboration with St Ignatius College plans to make a demonstration produce garden at SpringDale Neighbourhood Centre, using wicking beds. We hope that, by using wicking technology, the beds will look good and be productive even if they only get watered once a week or so. We expect to learn a lot from this project, so we can then help other gardeners who are interested in building their own wicking beds.

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Basic specification Frame: 3 metres long x 1 metre wide x 60 cm high. Constructed from Ecowood pine sleepers (no arsenic like in CCA treated pine). Water reservoir: 200 mm deep (pond liner filled with washed river sand) Growing medium: 300 mm deep (sandy loam + organic compost in a ratio of about 60:40)

Materials to make one 3m x 1m bed 8 x 3m Ecowood pine sleepers 12 x heavy duty galvanised steel right-angle brackets 48 x 50mm M10 coachscrews 3m x 4m pond liner (good quality, you really don’t want this to leak!) 4m x 1m felt underlay 4m x 90mm stormwater pipe 1 x 90mm stormwater pipe elbow 1 x 90mm mesh cap (as used on rainwater tanks) 200mm x 30mm PVC water pipe (for the overflow) 8m roll of 1.5m mulch mat (the fine-woven fabric, not the plastic coarse-woven stuff) or geofabric Approx. 0.5 m3 washed sand Approx. 0.5 m3 sandy loam Approx 0.3 m3 (= 10 x 30 litre bags) organic compost Worm farm: 25 litre food-grade white bucket with close-fitting lid Marine/Outdoor-grade silicone sealant (Sikaflex or similar)

Construction sequence 1 Cut the sleepers to size. (If you’re using 3-metre sleepers, simply cut two of them into 1-metre sections to make the endpieces.) 2 Assemble the bottom course of sleepers in situ. Secure the sleepers with an angle bracket and 4 coach screws inside each corner. 3 Use a spirit level to make sure the bottom course of sleepers is horizontal. 4 Assemble the second course of sleepers on top of the bottom course. 5 Use a spade bit to drill a 32mm hole for the overflow pipe. You want this hole at the bottom of the second course, so that it’s about

200mm from the bottom of the reservoir. 6 Line the bottom of the bed with felt underlay to prevent stones etc. from puncturing the pond liner. 7 Staple felt underlay over the angle brackets in each corner. Important!! Otherwise the weight of water will puncture your liner. 8 Insert the pond liner, ensuring that there are no voids in the corners to create weak spots. (The liner may be partially filled with

water to help with this.) There should be enough liner to go to the top of the second course of sleepers. Tape the liner temporarily into place, but don’t trim the liner until the reservoir has been filled!

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9 Place the distribution pipe (see below) in the reservoir. 10 Fill the reservoir with sand to about 180mm depth. 11 Run enough water into the reservoir to fully stretch the pond liner into place. 12 You can now staple the pond liner to the top of the second course of sleepers and trim off any excess. 13 Install the 30mm overflow pipe, piercing the pond liner at about 200mm from the bottom of the bed. Wrap the in-bed end of the

pipe in mulch mat to prevent it from becoming clogged with sand. 14 Seal the junction between pond liner and overflow pipe with silicone. Leave to dry. 15 Fill the reservoir with more sand until the overflow pipe is just covered. 16 Fill the reservoir with water to check that the overflow is working properly. 17 Lay mulch mat on top of the sand. It should overlap the second course of sleepers on all sides. Tape it temporarily into place. 18 Place the worm farm (see page 4) in the middle of the bed. 19 Mix your growing medium (loam + compost). 20 Partially fill the bed with growing medium so that the mulch mat is stretched tightly into place. 21 Staple the mulch mat to the top of the second course of sleepers. 22 Assemble the top course of sleepers on top of the second course. 23 Fill the bed with growing medium, leaving a 50–100mm gap at the top for mulch. 24 Leave overnight, then check the water level in the distribution pipe. You should find that it has gone down a lot as the water has

been drawn up into the growing medium. Top up, then leave to settle for a week or two before planting.

construction detail distribution pipe worm farm completed bed

Distribution pipe The distribution pipe comprises a 0.6-metre downpipe, an elbow and a 2.5-metre horizontal pipe with holes or slots cut in one side to allow the water to flow into the reservoir. A mesh cap on the top keeps debris and mozzies out. When you assemble the distribution pipe, make sure that the holes are on the bottom. Wrap the holed/slotted section of pipe in geofabric / mulch mat to keep sand out.

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Worm farm This is just a 25-litre food-grade white plastic bucket with a close-fitting lid. Drill lots of large holes in the bottom and in the sides below the soil level. When the bed is complete, you can partially fill the farm with compost and 100 or so compost worms, then put a pad of moist newspaper on top to prevent the compost from drying out. Put the lid on and leave to settle for a week or so, then start to feed the worms regularly with small quantities of veg peelings and other finely chopped (non-meat) scraps.

Cross section through wicking bed

pond liner felt underlay

20 cm

growing medium

reservoir filled with washed river sand overflow pipe

water distribution pipe

mulch

geofabric

worm farm

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Further information Colin Austin is the inventor of the wicking bed concept. For more background and for various types of wicking bed, see his website: http://waterright.com.au/wicking-bed-history.html Mairead Sullivan built wicking beds in her Melbourne garden in May 2009. She tells me that she’s delighted with the results so far. Read about her beds at: http://www.maireid.com/wickingbeds.html Milkwood Permaculture in Alice Springs have produced a useful step-by-step photo sequence for their version of a wicking bed. See it at http://www.milkwood.net/content/view/103/1/ Transition Bell in Geelong has constructed a wicking bed. See their website for photos: http://transitionbell.groupsite.com/gallery/7803?page=1 (You’ll have to sign up to access the forum.) Costa’s Garden Odyssey on SBS featured a very high-tech (expensive-looking) wicking bed design. See the webpage and video at: http://www.sbs.com.au/shows/costa/listings/detail/i/1/article/6172/Wicking-Garden-Beds Yet another version at: http://www.easygrowvegetables.com/html/wickingbed.html.