How to build a wicking bed

If you are thinking of adding a raised bed to your food garden, why not make it a wicking bed?  This blog post explains how to turn the normally useless bottom half of your raised bed into a water reservoir that will save you watering-time and water.

At a recent food garden visit host Tara explained the use of wicking beds and showed us several wicking beds in action.  People wanted to know more and the idea was born for a blog post that would explain how to construct one.

Introduction

What is a wicking bed?

A wicking bed is a garden bed with a water reservoir under it, and water gradually siphons up to the roots of plants by capillary action.  Wicking beds can be small boxes or large raised garden beds, or even be below ground.  In Western Australia where sandy soils are common, wicking beds are sometimes in-ground because surplus water will flow away through the sandy sub-soils.

What are the advantages of a wicking bed compared to a conventional raised bed?

• The bottom area of the raised bed is a reservoir from which water does not evaporate.  
• The gradual upward movement of the water means that you don't have to water daily.  Replenishing the reservoir once a week is all that is needed.  You can therefore be away from the garden bed for longer times than with normal raised beds without using any electricity or a timing device.  
• Plants chase the moisture coming from below and may therefore develop stronger roots.
• Soil nutrients may remain in your garden bed more than in a normal raised garden bed because they don't leach out through the low part of the bed.

Are there disadvantages of wicking beds compared to conventional raised beds?
Yes, there are, but nothing dramatic.

• The water reservoir can be as big and high as you like, but the soil layer can not be deeper than 30 - 35 centimetres.  If it is, the water travelling up from the reservoir will not reach the top layer of the soil, and that would defeat the purpose.  This limitation of 30 - 35 centimetres of soil is fine for food gardeners, because most vegetables, including root vegetables, won't grow deeper than around 20 centimetres.  It means, however, that you can't use a wicking bed for fruit trees or other deep-rooting plants.  The roots would soon penetrate the water reservoir area below and destroy it.
• Putting together a wicking bed is more work than a conventional raised bed, and the extra materials increase the cost of the bed.  For a list of what we used and what it cost, see below.
• If you dig really deep in a wicking bed, you could damage to top of the reservoir.
• In regions subject to torrential rain wicking beds can become mud pools if you don't have a sufficiently wide outlet pipe (but so can normal raised beds if drainage is insufficient).

The wicking bed we built

We had just bought a new galvanised iron raised bed when the idea for a blog post about the subject was born, and decided, with Tara's advice, and after a lot of research on the net, to make one in our own garden.  Here is the wicking bed we built:

Nothing unusual except for two pipes.  The vertical pipe that sticks out of the top of the bed is the wicking bed's inlet.  To (re-)fill the bed's water reservoir, we stick a hose in the top of the pipe, switch the tap on, and keep it open until water begins to come out of the outlet (the small white pipe sticking out further down).  When water comes out of that pipe, the water reservoir is full and you switch the tap off.

The materials we used
Many examples of wicking beds made on the mainland and outside Australia use materials that are not available in Tasmania or are hard to get here.  Our wicking bed was constructed using materials readily available in Tasmania.  The list below shows details of the materials used and their cost.  Looks complicated, you might say.  Keep reading.  All will be explained in detail.

You may be able to source some or all materials cheaply from a tip shop.  If you do, check they have not been in contact with poisons or contaminated substances, because that would make the produce grown in the bed unsuitable for consumption.

      

Step by step how we built our wicking bed

Step 1: Choose the enclosure

We used a conventional galvanised iron raised bed that was 63 cms high, 90 cms wide and 1.5 metres long to fit a space in our garden.

The enclosure you choose will have to be robust, so it does not fall apart after a few months.  Decide how deep you want your soil to be.  That will then tell you what the approximate height of your water reservoir will be. A container that is too shallow will be no good.
If you can source a completely non-porous container with a bottom, you will be able to skip some of the steps below and save money in the process.  Using a container with bottom is discussed at the end of the post.
If you can source a container with flat walls rather than one made from corrugated iron, this makes attaching the outlet (see Step 7) a lot easier.
Intermediate Bulk Containers (IBC) are used to transport liquids.  Used ones that have not been used to transport chemicals are ideal to make wicking beds because they have flat walls and are completely non-porous.  However, I could not get one for our experiment, and when available in Tasmania they are often so dear (and you need two go create the same soil surface) that usage here is not really viable.


Step 2: Determine where the inlet and outlet are going to be
The aim was to have a soil layer of 30 centimetres.  Our enclosure was 63 centimetres high.

A water reservoir 25 centimetres high, would leave 8 centimetres at the top of the bed for mulch, so the outlet was going to be 25 centimetres from the bottom-edge of the galvanised iron bed.

Next we worked out where on the perimeter the best spot was for the inlet and outlet:

• Position your inlet where you can easily reach it with a hose. 
• Position your outlet where you can see it while filling the reservoir, so you can stop filling as soon as water comes out of the outlet.  Ideally when water comes out of the outlet it waters some nearby plants, rather than a path or your shoes.

After determining where the best spot for the outlet was going to be on the perimeter, we drilled a hole with a hole saw (see photo below) that was exactly the diameter of the thread of the bit we were going to put through it.

Drilling the hole before installing the bed is easier than doing this when the bed is already in place.  We learnt the hard way that drilling the hole at this time is much easier than drilling it once the bed is in place.


Step 3: Prepare the ground and walls of your enclosure

Level the area where the wicking bed is going to be.  An area that is not level may ruin your effort because the soil in the bed may not be watered evenly.  When you have levelled the ground, put the enclosure on it, and then cover the ground inside your enclosure with old carpet.  The old carpet ensures that the plastic you are going to put on top of it will not be punctured by sharp bits in the soil.

We now had the following:

Now search the inside of your enclosure for sharp bits that might tear the plastic that will rest against it.  In our case we found small protruding ends of rivets (see first photo below) and covered them with corflute (see second photo).  There was no need to glue them in because the content of the tank will keep the corflute in place.

Step 4: Fill the enclosure with plastic lining

It is now time to construct the outside of your water reservoir.  We put in a double layer of new black plastic called 'builders film' (see list of materials above). Whatever material you use, it needs to be wide and big enough to form the bottom and sides of your water reservoir and it needs to be without any holes and be no-porous.

It is better to have a piece of material that is too big than one that is too small, because you can cut it to size later. Our bed now looked like this (yes, it looked very messy):

Some people use 'food-grade' plastic, because it might be safer.  We could not find this and used builders film.  Food-grade plastic would cost more.

Step 5: Fill the base of the reservoir
The water reservoir in a wicking bed can not just be water because you would not be able to put soil on top of that.  Pebbles or rocks or scoria or other materials that do not float, with lots of open spaces in between where the water can go, are good to use.  Tara had a good story about using polystyrene that floated.  Better not to use this material.

A reservoir with just pebbles will work in small beds, but in larger beds the use of ag-pipe + pebbles is recommended, so the water that comes in through the inlet evenly disperses over the entire reservoir area.

We used 10 metres of 50 millimetre wide ag-pipe.  The blue bit in the photo is where the ag-pipe starts and where we wanted the inlet.  We laid the pipe in circles from the outside to the middle, leaving an area without pipe in the middle.  We did not cover the start or end of the ag-pipe.

Things now looked like this:

The bricks in the photo are just temporary to keep the pipe in place. We removed them later.

Next we covered the ag-pipe with weed-mat.  The weed-mat makes sure the holes in the ag-pipe will never clog up with dirt.  We used ample weed-mat, so the ag-pipe will be completely separated from the pebbles and gravel and small particles that will be added next.

On top of this we carefully spread 'river pebbles' (see materials list above).  We used these smooth round pebbles because we wanted to make sure there wasn't any chance that the plastic reservoir lining be torn by sharp edges.

We used the pebbles to secure down the weed mat. We put the pebbles in between the pipe segments, and filled spaces between pipe and outer wall.  The end result was a solid layer to about the height of the ag-pipe that looked like this:

The bricks were still there to keep it all in place.  There is water there because the photo was taken after it had rained heavily.  The water could not escape because the plastic lining was doing its work!


Step 6: Attach the inlet
Find a piece of pipe that is slightly longer than the height of your bed and that is just a bit wider than the blue bit at the end of your ag-pipe.  Add an elbow of the right size (for details of what we used see the table above).

Attach elbow to pipe and to the end of the ag-pipe.
Stand the pipe up against the inside of the tank and then saw it off, if needed, so the inlet is at a convenient height, e.g. a few centimetres above the rim of the raised bed.
We used orange cord (of a type that won't rot away) to make sure that you can not accidentally pull any of these bits apart when they are all buried deep down the bed.  You could glue it all together.  It does not have to be water-tight.  We drilled holes in the tank wall near the top-edge and attached the top of the pipe to the rim with the same orange cord.

Step 7: Attach the outlet
The photo below shows the bits that made our outlet (for details see parts list above).
If you live in a high rainfall area (e.g. North Queensland), make sure that the outlet pipe is wide enough to quickly drain excess water.  The pipe we used had a 20 millimetre diameter.

From left to right: bit A, bit B and pipe C

Here is how we assembled the outlet, using the hole drilled in Step 2:

1. We cut a bit of rubber (orange in the photos below) into two rings that were just big enough to fit around the thread of bit A and act as washers.
2. On the outside of the tank we put bit A with an orange rubber around it through the hole.
3. On the inside of the tank we cut a small cross in the black reservoir liner precisely where we could feel the thread of bit A.
4. On the inside of the tank we put the reservoir liner over the thread without making the tear any bigger, put the orange rubber ring on the thread, then screwed bit B on the thread of bit A.
5. On the inside of the tank we fitted ten centimetres of pipe C into bit B.
6. On the outside of the tank we fitted ten centimetres of pipe C into bit A.
7. On the inside of the tank we wrapped weed mat around the opening of the pipe. The weed mat is meant as a filter, so nothing can clog up the outlet pipe.  We secured the weed mat to the pipe with a few plastic straps called a cable ties (see second photo below ; any non-perishable tie will do).

On the outside of the tank the result was:

On the inside of the tank the result was:

If you had a raised bed with flat walls, rather than one made from corrugated iron, drilling the hole and attaching the outlet would be a lot easier.


Step 8: Fill the rest of the reservoir
The rest of the reservoir, up to a height of 25 centimetres, was filled with small 7 millimetre blue metal pebbles (the cheapest material that would not clog that we could lay our hands on).

The finished reservoir looked like this:

In the photo this layer may look solid, but there is a lot of air between the pebbles. There is therefore a lot of room between the pebbles for water.  When the reservoir is filled, water enters via the inlet.  It will then go through the ag-pipe and disperse itself around the pebbles.


Step 9: Test the inlet and outlet
Really important, this point!  It is a lot easier to correct problems at this stage than later on when the bed is finished!

We put a hose in the inlet, opened the tap and watched the outlet.  After a little while we saw this:

We switched the tap off, but it took a few more minutes before the trickle of water stopped.  The system worked, but the water does not stop flowing the moment you close the tap. We therefore later added a blue-metal drain where the water lands, so overflowing water will go to a black currant bush and lemon tree that are next to the wicking bed.

We decided to let the reservoir rest for a fortnight, before continuing the project, so everything could settle down.


Step 10: Finish the bed with soil
We started this stage by removing gravel where it was higher than the outlet, making sure with a spirit level that it was level throughout the bed.

Next we put a layer of weed mat on top.  The idea is that this will act as a filter, so soil particles will not enter the water reservoir.

We placed a 30-centimetre long stick above the weed mat (see photo below), so we would always know how high the soil layer above the weed mat was.  This is important, as moisture may not reach ground level if the soil layer is deeper (see 'disadvantages'  section of this blog post).

Next we put a 10 centimetre layer of sugar cane mulch on top. This is not essential, but once again it acts as a filter between soil and water reservoir. The thick layer will compress and over time turn into soil. You could have a layer of sand instead, or just soil.

Now it was time for the top soil layer.  We took good soil from our garden and mixed it thoroughly with sand, coir, sheep poo and sugar cane mulch, to create a soil mixture that would be fertile, yet not dense.

Here is the situation after adding some of the soil:

When the soil level was up to the top of the stick, I cut off the plastic around the perimeter just above soil height.  Here is the end result:

Some additional comments

Put a cap on the water inlet
It is a good idea to find a cap for the water inlet (we used a jar), so insects etc. can not enter the water reservoir.

If you would use a non-porous durable box with a bottom
If instead of a galvanised garden bed without bottom you would be able to source a non-porous durable box with bottom, making a wicking bed would be easier.  You would not have to use carpet as explained in Step 3, and not need to do Step 4, because you would not need the plastic layer to make the reservoir.

If your wicking bed would be small
You could leave out the ag-pipe coiled in the base (Step 5), because water will spread over the entire reservoir without a problem.  You can use polystyrene or wooden boxes if they are deep enough to allow for a 20 centimetre soil layer and a reasonable size reservoir.

Some designs add an outlet at the bottom of the reservoir
There are designs by garden professionals that include an outlet at the bottom of the reservoir.  This would allow you to completely drain the reservoir of all water. This might be useful in warm climates where bacteria might enter the water reservoir.

The last word on wicking beds has not been said
Modern wicking beds are a relatively young technology.  Some designers have reported that their design did not work in extreme weather conditions.  Better still, some designers then shared their improved designs that should cope with those conditions.  There is a wide variety of designs out there, and different people use different materials.  Our design is based on the following sources:

• A great series of YouTube videos at Rob Bob Backyard Farming
• Wicking bed workshop notes provided by Terra Perma (thank you, Tara)
• Notes and advice provided by Food Garden Group member Tara

Our experiences with this design
I hope to report on this blog how our wicking bed performs this summer.

Happy Wicking,

Max Bee