Capillary Action Plant Waterer




Introduction: Capillary Action Plant Waterer

I wanted something that would provide moisture to the flats of seedlings in my greenhouse. Top-watering can be messy and often puts the young plants in a "feast or famine" situation. Watering from the bottom up is better. There are battery operated systems that will parcel out water from a reservoir to plants. Watering mats are also sold that provide water to the bottom of plants.

I wanted something that was inexpensive, relied on natural forces, easy to set up, and easy to use. All of the supplies used I had on hand or got from recycling bins. A side benefit of the system is that humidity is added to the air of the greenhouse.

Note: this will only work for pots or seedling trays that have flat bottoms with holes that will provide contact between the mat and the seedling's soil. Peat pots and the plant bags I use for tomato seedlings are ideal. The pots or trays need to be in a leak-proof tray that can hold the watering mat.

This can also be used indoors to water seedling flats.

Step 1: Tools and Materials

Tools: you will need scissors and a small (1/16 inch) drill or other hole maker.


  • seedlings in pots or trays with porous bottoms
  • a water-holding tray for the pots or trays
  • a separate water-holding tray bigger than your water reservoir (I used an old roasting pan)
  • water mat material: this should be non-woven and capable of absorbing water. I used some older synthetic chamois cloths I had on hand. These should be clean, but minor stains are okay.
  • a gallon water jug with a cap at least 1 inch in diameter (for ease of filling)

Step 2: Operating Principle: Capillary Action

Straightforwardly expressed in Wikipedia:

Capillary action (sometimes capillarity, capillary motion, or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied carbon fiber, or in a cell. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container wall act to lift the liquid.

The non-woven material of synthetic chamois and some camp towels dry by capillary action. They can also be used to conduct water from place to place. Conduction will occur as long as a source is providing water.

Step 3: Prepare Your Reservoir

The jug you use should be somewhat transparent to make checking of water level easy. I removed the outer label on mine. It should have a cap to limit evaporation and keep insects out. I used a gallon size for ease of handling.

Drill or pierce a single small (1/16 inch) hole in the bottom of the jug where it contacts the surface it is sitting on. You want the water to slowly feed out of the reservoir based on contact with your water mat. Setting the cap on the jug when it is filled cuts evaporation but its looseness provides enough air pressure for the slow feed to occur, If tightened, the flow will draw in the jug's sides slowly and then gradually stop.

Step 4: Prepare Your Water Mats and Feeder Strings

The reservoir should be situated in the center of the space occupied by the seedling trays. Cut 1 inch wide strips from your mat material long enough to reach from the reservoir mat to the mat under the seedlings; the feeder strip should fit one inch under each mat.

Cut a mat to fit inside the pan that will hold your reservoir jug. Install and dampen with water.

Cut mats to fit under your seedling pots or trays. Install (remove seedlings first) and dampen with water. Replace your seedlings.

Wet your feeder strings. Put one end of each 1 inch under the reservoir mat and one inch under the seedling mat (you can gently lift a seeding pot and mat together to insert the strip)..

Add water to your seedling trays to just above the mat so that your seedlings get a good preliminary watering. The system is best for maintenance watering.

Step 5: Place and Fill Your Reservoir Jug

Your reservoir pan doesn't have to be level (capillary action wets the entire mat) but I leveled mine for esthetics.

The flats being fed can be slightly higher or lower than the reservoir pan since they are being fed by capillary action rather than gravity -- handy for the multiple surface arrangement I have in my greenhouse.

When I initially filled my reservoir jug, I held it with one finger over the bottom drip hole under the rain barrel spigot. Realizing how cumbersome that was, I got another jug of the same size with no hole in it to fill and store water. Another advantage is that the cool rain barrel water warms in the greenhouse and is thus less shocking to tender seedlings.

Place the jug on the reservoir pan's mat. Nothing will ever get dripping wet, but everything will stay damp, including the seedling soil. I noticed that the seedlings really perked up and stayed that way over the first day of watering like this.

Be sure to leave the cap loose so that a little air can get in. The air pressure is needed to ensure water flow through the jug's bottom hole without overflowing the mat and its pan.

I'm watering 14 tomato seedlings, 50 kale seedlings, and 18 flower seedlings over two days with a jug this size. Just monitor your watering situation the first few days to see how frequently you will need to top up your reservoir jug.

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    4 years ago

    Can u use more than 1 jug?


    Reply 4 years ago

    If you do, I suggest breaking up the feed lines to two sets. Otherwise you are delivering twice the amount of water to the one set of feeds plus the plants they serve.