Wicking worm beds are similar to raised vegetable garden beds. They were developed by Colin Austin, a retired engineer living in Queensland, Australia. Wicking worm beds have built-in water tanks located below the soil and built-in worm farms, where the worms have access to the whole of the growing area as well as the worm farm itself. Various methods may be used to construct a wicking worm bed, but the one described here is the most suitable for an average suburban back yard.
1Make the bed with copper treated timber sleepers.
- Using 200mm x 75mm sleepers, cut 6 lengths 2700mm long and 6 lengths 1500mm long.
- Pre-paint the sleeper to minimise leeching of the chemical treatment.
- Set the first string of sleepers 100mm into the soil using a 2700mm length on each side and 1500mm length on each end.
- Join them using 2 x 125mm bugle head galvanised wood screws at each corner. Make sure they are level with a gauge.
- Their external measurement should be 2700mm x 1650mm.
- Repeat this for levels 2 and 3 of the wall so that the total depth is 600mm.
- Tie the three levels together in each corner using 50mm x 50mm slotted galvanised angle and 30mm button headed galvanised wood screws.
2Dig all the soil out of the bed.
- Dig down to the bottom of the wall and rake the soil to remove any sharp objects.
- Lay an 1800 mm x 3750 mm piece of heavy duty shade cloth in the bottom of the bed so that it covers the soil and all horizontal and vertical angled joints.
- This is to protect the plastic liner from damage during installation.
4Fit the tank filling, distribution and overflow system.
- Lay a 50mm slotted irrigation pipe around the inside perimeter of the bed to provide rapid distribution of water in the tank when it is being filled.
- Cut the filler tube (700mm long), vertical overflow tube (about 230mm long) and horizontal overflow tube (150mm) from a length of 40mm rigid PVC tube.
- Using 3 way couplers, join them into the distribution pipe.
- Secure the filler pipe to the bed wall using galvanised saddle clamps and 30mm button head galvanised wood screws (above the overflow point).
- Drill a 50mm hole through the bed wall just above the 250mm depth of the water tank.
- Fit a 90 deg angle rigid PVC tube to the top of the overflow tube, and then connect the last piece of tube to the other end of the angle. Push this tube though the bed wall to provide an overflow route out of the bed.
5Make a filter to fit on the end of the overflow tube.
- Cut down a 40mm PVC cap to form a sleeve.
- Put a small piece of UV protected plastic mesh Inside the sleeve, covering the open end of the overflow pipe.
- The filter will keep insect pests out of the water tank.
6Fill the bottom half of the bed with water and wicking media.
- Use crushed volcanic rock (20mm screenings) as a wicking medium.
- The rock should be just submerged when the tank water starts to overflow.
- Make sure the rocks are level with the water.
7Place a layer of shade cloth over the crushed rock.
- The shade cloth should extend about 100 mm up the sides of the bed.
- Fill the rest of the bed up to 50 mm from the top with good quality garden soil with plenty of organic matter worked into it.
- The shade cloth works by preventing soil from getting into the water tank, while at the same time allowing water to wick upwards out of the tank and into the soil.
- Water wicks up to 300 mm from the surface of the water in the tank, maintaining ideal soil conditions for vegetables to grow.
8Build a small worm farm.
- Cut a piece of 230mm x 6mm x 750mm long fibro cement sheet and a 200mm long piece of 200mm x 75mm treated timber sleeper.
- Paint the treated sleeper.
- Fix the piece of sleeper to the top string of sleepers in the middle of the end wall near the filler tube using 2 x 125mm galvanised bugle head timber screws.
- Use a piece of 50mm x 50mm galvanised angle bracket to secure one end of the fibre cement board to the adjacent side of the bed 200mm from the end wall.
- Secure the other end to the end of the sleeper stub sticking out from the end wall.
- Use galvanised M6 nuts and bolts to secure the board to the angle bracket and 30mm galvanised button head wood screws for the other fixings.
- There is about 100mm of soil under the worm farm on top of the water tank for the worms to migrate to and from the growing area of the bed.
- Composting worms are used in the worm farm because they tend to stay where the food is.
- The earthworms are much more mobile, taking food from the worm farm and distributing it widely in their tunnels throughout the bed.
- Use a thick layer of mulch and a small sheet of heavy duty shade cloth over the worm farm to provide protection from too much light and from predators.
- Well maintained wicking worm beds use very little water and fertiliser and the soil's uniformly moist conditions rich in bio-activity is an ideal environment for vigorous and healthy plant growth.
9Add a handful of finely chopped worm food to your worm farm every 2-3 days.
- For rapid results, chop suitable kitchen and garden waste in an old food processor. Avoid citrus and onion family waste and meat scraps.
- This well-chopped mix is easily digested by worms and soil microbes and provides very rapid decomposition.
10Make an indicator to keep an eye on the water level in the tank.
- Cut a 750 mm long piece of 6 mm Tasmanian oak dowel (or equivalent) and glue a table tennis ball to one end using a blob of silicone.
- Paint the assembly with hard-wearing line marking paint, and with black painted rings showing the full and empty levels in the tank.
- The indicator slides up and down the filler pipe as the water level rises and falls.
- Make a guide for the indicator with a 40mm PVC cap and an 7mm hole drilled in the centre.
- As well as guiding the level indicator as it slides up and down in the filler pipe, this cap keeps pests (snails, mosquitoes etc) out of the water tank.
11Build a pest exclusion system for the wicking worm bed.
- Cut the frame timbers from 42mm x 19mm untreated plantation pine.
- Cut 2 x 2700 mm lengths, 5 x 1610 mm lengths and 10 x 700 mm lengths.
- The 700 mm lengths control the height of the frame, and can be made longer for tall plants.
- Paint the frame timbers with water based acrylic paint.
12Install the vertical supports.
- Fit the vertical supports on top of the 2700 mm long top sleepers of the bed at 664.5 mm centres flush with their outer face. Use 50 mm x 50 mm x 40 mm galvanised steel angles and 30 mm galvanised button head wood screws to secure them.
- Add a short piece of galvanised steel builders strapping to each corner securing with 30 mm galvanised button head wood screws as shown in the photograph. Use a light hammer to flatten any sharp edges.
- This strapping helps stabilise the structure.
13Assemble the top of the frame.
- Use 2 x 2700mm lengths and 2 x 1610mm lengths of the painted timber.
- Clamp each corner (one at a time). Pre-drill for 2 screws and countersink. Screw the corners together using 60mm x 8g galvanised CSK wood screws.
- Mark the 2 x 2700 mm longitudinals at 9.5 mm from one end, and then at 670 mm spacings along their length. These are the centres at which the 1610 mm infill rails must be fixed.
- Clamp each joint (one at a time). Pre-drill for 2 screws and countersink. Screw the joints together using 40 mm x 10 g galvanised CSK wood screws.
14Sit the top frame on top of the vertical supports.
- Clamp one of the centre joints. Pre-drill for 2 screws and countersink. Screw the joint together using 60mm x 8g galvanised CSK wood screws.
- Note that 2 sets of screws are used in this joint at 90 degrees to each other. Take care to leave adequate space for them to pass without getting to close to each other.
- Finish the rest of the joints in the same way, but not the corners.
- The corner joints are complex, and there is only room for one screw in each corner to hold the top frame in place. So repeat the above procedure in the corners but for only one screw in each. (See above photo of top corner joint.)
15Make a simple system for fixing pest exclusion netting.
- Cut a piece of heavy duty shade cloth 2700 mm long x 900 mm wide.
- Fit 9 x 12.5 mm brass plated steel eyelets along each of the 2700 mm sides evenly spaced and centred 30 mm in from the edge.
- Eyelets and eyelet swaging tools are usually available from good hardware stores.
- Start at the top left corner of the south side of the bed (north side in the northern hemisphere). Screw a 25mm Galvanised Tek screw into the frame 20mm in from the top and left side of the bed.
- Leave 10mm of Tek screw thread exposed so that 2 eyelets can be accommodated on the "hook".
- Repeat the process in the top right hand corner of the frame, but gently stretch the fabric horizontally before locating the Tek Screw, so that the material is not loose on the hooks.
- Repeat the process for the bottom corners, this time gently stretching the fabric vertically before locating the Tek screws in the top sleeper of the bed wall.
- Screw the rest of the Tek screws in place making sure the fabric is kept taut.
- Repeat the whole process for the north side of the bed (south side in the northern hemisphere), but use lighter 21% shading bird and insect netting (VegNet).
- Cut a larger piece of the lighter netting 4500mm long x 1750mm wide. This netting covers the top and ends of the frame, and overlaps the other netting. It should be applied last and uses the hooks already in place for the side netting.
- Drape the netting over top and ends of the frame making sure the overlap is even between opposite sides of the net. Hook the netting over the Tek screws ensuring it is evenly stretched all round, and mark the places where the eyelets need to be attached.
- Remove the netting and fix eyelets at the points marked.
- Install the pest exclusion netting sides first, then top and ends.
- You will find that some of the Tek screw "hooks" only accommodate one eyelet and not 2. You can screw these hooks in a bit further so that only 5mm of Tek screw thread is exposed.
16Add snail and slug exclusion.
- Apply a band of self adhesive copper tape continuously around the base of the bed 300mm up from the ground.
- This measure is a very effective humane way of deterring these voracious mollusks.
- The worm farm is very effective at maintaining the fertility of the soil and very little organic fertiliser needs be used.
- The heavy duty shade cloth referred to in Step 15 absorbs heat from the sun in cooler months, moderates the air's temperature in the wicking worm bed and is an effective windbreak. Locate it on the side of the bed where it can collect the sun's rays without shading the plants from the sun. It can be left in that position all year round.
- Wicking worm beds use water very sparingly since they lose nothing to the subsoil and very little through evaporation, especially if you use mulch.
- Watering in summer is usually a weekly chore, but for most of the year no watering is required and you can get by with occasional rain.
- The pest exclusion measures are invaluable, and very little organic pesticide is needed in wicking worm beds.
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