Wicking gardens 101
In the name of science, Sheryn Clothier built and tested a self-watering – aka wicking – garden bed.
Sheryn Clothier built and tested her own self-watering garden bed.
Awicking garden is simply a raised garden bed above a fabricated water reservoir. Water is added directly into the reservoir and is wicked (transported by capillary action) up through the soil to the roots of the plants.
The benefits are well extolled on the internet (and by those that sell ready-made wicking gardens!). They require less maintenance in that watering is less frequently required and require up to 50 per cent less water overall because none is lost through evaporation. In the humid Waikato, I reasoned they would also mean less fungal diseases as they are escalated by overhead watering.
Building it yourself
To build one you need a waterproof reservoir to which water can be added directly, with an overflow hole for drainage and establishing the maximum water level, topped by a layer of garden soil.
The depth of soil can vary, but most vegetable roots grow 300-500mm deep. The bigger the reservoir, the less often it will require topping up.
What is important and seemed illogical in some references was the contact between the dirt layer and water level. The water table needs some permanent fill to support the soil layer. Frequently suggested were rocks or stones and then a layer of weedmat, shadecloth or carpet to hold the soil above. The flaw here is that when the water level drops, there is no wick for the water to travel up as there will be a gap between the water and soil.
Some suggested making depressions in the weedmat, but I reasoned these depressions needed to go right to the base to be fully effective.
It was a friend who gave me the answer. She used flexible plastic drainage pipe inside her water reservoir. While I would much rather use a natural material than plastic, this was a simple way to provide space for the water which would be fully in contact with the soil at any level.
Next I had to create a waterproof reservoir. References cited wooden raised garden beds lined with plastic pond liners but this seemed like a lot of work. A ready-made container seemed a much simpler solution.
My friend had used plastic drums cut in half, but I opted for a larger IBC (intermediate bulk container) – a 1000-litre plastic container measuring 1000mm x 1200mm x 1200mm high in a supporting metal cage – on top of a pallet.
Used to transport liquid, IBCs are available second-hand at a reasonable price (about $100) – just ensure they were used for something benign and wash well before use.
Making the wicking bed
My pragmatic husband cut the drum at 560mm high. He also cut the cage so I could access the garden, but left the top frame to support a cover.
Then we coiled some 65mm drainage pipe as tightly as we could inside, leaving the top end upright in one corner for a filling point and covering the bottom end with weedmat. These drainage pipes can also be purchased with a mesh sock to prevent any soil working its way inside, but we used the cheaper punched drainage coil as it was what we had to hand.
It was a bit tricky to coil the pipe so tightly, so we ended up tying it to a crossbar of bamboo to hold it while we worked on the next coil. Even so, it was impossible to completely fill the space.
I had some of the clay balls used in hydroponics which had been sitting around since the kids used them for slingshot ammunition 15 years ago (I knew the clay balls would be useful again one day!) and used these to fill some of the space, though I ensured there was still enough gap for the dirt to get down to the base. You can use any rocks or stones as long as they are smooth enough not to damage the outer wall.
The benefits of wicking beds are well extolled on the internet (and by those that sell ready-made wicking gardens!).
Filling in the bed
Next came the soil and I debated on the best type to use.
The water is wicked up by capillary action, which is when liquid goes up a narrow tube against the force of gravity. The narrower the tube, the better the wicking effect so it stood to reason a clay-type soil with its smaller micropores would be more effective than a sandy soil with its larger pore sizes. But then the clay might compact too much and the macropores of a sandy soil would increase the water-holding capacity – but would the wicking effect be enough? I gambled that it would and used a sandy compost dirt mix amongst the pipe, and a compost and soil mix above.
Watering needs
For you readers, I decided to measure the first fill of water so you knew how much water my garden held. I could then record how
The soil looked dry on top, but when I dug down to about finger depth, I could feel the moisture…
long this lasted and supply you with some accurate data to calculate your own garden size requirements.
However, when the first rain arrived, I realised this would affect my records and rushed outside filling and counting buckets, hurriedly filling up my water reservoir. A mere 300 litres later, I became suspicious that something was wrong.
My water reservoir is 200mm high by 1000mm x 1200mm. That is a maximum of 240 litres minus pipe, clay balls and sandy soil. I stopped. Over the sound of the rain I heard another, louder trickle and realised the IBC’s tap, located at the bottom of tank, was wide open.
I went inside to get dry and “guestimated” my water reservoir holds something around 100 litres.
With Christmas looming I planted a range of gourmet lettuces, a cherry tomato, an eggplant and a chilli, and left them to their own devices.
They grew wonderfully and I had lettuce to excess. Cherry tomatoes were prolific (aren’t they always?) and even the eggplant produced. We couldn’t eat it all.
But I cannot credit this solely to the wicking effect as everything had grown fantastically. The warm and wet Waikato weather last November and December had the vegetables, grass and weeds flourishing.
Thankfully for this experiment, Waikato then decided to cooperate – by then lapsing into a drought of major proportions.
I let the garden dry out to see what happened (OK so I forgot to water it). I noticed things starting to wilt and simply poked the hose into the intake pipe and flooded the whole garden up to the top of the soil surface. I could see the water level rising up the side of the plastic, and by holding my finger over the drainage hole it rose even faster.
While the sprinklers were going for ages in my other raised beds in an effort to give them a deep soak, this just required about 15 minutes. Then I left it to drain the excess out the drainage hole.
With lettuces going bitter faster than I could eat them, I decided some shade cloth on top of the frame would be beneficial. In retrospect, the frame is too low and access required some tricky yoga poses. Done again, I would use my removable alkathene pipe arches to support any cloth, net or plastic needed.
As the gorgeous hot summer continued, the other gardens required long and frequent irrigation but the wicking garden just quietly wicked away. Neglected in the smelting summer sun, the plants grew and produced and showed no sign of water stress. The soil looked dry on top, but when I dug down to about finger depth, I could feel the moisture, and the water
reservoir only needed topping up once every couple of weeks or so, and that only took a few minutes.
Weeding & care
Another oft-cited benefit of a wicking garden bed is the reduced weeding due to the lack of surface moisture.
This could be a negative in my type of gardening where vegetables are left to seed and I rely on finding them self-sprouted the following season. Could seed survive?
In the name of research, I left a few of the excess cherry tomatoes laying on the soil to see what happened. Success! A couple of hundred seedlings self-sprouted. However, if I were planting seeds in summer, I would water regularly until they were established. As for the weeds, there probably are less.
I have a tea concoction bubbling away near my gardens at all times and I frequently apply this when irrigating. Cow manure, seaweed, comfrey and other goodies get brewed into nutrition for my plants.
But frequent applications of this in the wicking garden can apparently cause problems. Whereas in a natural situation excess liquid nutrients will be leached through the soil, in a wicking garden these can accumulate in the reservoir to excess. So I mixed the cow poo straight into the soil and left the plants to access it at will.
So would I recommend a wicking garden? Yes. Would I dig out my existing raised gardens and construct a water reservoir? Preferably, but since these and their irrigation systems are well established, it’s not worth the effort.
However if I were building from new, I’d definitely build a wicking bed. ✤