Step 1 Line bottom and sides of trough with geotextile fabric, leaving an overhang all round.
Step 2 Place a lining of builder’s plastic over fabric, pushing it down against inner walls and base.
Step 3 Using hacksaw, cut a length of 50mm-dia. PVC pipe a little shorter than length of trough. Clamp it to work surface then, using drill, make evenly spaced holes all around.
Tip If the drill slips, use a small gauge bit to drill pilot holes, then follow with larger-gauge bit.
Step 4 Place 1 of the 50mm-dia. PVC end-caps on 1 end of pipe, and PVC 90° elbow on other end. Cut another length of 50mm-dia. PVC pipe, long enough to reach from bottom of trough to about 10cm above top. Attach 1 end of this length to 90° elbow.
Step 5 Position pipe assembly in the base of the trough.
Step 6 Drill a 13mm-dia. hole in side of trough adjacent to vertical section of PVC pipe, 200mm from base, for overflow pipe. This pipe removes excess water and will ensure the trough doesn’t flood when it rains.
Step 7 Remove pipe assembly from trough. Measure, mark and drill a 13mm-dia. hole in vertical section of PVC pipe, also for overflow pipe.
Step 8 Reposition pipe assembly in trough. Cut a short piece of 13mm irrigation pipe (about 8-10cm long) and push through hole in trough into hole in vertical section of PVC pipe.
Step 9 Fill the water reservoir section of trough (here, the bottom 200mm) with scoria.
Tip If the scoria is unwashed, first spread it on the ground and hose well to remove any fine dust.
Step 10 Cut a piece of geotextile fabric to size and place it over the scoria layer. This ensures no soil will filter down through scoria and clog-up irrigation feed pipe.
Step 11 Fill up remaining trough space with potting mix.
Step 12 Using hacksaw, cut a 40cm length of 100mm-dia. PVC pipe. Drill a network of holes in lower half of pipe and place 100mm-dia. PVC end-cap on top. Bury pipe in middle of trough, where it will work as the ‘worm tunnel’.
Step 13 Using sharp scissors, trim excess builder’s plastic and geotextile around lip of trough.
Step 14 Plant trough with herbs and vegetables. Fill reservoir with water via inlet pipe, then place remaining 50mm-dia. end-cap on top. This endcap only needs to sit loosely, as it’s removed regularly for water top-ups.
Step 15 Remove cap from top of worm tunnel pipe and place fresh vegetable and fruit peelings in base, followed by a handful of composting worms. Top up regularly with peelings – the worms will consume them, turning them into plant food, which will leach out into the surrounding soil to feed the plants.