What is a wicking bed?

Adam_W

A wicking garden bed makes watering easier to manage and creates an ideal environment for plants to thrive.

How it works

At the base is a lined, gravel-filled water reservoir, then a porous fabric sheet above the water to allow water and air movement, then the soil and plants.

The water in the reservoir is drawn up into the soil by capillary action, a complex natural process involving the surface tension of water and the tiny channels in the soil. It is aided by other natural factors such as suction created by plant roots and soil temperature changes.

Three important parts of a wicking bed are a simple pipe system that runs from just above soil level down into the gravel reservoir, an overfill outlet and liner. The result is a garden bed that has low water use and with minimal wastage, that keeps soil moisture at just the right level for optimum plant growth.

Pipe system

A wicking bed has a pipe system that runs from above soil level to a network of drainage-type pipes in the base of the bed’s reservoir. These pipes serve two purposes:

Watering – when you water your wicking bed you simply run the hose into the pipe that sticks up above the soil, the riser pipe, and fill until water comes out the overflow.

Breathing – these pipes act like a snorkel, allowing air to flow into the water reservoir when it’s not completely full. This helps prevent the water and soil becoming “sour” and also allows for air movement into the soil.

Drainage outlets

Every wicking bed must have at least one drainage outlet and that’s the overflow. This is positioned just above the gravel out through the side of the bed and is intended to drain off excess water during heavy rain or in case of accidental overwatering.

It’s a fantastic idea to add a second outlet with a tap at the lowest possible point to allow you to completely drain the bed should the need arise. This drainage option can be useful in areas with wet winters or high rainfall where you want to avoid an excess of water around plant roots. With the tap on this drainage outlet, it can be opened and closed as required.

Bed liner

Without a quality, robust liner your wicking bed simply won’t hold water. The material you select needs to be strong enough to resist tears or punctures, especially during the building process, and it needs a good lifespan. You don’t want to use cheap plastic film that breaks down in a year or two. You can use a builder’s polythene or plastic film but ensure it is heavy-duty and designed for use as a fully waterproof membrane in situations such as underneath concrete slabs. The best option is to use pond liner membrane. This can be purchased in pre-packed sizes or bought off the roll by the metre. Technically you only need to line your reservoir area to 20 or 30mm above the top of the gravel however it’s considered good practice to take the liner almost to the top of the bed.

Bed considerations

Size is the first thing you need to consider. To keep wicking beds functioning well, and make construction easier, you are better off creating a number of smaller beds and positioning them to look like a larger bed.

The most common materials used for wicking beds are treated pine sleepers. They’re light weight, making them easier to handle, are easy to cut, drill and screw, are long lasting and are very cost effective. To make your build easier, stick to the available lengths and use cuts that avoid wastage. For example, you might make your individual beds from 2.4m long by 1.2m wide lengths. This way each section of the bed frame only requires three sleepers – two at full length for the sides and one cut in half for the ends.

If your bed is going to sit on the ground then the base can be open. You’ll just build the walls, or frame, of the bed. You will need to ensure the area is level and cleared of any material that may puncture your bed liner. You can prepare the base area by adding a layer of paving sand or cutting a piece of old carpet to size and laying this down first.

If your bed is going to be elevated on a stand then you need to ensure the stand is capable of supporting the finished weight. Let’s assume you build a bed that is 2.4m x 1.2m. The regular depth of a wicking bed will be .5m (500mm). That gives your bed a cubic capacity of around 1.32cu/m (internal 2.4 x 1.1 x 0.5). Your bed will have a layer of gravel and then be filled with soil. Dry weight of a typical planter box soil is around 1 tonne per cubic metre. As gravel weighs a little more than soil, and you need to account for the weight of your sleepers and plants, the dry weight of your bed will be around 1.5 tonnes. After heavy rain, when the soil becomes saturated and the reservoir full, this could hit 2 tonnes so it is very important that your stand can support this. If you are unsure about the engineering required, then seek professional advice for this stage.

Want to build your own wicking bed? Check out the step-by-step guide How to build a wicking garden bed.