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Wicking Bed Construction, How to Build a Self-Watering Wicking Bed

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A wicking bed is a self-watering raised garden bed, and even though the design is a relatively new innovation that is catching the attention of many produce gardeners worldwide, it is essentially nothing more than a large-scale version of a self-watering pot. Self watering pots have been around for decades, and are based o the principle of sub-irrigation, where the water supply sits below the pot that is wicked upward into the soil in the container above.

This article provides detailed step-by-step instructions on how to build a wicking bed, but before we start building anything it is important to understand how wicking beds work, so we know exactly what we’re building and how to modify the design to our needs if we need to.

Also, when considering wicking beds, it  is really important to determine whether this system of gardening is suitable for our needs as gardeners. By understanding the pros and cons of wicking bed gardening, we can make the right choice and get the best results gardening with this wicking beds.

The Advantages and Disadvantages of Growing in Wicking Beds

Wicking beds are great for situations where watering is infrequent, such as community gardens and school gardens, where nobody is present over holiday periods to water the garden beds. The water reservoir in a wicking bed can carry enough water to keep plants alive for up to several weeks, depending on climate, season and location. They’re also useful for gardening under and around trees with invasive roots, that extract every last bit of moisture from the soil, such as Australian eucalyptus trees.

Growing plants in wicking bed systems is a useful technique that increases the range of possibilities of what you can grow where, and wicking beds have their place and purpose in a gardeners repertoire, but they also have their limitations, just like any artificial gardening system. What are these limitations you may be asking?

Most plants require a wet-dry cycle to grow, but wicking beds create an environment with constantly moist soil, which is unsuitable for many plants. Since water is retained in a wicking bed, this leads to a build up of fertiliser, and the evaporation of water from the soil combined with the upward wicking creates a situation where the concentration of salts can build up to dangerous levels in the soil, that can burn plant roots. The upward moving water also carries the excess salts upwards, so they accumulate at the upper soil levels, where shallow rooted seedlings are planted.

Another issue is that the bottom level of soil in wicking beds is always wet, while the upper levels can be fairly dry, because the soil can only wick moisture up so high through the forces of capillary action, adhesion and cohesion against the forces of gravity. As a consequence, the moisture available to a plants depends on the height of the garden bed and the depth of the plant roots. Deep rooted plants which dislike ‘wet feet’ (constantly waterlogged soil) will be very unhappy in a wicking bed, and will usually fail due to root rot.

It is also important to consider that a wicking bed in in fact a container garden, and all containers of soil or potting mix/medium have what is termed a perched water table, a layer of water-saturated soil at the bottom of the container that never drains away. If this layer is constantly wicking up more water, it can never dry due to evaporation or uptake by plant roots, and becomes a soggy, anaerobic (without air/oxygen) sludge that may promote root diseases.

The main drawback for most people is cost. Large, properly-built wicking beds are expensive to construct. In my mind gardening should be simple, cheap and sustainable.

My basic rule of gardening construction is as follows.

The cost and construction effort involved, going from gardening in the ground, to raised beds, and then to wicking beds, jumps astronomically with each step.

Wicking beds are not a universal gardening solution, and if we keep this in mind and use them where they perform best, we can best make use of the benefits while avoiding the disadvantages.

Wicking beds in my opinion, much like hydroponic systems, are best suited to growing annual vegetables, which are so short lived they don’t live long enough to develop long term problems due to the soil conditions, and require large amounts of nutrients in a short period.

Since wicking beds retain fertiliser all too readily, less fertiliser can be used for annual vegie growing. As such, wicking beds make great intensive vegie beds and kitchen garden beds.

Now that we understand the benefits and limitations of a wicking bed system, let’s have a look at how they work.

Wicking Bed Design Theory

A wicking bed is quite a simple design as shown in the diagram below

The size of the wicking bed is essentially down to personal preference, it can vary from a small tub which sits on a table or stand, all the way through to a full-sized garden bed wicking bed system.

Common wicking bed sizes are determined by the dimensions of commercially available raised beds or construction materials.

Wicking beds design showing the components of the system

If we look at the wicking bed design shown in the diagram, we can see it is constructed of several layers or levels. The best way to explain how the wicking bed system work is layer by layer, in the same way that it is constructed.

  1. The ‘shell’ of a wicking bed is a pre-constructed or pre-fabricated raised bed, it can be made of steel, wood, plartic, whatever is strong enough to hold the required amount of soil.
  2. The raised bed ‘shell’ is lined with pond liner so that it can hold a large volume of water. It turns the raised bed into a very large  watertight container.
  3. A hole is drilled through the side of the raised bed and pond liner to fit the overflow pipe (threaded tank inlet or bulkhead fitting), at a height of 20cm (8”), which allows the water to flow out when the water level gets too high.
  4. The pond liner water reservoir is filled with coarse scoria (a porous red volcanic rock) to the height of the overflow pipe. This layer will hold the water. the water sits in the spaces between the scoria, and can be wicked upwards by the soil. The scoria layer also serves as a structural support to hold up the soil above it away from the water below.
  5. The L-shaped inlet pipe is put into place before the scoria is laid down, it serves as a water inlet to fill the water reservoir with water. The vertical pipe is joined to the horizontal pipe with a 90-degree elbow join. The lower horizontal pipe has holes drilled right along its length so water drains out more easily.
  6. The scoria layer is covered with geotextile fabric or shade cloth to keep the soil layer above it from falling into the scoria layer water reservoir – essentially it is a barrier that separates the water below from the soil above.
  7. The soil then fills the bed to a level just below the edge of the pond liner, so the pond liner sits slightly higher than the soil level.

The wicking bed is filled with water from the inlet pipe, to fill the water reservoir. When it is full, some water will flow out of the overflow outlet. The water will then wick upwards as high as it can to keep the soil damp.

By understanding how a wicking bed  works, we can get a better idea about how we want to design one and determine the quantities of materials required tor construction.

Wicking Bed Materials

Wicking beds require a lot of materials to construct, and as a result, they are not cheap. To construct a wicking bed you will need the following materials:

1. Raised garden bed

Use either a prefabricated galvanised steel raised garden bed or construct your own DIY timber raised garden bed

2. Pond liner

Many gardeners use cheaper PVC pond liner for the purpose, which isn’t actually rated as a food-grade plastic. The more expensive option is butyl rubber pond liner, which has a very long warranty period, and is designed not to leak for a very long time.

It’s also possible to use black builders plastic that’s used for damp-proof courses. Polythene damp course plastic sheet is quite thick, and very affordable, but it’s not as durable as pond liner.

You will need enough pond liner to line the sides and bottom of the raised bed. So, the width of materials required will be (width of bed + 2x the height), and the length of pond liner will need to be (length of bed + 2x the height).

3. Coarse grade scoria

Scoria is a porous, light volcanic rock, and in wicking beds it’s used to fill the water reservoir, so it can hold up the weight of soil up above it, while still providing enough empty space for the water to be stored in. The water stored in the scoria layer wicks upwards into the soil, but the scoria does not do the wicking!

Why use scoria rather than any other material?

Scoria is used in landscaping for improving drainage, and is often used for back filling aggie pipes (agricultural slotted drainage pipes) and layering under ground, around wet areas. The reason scoria is used for drainage is because it’s highly porous and very light, and this is because it’s filled with air holes, much like a sponge, which can fill with water, and let water drain through. This means there is less rock, and more space for water in any given volume of scoria, compared to other materials.

In a wicking bed, the water reservoir underneath is a fixed size, and for it to work efficiently, it needs as much space filled with water as possible, with the least amount of that space filled with rock. It is this property of scoria that makes it the ideal material for this purpose.

You will need enough coarse scoria to fill the raised garden bed to a height of 20cm (8”). To work out the volume in litres, use the following formula: (length of garden bed (cm) x width of garden bed (cm) x 20cm)/1000, so for example, a 2m x 1m garden bed will take (200*100*20)/1000 = 400L of scoria. or 0.4 cubic metres.

4. Geotextile fabric

The geotextile fabric layer is used to separate the soil from the scoria-filled water reservoir beneath it, to prevent it clogging up and filling the space where the water is stored.

Geotextile fabrics are synthetic fabrics used in landscaping, which do not break down. They are usually sold as fine grade weed mat, which looks like see-through cloth (it does NOT look like plastic), are quite cheap in price, and often grey-black or white in colour.

Since geotextile fabric is quite thin, it’s a good idea to use a double layer of it when constructing a wicking bed. If geotextile fabric is unavailable, as a substitute it’s possible to use shade cloth with a high shading factor, such as 90% shading shade cloth, as the holes are smaller.

Whichever material is used, you will need enough to cover the bottom of the raised bed and extend a little up around the sides by at least 15cm (6”).

5. Soil mix

When filling wicking beds, use a high grade soil with a good level of organic matter in it. Ideally, a mix of 50% premium soil, 25% organic compost and 25% organic cow manure will give your vegetables and herbs a good start.

If you’re going to the expense of building a wicking bed, the worst thing you could possible do is skimp on the most important component to plants, the soil!

6. Water overflow outlet fitting

For the water overflow outlet fitting, use a 20mm (3/4″) Threaded Tank Inlet (Bulkhead fitting), and get a drill with the appropriate drill bit or hole cutter to drill a hole to fit the fitting into the side of the raised bed.

A 20mm water tank bulkhead fitting is used as a wicking bed overflow outlet

7. Water inlet pipe

The water inlet pipe is where the wicking bed reservoir is filled with water, and it’s constructed from two pieces of 50mm (2”) PVC pipe, joined by a 90-degree elbow joint.

The vertical segment of pipe should extend above the soil line at a height that allows the gardener to comfortably pour water into it (not too high), but not so low that it is lost amongst the vegetation.

The horizontal section should be approximately half to three quarters the length of the bed, and should be drilled all around, and over its length, with holes approximately 10mm-12mm (3/8”-1/2”) in size.

Tools Required to Construct a Wicking Bed

A few tools are required to build a wicking bed. Below is a list of the tools needed, and what they’ll be used for.

After we have gathered the required materials and tools, construction can commence. Allow a few hours for construction if you have never built a wicking bed before, as you’ll be learning as you go.

Once you are familiar with the process, and have developed and refined a system that works for you, you’ll be able to build wicking beds rather quickly.

The two wicking beds in the following instructions were built at a kindergarten, with a colleague (and a lot of little children with tiny plastic spades and buckets) in a little over an hour!

Wicking Bed Construction – Step by Step

In this instructional article we’ll be using a galvanised steel raised bed as the base for our wicking bed.

Also, for ease of construction, we’ll use a slightly simpler design where the water inlet tube is just a straight pipe with no bend in it, as shown in the diagram below:

Simplified design which uses straight inlet pipe rather than right-angled inlet pipe

Step 1 – Place or construct the raised bed in the desired location, once it is filled with soil it will be immovable!

Step 2 – Ensure that it is level by using a spirit level – place the spirit level cross-ways and length-ways and at various other angles to determine if it is level.

The wicking bed needs to be level because water naturally sits level, and if the wicking bed is angled, one side will be drier and the other side wetter than normal.

If a side of the bed is low and needs raising, lift the bed slightly and pack more soil underneath to elevate it, if it is high, dig some of the soil out to lower it.

Wicking bed must be leveled on the ground, otherwise the water level and wicking wont be even across the bed

If you’re wondering about the four  white tubes fastened inside the top edge of the raised bed in the picture above, they are part of the product, it was purchased that way. The vertical tubes are used for attaching flexible pipe from one side of the bed to the other to make a half-circle tunnel -shaped frame, to support a covering material, such as bird  or insect netting. shade-cloth or clear plastic.

Step 3 – Drill hole in the side of the raised bed, 20cm (8”) above the ground.

Hole drilled in the side of the galvanised steel raised bed, 20cm (8”) above the ground, using a cordless electric drill fitted with a 20mm hole saw.

Step 4 – Lay down the pond liner inside the raised garden bed and check that it fits properly.

Note, in case of mismeasuring and falling a bit short, the pond liner doesn’t need to come all of the way to the top of the raised garden bed. If it falls short in parts, but is still well above the scoria layer, that will be okay.

Fitting the pond liner into the wicking bed

Step 5 – 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 place the rubber washer of the fitting on the inside, against the pond liner, to ensure a watertight seal.

Water outlet bulkhead fitting fitted to wicking bed

Step 6 – Prepare the water inlet pipes.

If using the simplified single-pipe inlet design, drill 10mm (3/8”) holes all around the last 30cm (12”) of the 50mm (2”) wide PVC pipe, length should extend high enough above raised bed to allow efficient watering without reaching too high or pushing plants out of the way to find the watering inlet.

Drill holes into the water inlet pipe, on the lower section only if a single, straight pipe is used, or along the length of the second pipe, if an ‘L’-shaped inlet pipe is used

If using the more common “L”-shaped water inlet design, use two lengths of pipe and an elbow join. The horizontal section should be approximately half to three quarters the length of the bed, and should be drilled all around, and over its length, with holes approximately 10mm-12mm (3/8”-1/2”). Join the two pipes with the elbow join, just push-fit the pieces together, a friction-fit is sufficient.

Step 7 – Lay down a thin layer of coarse scoria  over the pond liner. This will bed it down and put a protective layer of scoria over the pond liner so that it can’t be damaged by the PVC inlet pipe.

Important – This simplified water inlet design, which uses only one shorter piece of inlet pipe and no elbow joint, is ‘I”-shaped and not “L”-shaped as in the previous design, so the rough cut end of the pipe points straight down onto the pond liner.

Make sure there is an adequate layer (5cm or 2”) of gently packed-down scoria between the end of the pipe and the pond liner to avoid puncturing it if the pipe is accidentally pushed down!

Lay a thin layer of scoria over the pond liner if using a straight water inlet pipe design to protect the liner from the sharp end of the inlet pipe

Step 8 – Clamp the pond liner to the top edges of the garden bed right around using small spring or screw clamps to keep the pond liner  in place while the bed is filled with materials.

Clamp pond liner to top edges of the wicking bed to prevent it slipping when filling with scoria and soil

Step 9 – Fill the garden bed with coarse scoria to the height that is level with the overflow outlet. Make sure that the scoria layer is fairly level and even.

Fill wicking bed with coarse scoria to height of overflow outlet

Step 10 – Lay down geotextile fabric (or shade cloth) over the scoria layer.

Lay geotextile fabric over the scoria layer

Step 11 – Wrap well around inlet pipe, allow the fabric to come up around the pipe to prevent any soil entering the scoria layer.

Wrap geotextile fabric well around inlet pipe

Step 12 – Cover the scoria layer with two layers of geotextile fabric (or a single layer of dense shade cloth), and tuck it in, pushing it down the sides between the scoria and pond liner to a depth of at least 15cm (6”).

For a wicking bed to work, the soil at the edges needs to extend into the water to wick it upwards

The geotextile fabric must be tucked in all around the sides, where the scoria meets the pond liner along the raised bed walls. This will secure the geotextile fabric in place, and allow some soil to fill the narrow, wedge-shaped gap down the side, permitting wicking to work properly.

It’s the soil that does the wicking in a wicking bed, not the scoria, as some mistakenly believe. A fold of the geotextile fabric, with soil inside it, needs to extend into the water to act as a ‘soil wick’, so the water can be wicked up. Without such a soil wick, the water can’t jump the air space between the water’s surface and the bottom of the soil once the water level even drops slightly!

Tuck the geotextile fabric down well into sides of scoria layer

Step 13 – Check that the scoria layer is level. If there are any high spots, pat them down. Low spots can be raised by pressing around the sides of the low area to push scoria into the area to fill it.

Check that the wicking bed scoria layer is level

If the scoria is far too uneven, lift a section of the geotextile fabric, level the scoria layer, and then put the geotextile fabric back in place.

Level scoria by hand if it is uneven

Step 14 – Check once more that the scoria layer is level under the geotextile fabric.

Recheck to ensure that the scoria layer in the wicking bed is level

Step 15 – Begin filling the raised bed with the soil (50% soil, 25% compost, 25% cow manure mix or your own blend), ensuring that the pond liner is kept against the walls of the raised bed as you fill with soil.

Start filling wicking bed with soil
Keep filling the wicking bed, ensuring that the pond liner is kept flat along the sides of the wicking bed
Wicking bed filled with soil, the blue plastic pot at top of water inlet is there as a cap to prevent dirt and debris entering the water reservoir

Step 16 – Trim off excess pond liner with scissors leaving about about 3cm (1-1/4”) of pond liner the soil line.

Trim off excess pond liner

Step 17 – Moisten the soil, and fill the wicking bed with water, then it’s ready to plant!

To get the system to wick properly, the soil needs to be damp. Wet the soil evenly by gently watering the soil  from above, repeatedly until water starts running into the scoria layer.

Once the soil is evenly wet, fill the water reservoir through the inlet pipe until water begins to flow out of the overflow pipe.

Plants up the wicking bed and mulch your plants to conserve moisture, then sit back and relax!

Pond liner evenly trimmed just above soil level
Completed wicking bed construction, ready to plant!

Getting More Out of Your Wicking Bed

Mulch  – The reason why there is a considerable lip or raised edge above the soil level , around 10cm (4”), is to allow the bed to hold a nice thick layer of mulch above the soil. Mulch keeps the moisture in the soil, prevents evaporation, and conserves water. The purpose of building a wicking bed in the first place was to reduce watering after all! If you’re building your wicking bed in the warmer seasons, always mulch!

Use a layer of mulch around 5-7cm (2”-3”) thick so the water lasts longer, and plants roots stay cool. The mulch will break down slowly and add nutrients to the soil. For mulch material in a vegetable garden bed, you can use pea straw, lucerne, hay or sugar cane mulch. See article ‘Which Garden Mulch is Best for Improving Soil?‘ for more information.

In-soil worm farms – you can construct worm farms directly in wicking beds using worm tunnels (How To Build a Worm Tunnel In-ground Worm Farm) so that the whole wicking bed becomes a wicking worm farm. This way, the earthworms generate worm castings, one of the best known fertilizers, within the garden bed itself!

Water recycling – the water that flows out of the water overflow outlet will be loaded with fertiliser, which can be directed into a bog garden, reed bed system, or garden bed in the ground, for moisture loving plants (if enough water overflows!)

Extra growing space – wicking beds, like other raised beds, can support frames or trellises to grow climbing plants on such as beans, peas, cucumbers, watermelon and any other edible annual climber you fancy.

Keep in mind that you cant hammer stakes or poles into the wicking bed itself, that will punctuate the pond liner and destroy the watering system. The frame, trellis or support has to either be anchored into the ground or attached to a wall behind the wicking bed. See the article linked here for a better way to stake up pots, planters and wicking beds that can’t be staked.

Protective covers – as with other raised beds, you can make a frame to support bird netting or insect exclusion netting to protect the plants in your wicking bed from pests. Another possibility is to use clear greenhouse plastic to make a cloche tunnel for extended season growing.

Wicking Bed Maintenance

To maintain a wicking bed, flush out the whole system at least once a year. If the wicking bed is undercover, and not exposed to rain which would help flush it out naturally, consider carrying out the task perhaps twice a year.

To clear away high levels of salts that build up at the top layers of the soil, water from above to wash them away into the water reservoir and out of the outlet pipe.

Also, go easy with the fertiliser when feeding wicking beds in spring and autumn, as fertiliser levels can accumulate, because every bit added stays in the system, unless it’s washed out.

Other than that, maintain a wicking bed just like you would any other raised garden bed.

Now that you know whether you really do need a wicking bed, how it works, and how to build one, the rest is up to you! Happy growing!

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