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Self Watering Gardens - What is Wicking?
Written by Ian Collins
Most gardeners would be familiar with the term ‘self-watering pots’. Large hardware chains and nurseries have a range of these pots, which have small water reservoirs in their bases. In America, they have been called ‘sub-irrigated planters’, or SIPs for short, which is probably a more accurate description.
‘Self-watering’ gardens utilise the same scientific principle as sub-irrigated planters and self-watering pots. This is what, in Australia at least, is called “wicking” and refers to the method of watering plants using a store or reservoir of water underneath the soil.
The so called “self-watering” capability derives from the scientific principle of capillary rise, where the water in the reservoir is effectively drawn up into the soil above by the forces of atmospheric pressure and the cohesive and adhesive properties of water. The cohesive properties of the water molecules create surface tension, and their adhesive properties attract them to the particles of soil. The water molecules are drawn up and occupy the pore spaces in the soil, previously occupied by air.
To understand what is happening in this process it is worth taking a quick look at the science. Capillary rise was first observed by Leonardo da Vinci and then derived into the Young-Laplace equation of capillary action in 1805.
The consistent level of moisture in the soil created in a wicking bed provides the plants with a readily available supply of water to take up what they need when they need it. This same principle of capillary rise is what helps transport the water from the roots of the plant to the leaves. Capillary rise is a function of the size of the soil particles. Most well-structured friable soils can achieve a capillary rise of around 35cms.
The key benefit of wicking
Not only does wicking provide the optimal amount of water for plants, it also uses significant less water in the process and, because of a variety of other factors in the way a wicking bed is constructed, it results in healthier, longer-lasting plants.
Self-watering is centuries old
Archaeological evidence of gardens irrigated by capillary rise dates back about 2,500 years to near the city of Old Jerusalem. In more modern times, plant self-watering systems can be traced back to Ohio USA in the 1890s.
‘Popular Mechanics’ magazine in 1909 made mention of a “Self-Watering Flower Box” that “protects plants from neglect”. The sketch in the article on the left shows an inlet pipe and a water reservoir very similar in design to those seen today. A patent was granted in the United States in 1917 for a sub-irrigated planter.
Sub-irrigation systems cover a range of bottom watering methods, of which ‘wicking’ is now becoming one of the most widely adopted. Other types of sub-irrigation systems include flood floors, ebb and flow benches, trough trays, Dutch trays and capillary matts.
The terms “wicking” and “wicking beds” are credited to Queenslander, Colin Austin, who is widely considered the pioneer in this area. He was the first person to really consider the application of wicking beds for rainwater capture as a method of combatting erratic rainfall.
Wicking breakthrough that lets plants breathe
More recently WaterUps has led product development and research into wicking. WaterUps introduced the first modular wicking cell in 2017, manufactured from recycled polypropylene. This provided a major shift in wicking bed design, increasing water reservoir capacity and optimizing the wicking efficiency by reducing the surface area of the wicks and incorporating an aeration layer to eliminate the chance of the soil above becoming anaerobic. Until the arrival of the wicking cell, reservoirs were constructed using ag pipe and a range of inert media including scoria, river sand and gravel. The soil or growing media was kept out of the reservoir space using geotextile fabric. This method is still used by many people today but lacks an aeration layer or gap between the growing media and the reservoir.
Until recently, all wicking beds were closed systems for container gardens. In 2020 an open system – a sub-irrigation channel wicking system – was introduced by WaterUps for in-ground application. This has allowed wicking to be used in small scale horticulture and has opened up a range of new potential applications for rainwater retention.
Wicking Beds as a series of layers
In my view the best way to understand wicking beds is to view them as a growing system comprised of a series of layers.
Starting at the top we have a mulch layer. This separates the soil surface from the atmosphere and acts as an important soil insulator, which reduces soil moisture loss and evapotranspiration.
The soil layer is your growing medium. It is where your plants derive their nutrients and water. The depth of your soil layer can vary depending on what you are growing.
Well-functioning wicking systems should also have an aeration layer between the growing medium and the water in the reservoir. This helps prevent the soil layer from becoming waterlogged or anaerobic.
Finally, the reservoir layer at the bottom is where water is stored in a wicking bed.
Why are wicking systems useful?
Horticulturalists often measure the readily available water (RAW) around the root zone to indicate when to irrigate. The aim is to achieve what is called Field Capacity, which is probably most simply described as the amount of water that a well-drained soil will hold against gravitational forces. This upper limit of available water for plants, or Field Capacity, is what you are achieving in a well-functioning wicking bed. The best thing about a wicking bed is that you don’t have to keep measuring the readily available water near the root zone of your plants because Field Capacity is maintained provided there is water in the wicking bed reservoir. This provides plants with the optimum soil moisture conditions where it is needed near the root zone.
Apart from providing a healthy environment for plant growth, wicking beds have other advantages over other garden beds.
They save a very significant amount of water compared to surface watering. Studies, including one by Ecologist Peter Rutherford have shown up to an 80% saving in water usage.
Because they both store water and can also capture rainwater, they don’t require regular watering. Depending on the prevailing climatic conditions and what you are growing, wicking beds can be left unattended for long periods.
Recent research done by WaterUps, in conjunction with leading horticulturalist Angus Stewart, has also shown that the use of wicking can increase crop yield and reduce the occurrence of plant disease, particularly fungal related problems. Some of these research results will be published in Angus’ new book that is due for release later this year.
This is achieved due to the way in which the soil is watered by capillary action. The top layer of approx. 5cms soil in a wicking bed is generally dry. This dryness is well above the root zone and provides three distinct advantages to plants compared to a surface watering system.
- The reduced humidity at the plant base very significantly reduces the incidence of fungal, mold and mildew attacks.
- The drier surface soil will not readily germinate weeds and unwanted plants.
- The dry surface soil discourages plant pests from laying their eggs resulting in lower pest incursions.
The benefits of surface soil dryness and the healthy subsurface soil environment near the root zone, resulting from moisture consistency, are key reasons why superior growing performance and crop yields are achieved in properly functioning wicking beds.