A self-watering capillary tray is a wicking tray with its own water supply that keeps seedlings in pots watered. All you have to do is occasionally top up the main water reservoir, and that’s all!
They’re great for watering seedlings, freshly rooted cuttings, and other plants that require constant moisture.
How Does a Capillary Watering Tray Work?
This capillary watering tray works on the principle of wicking. Water seeps through the porous terracotta pot and is wicked by capillary action through the absorbent capillary mat. Plant pots sitting on the capillary mat will wick water up from the mat to the plant roots.
- The terracotta pot slowly oozes water 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 water from the damp capillary matting.
Here’s a simple explanation of how capillary action works in a capillary watering tray.
- One of the properties of water is that it sticks to itself. This property is called cohesion. We’ve all seen how this works when we put two drops of water next to each other. When they touch, they merge into one big drop.
- Water also adheres to other materials, which causes them to become wet. This property is called adhesion.
In the same way that small drops of water can join up to make a bigger one, what happens in the capillary watering tray 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. 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 water from the potting mix, or the mix dries out from evaporation, water will wick back into it to restore the lost moisture, so our plant gets its water supply and stays a happy plant, which is the way we prefer to keep them!
Construction Details
Here’s how to build your own capillary watering tray, step by step.
Step 1. Gather the materials
The capillary watering tray is built around a plastic tray with drainage holes.
Pictured below is a plastic seedling punnet tray that is used commercially in nurseries. Any plastic tray with drainage holes will work.
Next, we’ll need a sheet of capillary matting. It’s a grey synthetic fabric about 5mm thick that doesn’t rot when exposed to constant moisture.
Step 2. Seal the drain hole in the terracotta pot with silicone sealant
You can buy silicone sealant from stores that sell greenhouse or hydroponic supplies.
Note: It may be possible to use any absorbent material in place of the capillary mat, maybe even newspaper, which will eventually degrade but is easily replaced. I haven’t tried it yet, so if you’re keen to experiment, give it a go and send me some feedback!
Step 3. Cut capillary mat to size
Step 4. Sit sealed terracotta pot water reservoir in the centre of the capillary tray
The final component required is a small terracotta pot. The one used here is a 5″ (13cm) wide pot. Some silicone sealant will also be required.
Step 5. Relocate pot and tray to a protected location
The hole in the pot is sealed with silicone sealant (or any other means) so it can hold water, and does not leak. Allow the sealant to harden.
Step 6. Place pots of plants in tray and fill terracotta pot with water
The capillary mat needs to be cut so that it fits neatly into the bottom of the plastic tray.
Step 7. Water the plant pots to start them wicking
When the silicone sealant has dried properly, the capillary watering tray can be set up. Place the terracotta pot in the centre of the tray. This position ensures an even level of moisture right through the tray.
Step 8. Top up water level in terracotta pot as needed
Take the pot and tray to its final location, as it’s difficult to move it when it’s fully loaded with plants.
A sheltered location with part shade or dappled sunlight is recommended, as delicate plants don’t like being exposed to harsh sun or strong winds, and the water will dry out faster under such conditions.
Place pots of plants around the terracotta pot water reservoir, evenly spaced so they all receive a good supply of water.
Next, fill the terracotta pot with water.
Water the plant pots until excess water flows out from the bottom, reaching the capillary mat to start wicking into the soil.
Maintain the water level in the terracotta pot to ensure the plants don’t run out of water.
If the terracotta pot runs dry, refill it with water, then re-water the individual plant pots to re-establish the wicking.
Capillary Watering Tray Hints & Tips
This design for a capillary watering tray can be scaled up to create a larger setup if needed.
Pictured below is another capillary watering tray I’ve constructed, which is almost twice as large, approximately 45 cm x 60 cm (1.5′ x 2′), and has been set up for a few months. It has been located on the east side of the house, near a fence, where it gets some morning sun and dappled sun around midday.
I’ve been using this system to establish small seedlings and freshly rooted cuttings a bit further, until they’re ready for replanting.
This watering system, where plants are watered from the bottom up, is called sub-irrigation.
Since moisture wicks up from the bottom and the plant’s roots grow toward the moisture source, sub-irrigation helps seedlings develop strong root systems, making them much more resilient when they’re planted out in the garden later on.
Using this system, plants will grow really well… and may even root directly into the capillary mat! To prevent this, place a sheet of root control mat (such as Marix weed control mat) between the capillary mat and the pots. Greenhouse and hydroponic supply stores that sell capillary mat will also sell the root control mat.