Wicking Bed Construction

Gardening is the corner stone of independent living and therefor of preparedness. That doesn’t mean insuring your food security has to be back breaking work!

A raised garden bed can make gardening much easier for those with bad soil and or bad backs! Even those in wheel chairs or using walkers to get around have found these raised beds work for them. Add that a wicking garden bed is also self watering and it’s a big bonus!

See how to construct your own raised self watering wicking garden bed at the featured link below from Deep Green Permaculture.

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 the 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 the roots of the plants. Also, the upward moving water carries the excess salts upwards, so the salts accumulate at the upper soil levels where shallow rooted seedlings are planted.

Another issue is that the lowest soil  levels in wicking beds are 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 build. In my mind gardening should be simple, cheap and sustainable. My basic rule of gardening construction is as follows. Only use a raised garden bed when growing in the ground is not possible.  If using a raised bed is not possible, only then use a wicking bed. The cost and construction effort involved going from gardening in the ground to raised beds 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 personal preference, it can be vary from a small tub which sits on a table or stand, all the way through to a full garden bed sized wicking bed system.

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

  • For very large wicking beds, railway sleepers are often used, and the dimensions and efficient use of materials dictate the size. A railway sleeper is 2.4m (8’) long so a popular size is 2.4m (8’) x 1.2m (4’) as this uses three sleepers to construct a single level. A typical height when using such materials is 60cm (2’) high, as each sleeper is 20cm (5”) high, and if they are stacked three high, the total height is 3 x 20cm = 60cm (3 x 8” = 24” = 2’). Such a design uses 9 full-length railway sleepers. Another design option to save 1/3 of the materials is to dig a trench in the ground for the water reservoir level so the pond liner sits below the ground 20cm (8”) with the outlet at the same level as the ground, this way you can use only so the two levels of sleepers instead. By comparison, such a design uses only 6 full-length railway sleepers.
  • Galvanised or coated steel raised garden beds of various dimensions are also commonly used as long as they are of adequate depth. Typically something around 70cm (28”) high works well as this allows for a good depth of soil, and enough of a lip above the soil level to hold mulch in place.
  • If using a small container which isn’t that deep you wont be needing a 20cm (8”) deep water reservoir, you can scale it down to suit the dimensions of the container.

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, 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 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 wick it upwards. 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 liber 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:

  • Raised garden bed – prefabricated steel raised garden bed or DIY timber raised garden bed
  • Pond liner – either PVC or the more expensive butyl rubber (these have very long warranties on then and are designed not to leak. While some people use black builders plastic, it is not as durable as pond liner and not recommended). You will need enough pond liner to line the sides and bottom of the raised bed. So, your width of materials will need to be (width of bed + 2x the height), and the length of pond liner will need to be (length of bed + 2x the height).
  • Scoria (Coarse Grade) – this is a porous volcanic rock that fills the water reservoir, the water occupies the scoria layer and wicks upwards. You will need enough 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 2mx1m garden bed will take (200*100*20)/1000 = 400L of scoria. or 0.4 cubic metres.
  • Geotextile fabric – separates the soil from the scoria-filled water reservoir. Geotextile fabric is synthetic and does not break down, which is important. Geotextile fabric is sold as fine grade weed mat which looks like see-through cloth (it does NOT look like plastic) and can come in a few colours, often grey-black or white. As it is quite thin it is a good idea to use a double layer of geotextile fabric . If geotextile fabric is unavailable, you can use shade cloth with a high shading factor as possible such as 90% shading shade cloth, as the holes are smaller. Whichever material you use, 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”).
  • Soil Mix – 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 vegies a good start.
  • Water Overflow Outlet Fitting – use a 20mm 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.

  • Water Inlet Pipes – use two pieces 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”).
  • Tools required – spirit level for levelling the garden bed and scoria laver, a drill and appropriate drill bits and hole cutters as discussed above, a set of small spring or screw clamps for holding the pond liner in place while filling, and scissors to cut pond liner.

After we have gathered the required materials, we can then begin construction. 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 them 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!

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