Preparing a wicking box

A wicking box is a box for growing plants, which has a water well in the base such that water ‘wicks’ up from the well into the growing medium above. A bit like the self-watering pots you see for sale in nurseries. I’m putting a couple more together, so thought I’d document the process.

The idea of a wicking box is that you don’t have to water the plants in the box as frequently as you would plants in a normal garden, thus saving time and water (as all the water gets used by the plant and doesn’t drain away).

This site explains the basic water-wicking principles. I hadn’t looked at it for some time, so when checking it out to see if the link still worked, I found that there are a lot more links and info than I remember. It’s well worth a look.

Before I go any further I should mention that what I’m describing here is my way of doing it. I’ve read enough blogs about water-wicking boxes and beds to realise that everyone does something different—what is common to all is the principle of water soaking up (wicking) into the soil above the reservoir, so do check out what other people do and find the method that suits you best.

Water will wick up vertically against gravity, about 30 cm. Any more and the surface of the soil won’t ever get wet (unless it rains or you water it), so the depth of soil above the water well shouldn’t be greater than this. The well can be any depth.

I went to my local hardware centre to check out suitable boxes (that’s Bunnings if you live in Oz). I found these black plastic crates. They’re 45 cm wide x 65 cm long x 26 cm deep and hold 60 litres:

I would have preferred them deeper, but the deeper ones (at that time…they have a bigger range now), were clear plastic and my experience of clear plastic planters is that the light promotes algal growth around the sides. I bought quite a few at the time (more experience has taught me that if you find something good, buy more and put them away, because when you eventually go back to get another one, they won’t be stocking them anymore) and there’s still quite a store of them under the house (hence the cobwebs on the one in the picture). At the moment, I have 25 in service!

First thing I do is drill 2 drainage holes, one at each end, such that there will be not more than 30 cm of soil above them (since my boxes are only 26 cm deep anyway, you can see why I would have preferred them deeper).  By the way, I’m calling it ‘soil’ but as you will see, you don’t fill them with ordinary garden soil. In my boxes, I drill the holes about a third of the way up from the base and they’re about 1 cm in diameter:

I settle the box (level) in it’s final position, because there’s only one of me, and once it’s full of wet growing medium, it’ll take two to lift it! I put mine up on polystyrene fruit boxes to keep them above rabbit-browsing height and then discovered that a big rabbit can stand on hind legs and reach up to keep all the greenery nicely trimmed around the edges! There’s no problem with this one on the deck, though (at least until a rabbit learns to negotiate the steps):

I put a plastic tube into the corner of the box. This is where the water is added (initially…see below). I have a stock of tubes which came from the local greengrocer. They were the centres of the rolls of plastic bags you put your fruit and veggies in, and they just happened to be the right depth for my boxes:

I fill the box with water, up to the drainage holes and start filling the box with organic material—raked leaves, prunings, lawn clippings (not too much of those), anything that will eventually rot down. These two new boxes have the remains of the broad bean plants I cut down and casuarina needles raked from under the trees:

I keep adding material (stomping on it to compact it) as it rots down. The water in the bottom will help. I add good compost if I have it. Of course if I had enough compost to fill a box entirely, I would do that. I just never have enough of it, but I do put in what I can spare. I add compost worms. That’s important. They will chew their way through the material, aerating it and turning it into worm castings. Ordinary garden soil is not good enough. It will be too heavy and will compact down and hold too much water. I want a nice, well-aerated, nutrient-rich growing medium. This will shrink down as the plants utilise it and will need to be topped up regularly with more compost and mulched to stop water loss from the surface.

I’m now about ready to plant something. I’ll either plant seedlings or direct-sow. Here are butter bean seeds just germinating:

These are my observations from my particular system which I began about 4 years ago.

# There is no actual water well in the bottom of my boxes. I want the soil (I’m going to keep calling it soil, but you know what I mean) in the bottom third of the box to be pretty well always saturated with water. Water-loving roots will grow into this area. Above that, the moisture levels decrease towards the top. When the soil is saturated, water will wick up into the less moist layers above. Fine feeder roots will prefer this area. If I look into the tube at the side and I can see water in the bottom, I know the bottom soil is saturated. Sometimes I use a dipstick to check on the depth, although this needs care! I’ve found that frogs love this moist shady tube and make it their home, which is good because they probably help to control any plant-eating predators. This little tree-frog spent several weeks in the tube and used to come up and sun itself at the top of the tube each day:

# Initially, I just added water through the tube. After a couple of years of growing, the water was taking too long to soak into the soil, so now I water from above, just lightly, and anyway that’s what happens when it rains. Water drains into the bottom and keeps the soil saturated up to the level of the drilled holes, then excess water drains out the holes. Sometimes the holes get blocked by worm activity, but it’s easy to push in a stick and work it around to clear the blockage. I check on soil moisture by sticking my finger into it. At one stage I was going to put in a dripper system for the line of boxes near the water tank, so that water was constantly and slowly fed into each tube, but I just haven’t got around to that yet, because hand watering is so easy.

# After a crop is finished, I usually find the soil level in a box has dropped by a few cm, so I top up with fresh compost. Now that I have chicken manure compost available, the plants are really loving it.

# I check regularly to see that a box still contains worms by pulling aside the mulch on the top. That’s where they’ll usually be. If there aren’t any, I dig down a bit and if I still can’t find any, I add a few from the worm farm.

# When plants have finished I don’t pull them out, just cut them off at the base and leave the root systems in situ. Peas and beans will shed their nodules and put nitrogen into the soil and the worms will feed on the rotting root systems.

I’ve also created deeper wicking tubs from large plastic plant pots which, because of the greater depth, I use to plant tomatoes. The post about that is here, if you want to check it out.

The boxes and tubs are ideal for small spaces like patios or decks and perfect under trees where root competition would make traditional vegetable beds impossible.

I’m really pleased with the whole system. The plants grow well and watering is only necessary a couple of times a week in summer and usually not at all in winter.