This is a quick and easy project to construct a self-watering capillary tray.
It’s basically a tray with its own water supply, that you sit your pots in, and it automatically waters them for you. All you have to do is occassionally top up the main water reservoir, that’s all!
They’re great for seedlings, freshly rooted cuttings, and anything else that needs constant moisture.
How Does it Work?
This simple but effective device works by the principle of wicking, as explained below. In this setup, a terracotta pot (with its drain hole sealed) is used as a water reservoir. Beside it sits a plant in a pot. Both are sitting on capillary matting which absorbs water.
- The terracotta pot slowly oozes water out onto the capillary matting.
- The capillary matting, a synthetic absorbent material, soaks up water like a sponge.
- The moist potting soil in the plant’s pot wicks up water from the damp capillary matting.
Now, some basic science. A property of water it that it sticks to itself – this property is called cohesion. It’s easy to understand when you consider that if you put two drops of water next to each other and push them together, they become one big drop!
Similarly to the way that small drops of water join up to make a bigger one, what happens here is that the water in the potting mix connects to the water in the capillary mat which connects to the water in the terracotta pot, and all three objects act like one big wick, drawing an even amount of water across themselves, and maintaining it as long as there’s water in the terracotta pot to draw from.
So, when the plant draws up the water in the potting mix or it dries out from evaporation, water will wick back into it to restore the moisture lost, so our plant gets its water supply and stays a happy plant, which is the way we prefer to keep them!
1. The first thing you will need is a plastic tray with drainage holes in it to hold the whole thing together.
Pictured below is a seed punnet tray that is used commercially in nurseries. Any plastic tray with drainage will work.
Plastic seed punnet tray
2. The next important piece is the capillary matting. It’s a grey synthetic fabric about 5mm thick that doesn’t rot from constant moisture. You can probably find it at places that sell greenhouse or hydroponic supplies.
I’m guessing you can most likely use any absorbent material, maybe even newspaper (which will eventually degrade, but who cares, its recyclable), but I haven’t tried it yet, so if you’re keen to experiment, give it a go and send me some feedback!
Capillary matting, an absorbent synthetic, rot proof material
3. The third component required, a small terracotta pot, the one used here is a 5″ (13cm) wide pot.
A small terracotta pot, size is not too critical in this application, it just has to hold enough water!
4. Seal the drain hole in the terracotta pot with silicone sealant (or by any other means) so it holds water and does not leak, and allow it to harden.
Terracotta pot with drainage hole sealed with silicone sealant.
5. Cut the capillary matting to size so that it fits neatly into the bottom of the plastic tray.
Capillary mating cut to size to fit in tray
6. When the silicone sealant has dried properly, you’re ready to set the watering treay up. Place the terracotta pot in the centre of the tray. This ensures an even level of moisture all around the tray.
Best to move this setup to its final location at this point, as it may get too heavy or delicate to move, once the water and plants are put in. A sheletered location with part shade or dappled sunlight is recommended as you don’t want your delicate plants exposed to full sun or wind. The water won’t last as long either!
Pot place in centre of tray
7. Place as many plants as you want around the terracotta pot.
Japanese maple (Acer palmatum) seedlings in tube pots placed around watering pot
8. Fill the terracotta pot with water.
9. Water each of the plants so some excess water runs from the bottoms of the pots onto the capillary matting to establish the wicking into the potting soil.
10. All complete, a well watered collection of plants. Now, just top up the terracotta pot and it will do the rest.
If the terracotta pot accidentally runs dry, re-fill it with water and re-water the individual plant pots to re-establish the wicking.
Some Useful Hints & Tips
This can be scaled up in size too without any problems. this is another setup I have which is almost twice as large, approximately 1.5′ x 2′ in size, which has been in place for a few months. It’s located on the east side near a fence where it gets a bit of morning sun, and dappled or indirect light around noon.
This is where I put small seedlings and freshly rooted cuttings to keep them safe from drying out, and to establish them a bit further till they are ready to plant.
This system of watering, which is essentially what is termed “sub-irrigation” (watering plants from the bottom up), also allows seedlings to develop strong root systems, which make them much stronger when they are planted out in the garden later on.
Since the moisture wicks up from the bottom, and the plants roots will grow towards the source of moisture, the sedlings develop a deep, strong and extensive root system. Be careful because they will grow really well, and will set roots into the capillary matting!
You can add a sheet of root control matting over the capillary matting if you want to avoid the plants rooting into it, and the retailers that sell capillary matting also sell the root control mat .
Also, if you really want to give the seedlings and extra boost, you can add a nutrients or fertilizers to the water in the pot. In keeping it all organic, I’d recommend trying things like seaweed extract, worm casting liquid, or compost tea. You’ll be surprised at the accelerated growth rate due to the supply of constant moisture and nutrients!