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How to Build a Capillary Watering Tray for Raising Seedlings

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A self-watering capillary tray is a wicking tray with its own water supply, which can keep 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 anything other plants that require constant moisture.

How Does a Capillary Watering Tray Work?

This capillary watering tray works by the principle of wicking. Water seeps through the terracotta pot, which is porous, and the water is wicked by capillary action through the absorbent capillary mat. Plant pots sitting on the capillary mat will wick water up from it to the plant roots.

  1. The terracotta pot slowly oozes water out onto the capillary matting.
  2. The capillary matting, a synthetic absorbent material, soaks up water like a sponge.
  3. The moist  potting  soil in the plant’s pot wicks up water from the damp capillary matting.

Here’s a simple explanation how capillary action works in a capillary watering tray.

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, 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!

Construction Details

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.

Plastic seedling punnet tray

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.

It can be purchased 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 yet, so if you’re keen to experiment, give it a go and send me some feedback!

Capillary matting is an absorbent synthetic, rot-proof material that wicks water very well

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.

A small terracotta pot, size is not too critical in this application, it just has to hold enough water!

Step 2. Seal the drain hole in the terracotta pot with silicone sealant.

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.

Terracotta pot with drainage hole sealed with silicone sealer

Step 3. Cut capillary mat to size

The capillary mat needs to be cut so that it fits neatly into the bottom of the plastic tray.

Capillary mat cut to size to fit in plastic seedling tray

Step 4. Sit terracotta pot water reservoir in the centre of the capillary tray

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 5. Relocate pot and tray to a protected location

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.

Pot place in centre of tray for even watering of pots

Step 6. Place pots of plants in tray and fill terracotta pot with water

Place pots of plants around the terracotta pot water reservoir, evenly spaced so they all get a good supply of water.

Next, fill the terracotta pot with water.

Fill the terracotta pot water reservoir with water. In this picture, Japanese maple (Acer palmatum) seedlings in tube pots are placed on the capillary watering tray.

Step 9. Water the plant pots to start them wicking

Water the plant pots until excess water begins to trickle from the bottom of the pots, and onto the capillary mat to establish the wicking into the potting mix.

Watering pots sitting on the capillary watering tray

Step 10. Top up water level in terracotta pot as needed

Maintain the water level in the terracotta pot to ensure that the plants don’t run out of water.

If the terracotta pot accidentally runs dry, re-fill it with water and 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 in size to create a larger setup if needed.

Pictured below is another capillary watering tray I’ve constructed, that is almost twice as large, approximately 45cm x 60cm (1.5′ x 2′) in size, 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 to be replanted.

Larger capillary watering tray with plants

This system of watering, where plants are watered from the bottom up, is known as sub-irrigation.

Since the moisture wicks up from the bottom, and the plants roots will grow towards the source of moisture, sub-irrigation allows seedlings to develop strong root systems, which make 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 set roots directly into the capillary matting! 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.

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