What is Wicking?
Wicking is the movement of water in soil, cloth or other materials through very small channels called capillaries – tubes as thin as hairs. Wicking works best when there are many millions of capillaries (invisible to the eye). In soil, wicking can draw water up more than 300mm.
What Prevents Wicking?
Wicking is prevented when there are stones around the reservoir pipe and a waterproof barrier such as plastic lining above the pipe. Lining blocks the capillaries but plant roots may still grow between the wall of the bed and the edge of the lining it and suck up the water. This then lowers the water table to below the barrier and creates a layer of air. Even a small air gap is too big for the weak power between water molecules1For the science-minded, wicking happens because a molecule of water has a positive and a negative end. These ends attract each other and so water hangs together. The power that links the molecules is not very strong., and wicking stops.
Why Do People Put Liner and Stones into Their Wicking Beds?
People mistakenly assume, or salespeople tell them, that rain or excess surface watering will fill the water reservoir with soil. In a correctly set-up wicking bed the outlet holes of the reservoir pipe face downwards, not sideways or upwards, and the slow wicking process does not carry soil particles into the reservoir pipe. I have emptied many wicking boxes after years of use that included very heavy rain and floods, and very few had soil in the reservoir pipe – and then just a few handfuls.
Stones and pine-bark Waste Space
Stones and pine-bark do not store water or nutrients, and they occupy space where soil biology and plant roots should be allowed to grow the healthy plants that we want.
Wicking is Slow
Filling a wicking bed reservoir pipe from a tap or hose is fast and water soon comes out of the overflow. But wicking is slow. Especially when the soil is very dry, filling a wicking bed or box up to full water holding capacity may need to be done in stages. A 30-litre Styrofoam box may need 5 litres of water before it is full, needing four fillings over two days.
No risk of Water Logging
There is no risk of water logging at the bottom of the wicking bed. Once a wicking bed is at full water holding capacity, the bottom layer of the box up to the level of the overflow is filled with excess water only for a short time. Plant transpiration and evaporation from the surface soon uses the excess, and air with oxygen moves back in. Even without plants, there is still enough evaporation to prevent water logging.
No risk of Foul Water
Foul smelling sodden anaerobic soil may occur when beds are unable to wick because they are filled with un-improved sandy potting mixes and/or bark. Pine bark repels water and reduces biological action. Pine-bark based mixes are good for keeping plants small, not for growing plants to maturity. To improve local sandy soils so that they more effectively wick, see Soils for wicking boxes and tubs.
Happy growing!
Author
- 1For the science-minded, wicking happens because a molecule of water has a positive and a negative end. These ends attract each other and so water hangs together. The power that links the molecules is not very strong.