Wicking Bed Gardens

What is A Wicking Bed Garden?

Wicking Bed Gardens are self-contained, raised garden beds that have a built in water reservoir at the bottom. They are like a giant self-watering pot, making them incredibly water efficient and low maintenance.

Why Choose a Wicking Bed System?

• A Water wise vegie growing – uses up to 50% less water than conventional vegetable gardens
• Minimal water lost through evaporation
• Low maintenance
• Less risk of under or over watering
• Plants get the exact amount of water they need and their roots stay cool
• Soil remains at a fairly constant moisture level enhancing soil fertility
• They provide good drainage in the event of a large rainfall
• Avoids infiltration from tree roots and running grasses
• Allows for thick mulching which also decreases evaporation and reduces weeds
• Improves soil quality through consistent moisture levels and the cycling of nutrients (nothing washes away)
• Reduces the risks of fungal diseases
• Can be built on any surface
• Garden beds warm up more quickly in Spring
• Avoids issues of poor ground soil quality and soil contamination
• Wicking beds are said to be more effective at sequestering atmospheric carbon than many other traditional types of garden beds
• You can still enjoy holidays and have a productive edible garden!

How Does it Work?

Moisture is drawn up through the soil via a process called capillary action or wicking

Design Principle

The total depth of the garden bed should be around 50-60cm high, allowing 20-30cm for the water reservoir and 30cm for the soil. Too shallow and the soil will get too wet. Too deep and it won’t get wet enough.

Material Options

• Liner: pond liner or 6mm builder’s plastic. Unless the container itself is watertight such as an IBC. Always aim for food grade materials and if using an upcycled IBC consider what it was used for previously.
• Interface material – between the water reservoir and the soil: geotextile fabric or double layer of shade cloth
• Water dispersion pipe (for the base of the reservoir): 50mm PVC pipe with 10mm-12mm (3/8”-1/2”) holes drilled all around and over its length or 50mm ag pipe
• Media: 12mm scoria is ideal. It is porous and is excellent in assisting the wicking effect. Otherwise pea gravel is a great substitute. An alternative is to create a media-less wicking bed (no gravel), which require a support structure for the soil, such as upturned pots or trays. Commercial options are also available.

Method

• Choose a level site, or level it once your garden bed structure is in in place. Use a spirit level, place the spirit level crossways and lengthways and at various angles to determine if it is completely level.
• Line your garden bed, if it is not a waterproof container.
• Put a soft buffer of sand or carpet on the ground and fit the liner so there are no tears or holes. You can choose to extend the liner until just above the reservoir level, or you may extend it close to the top of the garden bed.
• To prevent any damage to the liner add a buffer of sand, old carpet or an old blanket on top of the plastic. Then add a 2.5cm layer of scoria or gravel to the base.
• If creating a bed on soil, you may choose to dig it 20-30cm into the ground and only have 30cm raised out of the ground.
• Create an inlet pipe. Install a length of 50mm PVC pipe vertically, attached to a PVC 90 degree elbow that will sit near the base of the bed on top of the scoria you have just placed.
• Attach a length of 50mm slotted AG drainage pipe (or 50mm pipe with holes drilled in it) to the elbow, and run this along the middle of the bed, adding a cap on the end.
• If using ag pipe, some people choose to loop it around the garden bed to maximise water dispersal.
• Add a cap to the top of the inlet pipe to prevent mosquitoes and other creatures getting in.
• Fit an overflow pipe. This allows excess water to drain out, thus preventing water-logging. The overflow can be inserted just below the interface material, ideally on the opposite side from where the water inlet is. Alternately, the overflow can be placed at the bottom of the tank with an elbow up to the level of the interface. The second option means that you can twist the pipe around to let all the water in the base of the reservoir drain out if required.
• A hole is drilled through the raised bed and liner to fit the overflow pipe (a threaded tank inlet or bulkhead fitting is ideal if using a lined system). Install the 20mm threaded pipe outlet (bulkhead connector). Cut a hole in the pond liner just big enough to fit it through the hole, no larger, and ensure that the rubber washer of the fitting is against the pond liner to ensure a watertight seal.
• Cover the water dispersal pipe and the bottom of the bed with scoria, to an overall depth of 20-30cm.

It’s ideal to fill your reservoir with water (fill until water comes out the overflow pipe) and leave for a few hours or overnight to ensure there are no leakages in the system.

• Place the interface material on top of the reservoir media, bringing it up the sides of your garden bed. Securing it to the sides of your bed will give the best result.
• This layer acts as a wick, while also stopping soil from entering the water reservoir.
• Fill the top half of the bed (approximately 30cm deep) with a blend of 50% good quality organic soil and 50% compost. For the wicking to work properly it is essential that the soil medium is high in organic matter.
• Top the bed with a layer of 5-7cm of mulch, taking care not to cover the PVC pipe opening.
• Water the garden from above and plant out.
• NOTE: The garden will need to be watered from the top regularly for the first couple of weeks until the roots of the plants are strong enough to start the wicking process.

How Much Media Will You Need?

To work out the volume of media required, in litres, use the following formula: 

(length of garden bed (cm) x width of garden bed (cm) x depth of media – 20cm)/1000

So for example a 2mx1m garden bed will take (200*100*20)/1000 = 400L of scoria. or 0.4 cubic metres.

Note: Adjust to x 30cm if building you reservoir material up to a depth of 30cm, rather than 20cm.

Wicking Bed Maintenance

You should only need to water your Wicking Bed via the inlet pipe about once every one or two weeks once established. You can do a visual check down the pipe, or use a dipstick.

Be aware that, as a closed system, everything you put into the garden bed stays in the bed. Overuse of fertilisers may see the soil sour fairly rapidly, leading to an increase in saltiness. Less is best, and observation is the key to success. Still use them, but not in excess.

The compost and soil mixture will need to be topped up seasonally, as will the mulch. 

You may want to insert a worm composting tower into the soil portion of the bed to enhance soil fertility.

If you need trellising for climbing plants, consider external fixation, to avoid piercing the liner.

Good luck! Happy gardening.

Words by Kerrie Anderson, Permaculture and Sustainability Educator at Synergy Permaculture Australia

Featured image by Sandi Eyles