Self Watering Tray

Self Watering Tray_07

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.

Watering Tray Schematic2

  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.

 

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!

   

Construction Details

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.

Self Watering Tray_01

 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!

Self Watering Tray_02

 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.

Self Watering Tray_03

 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.

Self Watering Tray_04

 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.

Self Watering Tray_05

 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!

Self Watering Tray_06

Pot place in centre of tray

 

 7. Place as many plants as you want around the terracotta pot. 

Self Watering Tray_07

 Japanese maple (Acer palmatum) seedlings in tube pots placed around watering pot

 

8. Fill the terracotta pot with water.

Self Watering Tray_08

 

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.

Self Watering Tray_09

 

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.

Self Watering Tray_10

 

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.

Self Watering Tray_11

 

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!
 
 
 
 

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27 Responses to “Self Watering Tray”

  1. Blackthorn Says:

    Hi Jay,

    It will be bit like leaving a damp sponge sitting on the spot where the tray is located. Even though there is a small air gap, the humidity is high below the tray and you do get a lot of moisture building up. There’s a simple solution if you want to protect the surface below. The seed punnet trays are also sold as the bases for small plastic propagators (with a clear plastic lid to grow seedlings in), and you can buy separate plastic drip trays that fit these. Put one underneath, and it doesn’t matter if you accidentally spill any water when filling the water reservoir pot. Hope this helps!

  2. frazzledsugarplummum Says:

    At the moment I am filling the tray the pots are in but I like the idea of using the terracotta pot as a reservoir. Now to track down some capillary matting. Thanks for this post. Very helpful and interesting.

  3. Sarah Adamson Says:

    Hi! Thank you for sharing this technique. So… The center pot is unglazed, non-draining terra cotta. Okay… Got that. Are the little outer black pots in your photo made of plastic? Do they have drainage holes? Could they be drainage-holed terra cotta? One other thing… Is the tray really drainage-holed too? I’m sorry, but that just doesn’t make sense to me. I’d think that either we’d want the water to hang out in the tray until it can be soaked up, or we’d expect there to be no excess water because it’s all contained in the absorbo mat, thus negating the need for drainage holes.
    Looks like I got a little carried away here. Sorry about that.
    I’m thinking about starting a collection of mini or micro mini african violets. They grow best with sub whatever watering, as they don’t like their leaves to get wet.
    Thanks again for sharing.
    Rock on with your bad self!
    -Sarah

    • Blackthorn Says:

      Hi Sarah,

      Great questions!

      The pots I’ve placed in the self-watering tray are regular, off-the-shelf small black plastic pots with holes underneath. You can place any pots in the tray as long as the water can wick upwards. I use all manner of pots in these trays. Even pots with side holes work as some of the soil spills out and creates a bridge for the water to wick into the pots. Either way, the plants roots will find their way to wet capillary matting and take hold! You can use another horticultural fabric, root control matting, over the top of the capillary matting if you want to limit this. You can use any pots that have drainage holes for the plants you intend to water, and teracotta ones would work great.

      The tray underneath has drainage for a simple reason, my trays live outside, out in the open. When it rains, the capillary matting stays wet and maintains the same level of moisture, the terracotta pots fill up with fresh water, but the plants don’t end up swimming knee deep in a tray of water.

      Must say though, brilliant idea to use it as a sub-irrigation system for african violets. Obviously, african violets live indoors, so you wouldn’t want the moisture from the capillary matting seeping down though a tray with drainage, you’re abdsolutely correct, you would use a watertight tray to retain the water. I’ve never thought to use it as an indoor system, and I think it’s a great idea. I’ve got some african violets indoors, and it’s a tricky watering them without wetting the leaves, and since I do have trays that don’t have drainage holes in them, I’m going to give this a go! Thanks for a great idea that extends the flexibility of this design!

      Regards

  4. spiritofsarah Says:

    African violets are supposed to be super difficult to grow, right? Nope. I must preface this by admitting that the only AVs I’ve grown so far were from a florist… i.e. of no particular variety. I guess watering is the tricky part. You need to avoid the leaves, or they’ll spot. You can water from the top with a watering can with a long spout, or you can use a syringe or turkey baster. I find it really easy to water from the bottom with an “African Violet Pot.” It’s a two-part self watering ceramic pot. You guessed it: the inside pot is unglazed. I think another name for this style is Oyama (see Rob’s AVs: violetbarn.com). There’s enough water in a, say, 6″ (outside) pot to last a whole week… Perfect for the lazy gardener. What’s even better is that if I find I’ve forgotten the poor dear for a few weeks, and the flowers have all died and some of the leaves too, all I have to do is give it some water. Pow! It comes back to life. After she’s had some to get over it, all the flowers come back. It makes me feel like a super gardener. Next I’m going to try using an ordinary (holeless) terra cotta as the inside pot. Will it do as well as the ceramic? Will it wick enough? These questions—and many others—will be answered in the next episode of “Soap.”

  5. brian farrugia Says:

    good, very simple and sensible. I would cover the free standing water as mosquitos especially the Tiger mosquitos will have a field day.

    • Blackthorn Says:

      Hi Brian, I use a completely natural solution to stop the mosquitos – just drop in some duckweed. Within a week it creates a thick floating layer that slows evaporation, and totally blankets out the mosquitos. I discovered this by accident when I dropped some duckweed from my water garden into one of the pots without realising it!

  6. rg Says:

    Hi, I am just setting up a small greenhouse (from an old window) and I have been thinking about how I can propagate seeds using a wicking method. I came across this post and am inspired to give it a try. First I just wanted to hear if anyone has had successful germination from seeds this way? Or any thoughts on starting off with seeds? Was thinking it might be possible putting a heating mat under it too for cold nights. Any thoughts?

    • Blackthorn Says:

      The setup you’re describing is what commercial production nurseries use to grow seedlings, a greenhouse with capillary mat sub-irrigation and bottom heat.

  7. Krr Says:

    I would use paper towels… Or even better old worn out shirt to stuff into the bottom of the water reservoir (any fibrous material works because of capillary action: the adhesion to other materials… Not just water itself:) once the shirt is corked into the water reservoir, pieces of material can be corked into plant containers extending up to the center of the container thus allowing all other hole to be covered so roots don’t grow out of the container plus this minimizes water loss because there is less material expose to the air than with the mat. I’m using a 2 liter soda bottle with hole just big enough for material as my reservoir. If you decide to go with a tc. pot you can also turn it into a fountain by attaching vinyl tubing to a small submersible pump placed under a smaller tc pot turned upside down through the smaller pots drain hole and add enough medium size stones to cover small pot but not fill the big one…. Now you have a Zen garden in a tray…

  8. dannywobble Says:

    Hi Blackthorn is it?

    Thank you for the helpful article, really cleared up a few things for me. Although I do have one question. How does the water get from the reservoir pot to the spongey material if the access/drainage hole to it is blocked by sealant or something else? I get the feeling I’m missing something incredibly basic here but could you clarify anyway please?

    Thanks Danny.

    • Blackthorn Says:

      Hi Danny,
      The missing bit you mention is simple fact that terracotta is actually porous, it will hold water, but it slowly seeps through. You must use plain teracotta, if the pot is glazed it wont work. That’s why a lot of earthenware is used in the kitchen is glazed, so the water doesn’t weep through. Hope that helps.

      Regards

      • dannywobble Says:

        Hi again Blackthorn,

        I was just wondering, if this method was used for germination and seedlings. Would the constant wet/damp conditions cause damping off, root rot, or rotted seed (I’m not sure which one applies but I think there all genuine problems)? Can you tell I’m a novice!

        Thanks again for the article and pics with it(invaluable in an article such as this)

        Danny

  9. dannywobble Says:

    Ahh thank you very much for the reply, I was completely stumped there. I had no idea terracotta was porous, again thank you for the reply and the very informative article.

    Danny.

  10. Mary Says:

    Hi,
    First great informative article. I am thinking about setting up a small greenhouse for a project for my sunday school kids age 8 and 9. Since we would only be in the class once a week I was worried about watering throughout the week. So this helped me a lot. Do you have any suggestions on easy flowers and plants to grow?

    Thanks,
    Mary

    • Blackthorn Says:

      Thanks Mary, I’m assuming you’ll be growing plants in fairly small pots, and not being sure of your climate, even in a greenhouse, a few easy to grow plants that you might like to try are things like strawberries, peppermint, spearmint, lettuce, spinach, thyme, chives,lamb’s ears, violas, violets, small daisies. Succulents are also very hardy and would be survive well with only weekly watering.

  11. Mary Says:

    Thanks for the info I think well use the violas and daisies.

  12. finchfamily4 Says:

    I’m going to try using fabric grow bags with this technique

  13. Sharon Marambe-Quye Says:

    I use a old towel as my capillary mat and it works great. Any cotton will do.

  14. Bede Says:

    Thanks for this. I had previously tried sealed bottom trays & tubs with an inch of sand in the bottom that I bedded my pots into and from there I watered the sand. Growth was fine and I never let the sand dry out though it was a tad heavy on my green house shelves, got my Melbourne late winter seedlings off to a great start.

    This year I repeated it without the sand (had none at the time) and left about 10mm of water free in the bottom of the tubs, no root rot issues and the tomato and sunflower roots grew into the water and were very happy, as are my grape seeds/seedlings, basil, & celery.

    Next I’ll try a hybrid of your unsealed pot idea with an inch or so of sand.

    Cheers,

    Bede.

  15. waterstar Says:

    What materials are good for the capillary mat?

    • Angelo (admin) Says:

      Anything that wicks water will do. If you want it to last a while, something that won’t rot too quickly as well.

  16. waterstar Says:

    Are there cheaper reservoirs I could use than terracotta pots? I don’t have any,,,

  17. Andrew Hughes Says:

    This might sound like a dumb question but how does the water drain from pot to matting? Trying to setup this up before I go out of town for my seedlings. Thanks for any info!

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