Wicking beds are quickly growing in popularity, particularly for growing vegetables. Water is a becoming a scare commodity in some areas as the climate changes and ecosystems adapt. We delve into the water savings of sub-irrigation and give you the low-down on how often you should top up your sub-irrigated planter.
Do Wicking Beds Save Water?
Are wicking beds the answer? Do they really save that much water? How do the compare to other irrigation systems?
Wicking beds can save thousands of litres of water over several growing seasons. The amount saved varies according to a number of factors including mulching, reservoir capacity, depth and rain frequency.
How Often Does a Wicking Bed Need Watering?
Wicking beds are self watering right? Why would I water it?
A wicking bed requires it's reservoir to be topped up when it runs dry. This top-up can happen naturally due to rainfall if the bed is exposed to the elements; however hot weather can make the bed dry out faster. The frequency of water top-up is therefore highly dependant on the weather, but 4 weeks or more after last rainfall is not uncommon.
4 Ways a Wicking Bed Loses Water
To make a judgement on how often your bed will need watering, we will need to explore the various ways a wicking planter loses water:
The number one way a wicking bed sheds water is through evaporation. Moisture likes to move from wetter soil to dryer soil to "even things out". This is what drives the wicking process. This means that water is continually wicked up to the surface and evaporates from there.
The amount of evaporation depends on a number of factors: how wet the top of the soil is, the humidity of the air, the ambient temperature, and the granularity of the surface of the soil.
However evaporation is a waste of water in this context, and can be slowed in two ways, both of which are ideally employed:
i) Preventing Evaporation by Over-Filling
By over-filling the bed by a couple of inches - research shows that water only wicks up to a maximum height of around 40 cm above the water line. This means filling your bed slightly higher than this maintains a dry surface which reduces evaporation.
Over-filling has many benefits which we have written about a length here - do have a read. Urban Veggie Crew always installs wicking beds to a height of 45cm above the lowest waterline.
ii) Preventing Evaporation by Mulching
Adding a generous layer of mulch on top of the bed will reduce any residual evaporation significantly. There are many types of organic and inorganic mulches available, pebbles, compost, wood chip, lucerne.
We favour organic mulches like sugarcane, pea-straw or lucerne because they break down and feed the soil. Wood chip, whilst organic, can suck nitrogen from the soil as it decomposes.
The next most significant source of water loss in a wicking bed is "transpiration" - this is the natural process that plants use to gain energy and grow.
Plants require carbon from the air to build roots, stems and foliage. So they use the hydrogen from water (H2O) in the soil to attach to the carbon from carbon dioxide (CO2) in the air, leaving the oxygen (O2) component of carbon dioxide behind.
This is the primary reason plants need water. It is what allows them to grow - much of the structure of a plant comes not from the soil, but from the air! It is also the reason we say that they "take in" carbon dioxide and "give out" oxygen.
A wicking bed containing lots of healthy, leafy plants will therefore lose a lot of water to transpiration. There is not much to be done about this - in fact this is what we want to use the water for.
3. Draining / Leaks
The worst case of water loss in a wicking bed is leaks; either from a liner puncture or a leaking or faulty drain fitting. Care should be taken when placing down the liner to protect it from anything sharp coming under the bed from below.
And obviously stakes and sharp implements should not be used inside the bed.
Generally if a leak occurs, there is no option but to dig out the bed contents on to a tarpaulin, locate the source of the leak and patch / replace the liner or faulty fitting.
This is a one source of water loss in a sub irrigated planter that we don't need to worry too much about.
A well designed wicking bed will have an overflow creating a high water-line in the base of the bed. When the bed is watered to higher than this point, water will flow out of the overflow pipe.
This can happen either from over-watering the bed - often the overflow is used as a sign the bed is full. That is a perfectly legitimate way to tell when to stop watering.
Overflow also occurs after extreme rainfall conditions (or less extreme rainfall straight after having filled the bed up!).
Prolonged overflow events are not desirable because a lot of the nutrition from the bed is retained in the reservoir water - it's a shame to lose that from an overflow. We sometimes direct the outflow from our beds onto nearby lawns and shrubs, although it's always important to ensure airflow into the base as well if the outflow is piped away.
If our recommendations on reducing evaporation are carried out (overfilling and mulching), there's no reason a wicking bed with a 100mm deep reservoir can't go 3-4 weeks without being topped up in summer, and a couple of months in winter. But note that super hot days may reduce that time, and rainfall will increase it.
Wicking beds offer immense water savings over the long term whilst giving the plants exactly the right amount they need to transpire.