One of the questions that many people have when constructing wicking beds is what are the best rocks to use in the water reservoir section underneath. Coarse scoria is the recommended material, but are there any reasonable substitutes?
As a bit of background, a wicking bed is a self-watering raised garden bed which works by the principle of sub-irrigation, where the water supply sits underneath and is wicked upward into the soil in the container above. It is essentially nothing more than a large-scale version of a self-watering pot, and as most gardeners know, self watering pots have been around for decades.
If you would like step-by-step instructions on how to build and maintain a wicking bed, please see my article – Wicking Bed Construction, How to Build a Self-Watering Wicking Bed.
This is a diagram of how a wicking bed works. The water reservoir at the bottom is filled with coarse grade scoria – this is a porous volcanic rock that fills the water reservoir, and holds up the heavy layer or soil above it. The geotextile fabric layer separates the soil at the top from the scoria water storage area below.
Why is Coarse Scoria the Best for Wicking Beds?
The purpose of the water reservoir at the bottom of a wicking bed is to hold water, not rocks!
Coarse grade scoria is used in the wicking bed water reservoir for the following reasons:
- Scoria is porous, it’s really light because it’s filled with air holes holes like a sponge, which can fill with water and let water drain through, so there’s less rock and more space for water in a given volume compared to other materials.
- Scoria is extremely strong for its very light weight, it can hold up a large volume of soil easily without any risk of collapsing, and being light it keeps the overall weight of the completed wicking bed down so it doesn’t stress any components of the structure of the raised bed, or overstretch the pond liner. If the wicking bed is used on a deck or rooftop garden, keeping the weight down is critical!
- Scoria in the coarse grades is made up of rather big pieces, which all grip onto each other and lock together leaving large air spaces between them, allowing more water to be stored in the water reservoir with less space being occupied by rocks.
Can Other Rocks be Used as Substitutes?
Scoria is classed as a structural lightweight aggregate according to the ASTM C 330 Standard Specification for Lightweight Aggregates for Structural Concrete, where:
- fine aggregates have a bulk density of less than 70 lb/ft³ (1120 kg/m³)
- coarse aggregates have a bulk density of less than 55 lb/ft³ (880 kg/m³)
Put simply, a cubic metre of coarse scoria weighs less than 880kg.
Many people ask if gravel can be used in wicking beds. The bulk density of loose coarse gravel is around 1522 kg/m³, in other words a cubic metre of gravel will weigh around 1.5 tonnes, almost twice the weight of the scoria, but without the water holding capacity, because gravel is sold, it’s not porous.
Using gravel will increase the weight and reduce the water holding capacity of a wicking bed, so it will need watering more often in hot weather, and can’t be left as long between watering visits. So, it will work, but badly…
How to Calculate the Volume of Scoria for a Wicking Bed
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.
The Other Wicking Bed Question – What Can I Use for the Geotextile Fabric Layer?
The geotextile fabric layer is there to stop the soil from falling into the scoria water storage area, and filling it with mud, it doesn’t do anything else. It doesn’t need to wick at all, it’s just a barrier to keep the soil and water layers separate.
A good geotextile fabric is Marix weedmat, used doubled over, as in two layers of the material are used. It feels like fabric and lets water through but keeps the soil out. It’s deigned to be buried and to last beneath the soil.
Even shadecloth would work if the holes are fine enough to prevent soil washing through, use the 90% shade rating shadecloth if choosing this material. Shade cloth isn’t designed to be buried, so I’m unsure how well it will last underground, but this would be the most often used second choice material in wicking bed construction.
Don’t use pond felt, that’s for lining ponds to prevent the lining being punctured, and don’t use capillary matting, that’s used to wick water to sub-irrigate seedling punnets in commercial greenhouses, it’s very expensive, way to narrow and is best used for making self-watering capillary trays.