Wicking garden beds are a method of gardening which uses a closed bottom system with a reservoir of water at the base of the garden bed to irrigate plants from the bottom up. There are commercial garden bed setups that use this system and in fact it is basically an oversized self-watering pot plant, but it is also an easy thing to build yourself from basic supplies or recycled materials.
The greatest benefit of wicking gardening is that it is highly water saving. Virtually 100% of irrigation water in a wicking system is available to plants and watering is down from the roots up, reducing instances of leaf mould and disease that can occur from top watering . This article will show you everything you need to know to decide if this gardening method is right for you and if so how to build or convert existing garden beds to wicking beds.
Deciding if A Wicking Garden Bed is Right for You
Wicking garden beds are ideal in certain gardening applications, but they are not the best choice in others. Situations where they are most ideal include:
- Renters who need a self-contained gardening method that can be easily removed when moving without impacting the existing garden.
- Paved or concreted areas without access to natural soil. Since wicking beds are a close system you can use them to grow plants where there is no soil and if any overflow is appropriately directed it shouldn’t cause any staining or damage to the ground surface.
- People wanting to growing edible in contaminated areas where the natural soil is unsuitable to use. If you live in an area with prior (including historical) industrial or mining activities. Get your soil tested before growing edibles in it for only a $20 donation through the Macquarie University Program Vegsafe.
- Anyone living in an area of low rainfall or wanting to reduce their requirements for irrigation.
- People living in suburbia or urban areas with smaller gardens.
There are however some downsides to this method of growing which you also need to weigh up which include:
- The method is resource intensive (from the materials it requires for construction) compared to traditional growing in the ground and therefore it may not be ideal in larger gardens/homesteads.
- The materials used can break-down overtime requiring the garden to be dug up, repaired and reset. This is largely dependant on the choice of materials used. Builders plastic and pond liner may break down in a few years, whereas an IBC tank may provide many decades of use and a cast iron bathtub may last a lifetime.
- There is potential concern about the leaching of different chemicals into the water irrigating the plant where plastic liners or IBC tanks are used. Additionally IBC tanks do not always have a known prior history and may have been used to store undesirable chemicals which could affect the health of your plants and yourself by eating them.
Personally for our family at Loganberry Forest, we previously used wicking beds when we lived in small rental properties in the desert of central Australia (see video at the bottom). We were inspired by the huge success the community garden in Alice Spring (not close to us but in a similar if not wetter environment than ours) which extensively used them. And we can confirm that they were ideal for our circumstances at the time and greatly increased the amount that we could grow in the desert.
Now that we are living in a wetter temperate climate in rural Victoria at Loganberry Forest which is our ‘forever home’ with virtually unlimited growing space (20 acres) for our food wicking beds are long longer right for us. Instead we now use other methods to try and save our use of water such as mulch, mini swales and collect rainwater in our tanks and dam for irrigation so we do not get big water bills.
The Physics of Wicking Garden Beds
This diagram shows you the basic essentials of any wicking garden bed. You have a water reservoir within a closed bottom system, a water permeable layer (but ideally not plant root permeable) layer (eg shade cloth or weed matting) and the soil where the plants grow.
Note the two 30cm depth measurements. These are important! This is because wicking garden beds work through capillary wicking action which allows the water to spread upward through the soil layer from the water reservoir. We have all seen these same forces in action elsewhere. For example if you hold a towel and the bottom touches a puddle, a much larger amount of the towel gets wet than is directly in the puddle. Capillary action occurs because the forces of surface tension within the water and the adhesive forces between the liquid and the towel (or soil) push the water upward. But this only occurs to a point, and ceases once the force of gravity is greater than the wicking forces. What height water can rise is largely dependant on factors like the size of the spaces between the grains of soil (ie. soil texture). But to keep things simple 30cm is said to be a good maximum soil depth and water reservoir depth to ensure that most soil types can wick the water high enough to the plant roots. FIY the Australian Colin Austin is credited as the inventor of the wicking bed concept and I can only assume he looked through soil science data models to arrive at this 30cm figure.
The risk of having a greater water or soil depth layer than 30cm is that the water will be stuck at the bottom fo the bed and not be able to wick high enough to reach the the plant roots. The one exception I think for this rule is then if you are trying to create a wicking bed for something like a fruit tree. As the roots of trees go very deep they will need more than 30cm of soil but therefore also be able to access water at a greater depth. The potential problem though with using wicking beds for trees is they may breakthrough your layers and ultimately may cause leakage in the system or root rot if they reach the water reservoir. So while some people have been successful with this method for tree, it is certainly more difficult to get it right than with vegetables and just going with the 30cm/30cm measurements.
Structural Materials for a Wicking Bed
There are a multitude of choices when it comes to the structural materials for a wicking bed. The best for you will be based on both what you have available as well as the longevity you are hoping to get out the garden bed. What you are aiming for is a watertight growing container and this can be achieved a number of ways. You can create you garden bed within an already water tight container which has an appropriate depth. This could include:
- Half an IBC tank
- A bathtub
- A pond
- A Styrofoam broccoli box
- Plastic or metal drums
- Plastic boxes
Alternatively you can line a non watertight or open structure with a waterproof material like heavy duty builders plastic or pond liner. These open structures could include:
- a raised garden bed kit (DIY or store bought).
- a garden bed/hole dug into the ground
The longevity of the wicking bed will be determined by the strength of the materials used and how quickly they may break down due to UV radiation, exposure to water and damage from plant roots. You obviously want to make sure anything you use is not going to leach toxic chemicals into the system and your food supply so a barrel that was formally used to store car oil or pesticides is a firm no.
You are also going to need a permeable layer between the soil and the water reservoir. Something water can get through but ideally not plant roots. Options include things like shade cloth, weed matting or repurposed materials like feedbags, wool carpet etc.
Optional additions include PVC pipe or slotted ag pipe to distribute water to the reservoir layer without wetting the soil plant leaves (the benefits of this are less mould issues with the plants, its not strictly necessary though, if you water the soil excess will travel through to the water reservoir layer).
Also you may wish to have a single point of overflow through another PVC pipe. This prevents water saturating the soil layer and drowning the plants if it becomes too high and allows you to direct the overflow to somewhere you can reuse it, like watering another wicking garden bed or a standard garden bed. You could in fact use this to create a series of terraced wicking garden beds that only required the top most garden to be topped up and watered.
An alternative to this is make the join between the water reservoir and soil layer non water tight so that excess water can get through (as we did in the video below). The benefit of this is less resources required and less effort is needed to make the entire wicking bed water tight (only the water reservoir section needs to be then).
Selecting the Best Growing and Water Reservoir Mediums
So in a wicking garden bed the soil layer needs to be suspended above the water reservoir and the easiest way to do this is usually to have the water reservoir filled with some kind of medium with a lot of pore spaces. Common water reservoir mediums are materials like gravel, scoria, crushed bricks or sand. The larger the grain size of the material in medium the more water the reservoir will hold. But even if you do need to use a finer grained material like sand (as we did in the video example below), you can boost the water storage capacity by added in pockets where extra water can be stored by adding in holding containers that are permeable to water but won’t be just filled with the sand. We did this by burying some pvc pipe offcuts wrapped in shade-cloth material.
For the growing medium (soil) there is less choice. You need the soil to be homogenous and fine grained to properly. This is not the time to be doing lasagna bed gardening (although soil created by this method and now entirely broken down is fine). Choose or create a soil blend that is rich in organic material to feed your plants.
Putting it All Together
To construct you wicking beds gather together all your chosen materials. Note that if you are using an open bottom structure like a regular raised garden bed and your natural ground surface has a lot or rocks in it which could pierce your waterproof layer then it it is a good idea to remedy that with something like repurposed carpet or a layer of sand.
Simply put together your garden bed layer by layer. Then fill your water reservoir. Once it is complete you need to get the wicking action started by saturating the soil layer at least initially or any time that it appears to be fully dried out. This is a good test of your overflow system too. If you are clever about working out the storage capacity of your system and how much water your plants are used it would be possible to put a wicking bed on a timed irrigation system which is good if you go away on holidays. That being said one of the benefits of a wicking garden bed is that it allows much longer time to pass between waterings.
To see how we constructed our wicking garden beds when we lived at our desert rental property watch the video below. Happy growing!