How to Build a Capillary Watering Tray for Raising Seedlings

self watering wicking tray for seedlings
Self Watering Tray_07

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.

diagram capillary watering tray
  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.

  • One of the properties of water is it that it sticks to itself, this property is referred to as cohesion. We’ve all seen how this works, when we put two drops of water next to each other, and they touch, they become one big drop.
  • Water also sticks to other materials, this is how it makes them wet, and this his property is referred to as 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, 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.

seedling punnet tray
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!

seedling punnet tray with capillary mat
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.

seedling tray, capillary mat and terracotta pot
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 drain hole filled with silicone sealer
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.

plastic seedling tray with fitted capillary mat
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.

seedling tray with capillary mat and terracotta pot
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.

filling water reservoir for capillary watering tray
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 on capillary watering tray
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.

pots on capillary watering tray

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
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.

58 Comments

  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. 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

    1. 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. 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 questionsand many otherswill 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.

    1. 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?

    1. 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 dont grow out of the container plus this minimizes water loss because there is less material expose to the air than with the mat. Im 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…

    1. Heather says:

      I would avoid natural fiber materials, they will rot quicker. The quickest capillary mat would be one of the reusable bags — walmart has them for 50c per. I would avoid red unless you’re growing tomatoes in this type system in larger containers – -the reflection of the red light waves is apparently helpful to tomato plant growth and production (I grow my tomatoes in the ground and haven’t been able to find red landscape fabric or permeable plastic.

      1. Sugarmags says:

        Reusable planting bags?

  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.

    1. 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

      1. 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

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

    1. Blackthorn says:

      Great tip, thanks!

  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?

    1. 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. 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!

    1. Angelo (admin) says:

      All plain (unglazed and unpainted) terracotta clay is porous, it weeps water slowly through its surface.

      1. thanks

  18. madmo says:

    I’m so happy to see that this is a current post. I am attempting using grow bags for tomatoes this year, as they don’t get enough sunlight in my garden.

    My plan was to put them in a tray and maintain a small amount of water and have the bag serve as the wicking material. Does this sound like it would work? I’m in California and don’t have to worry about rain in the summer. If this approach works, any ideas about an affordable large tray? I’ve been looking at boot trays, which still are not very cheap as I need three of them for six plants.

    If I want to use the terra cotta pot method, then I’d need wicking material as well, right? I assume I can use a larger pot if I want the water to last longer, right?

    Thanks so much!

    1. Angelo (admin) says:

      If the bags will be sitting in a tray of water, you’ll need to be careful not to waterlog the potting medium and rot the roots. Most plants need a wet-dry cycle, so you’ll need to let the water dry out for a while before you top up the water level if that’s what you intend to do.

      If you want to set up a large scale version of this project, yes, you can just use a larger terracotta pot, and you’ll need to use some kind of wicking material to sit the plants on.

      1. Heather says:

        I would NOT say you want a wet-dry with tomatoes. That’s how the skins split and you get diseases or ruined fruit. Even watering that’s why this, and the RGGS works (check out Larry Hall videos on YouTube and his FB page, Rain Gutter Grow Systems. Check out the construction materials section of your home centre, they may have some larger plastic trays, or a kiddie pool on sale will work too. (That’s the foundation of the first version of RGGS developed by Mr. Hall, it used rigid plastic kiddie pools with the non-woven reusable grocery bags placed inside, holes drilled about 2″ up from the bottom to ensure that the water doesn’t overwhelm the bagged plants, and you can add gravel to the water to be a bit higher than the level, to discourage mosquito growth and reproduction.

  19. Angelo (admin) says:

    Hi Heather, this system is for growing seedlings and cuttings to provide them with a controlled and steady amount of moisture, not for growing full sized fruiting tomato plants! What I mean by all plants (except aquatic plants) needing a wet-dry cycle, is that they go from damp to almost dry, the water moving through the soil carries air down into the roots, which all plants need. I don’t mean going from waterlogged to bone dry, both are bad for plants – too much water and roots suffocate, no water and roots die.

  20. Gerry Renting says:

    where can I buy a capillary mat Please advise Gerry

    1. Angelo (admin) says:

      Horticultural supplies stores and hydroponics shops are your best bet, it’s used for plant propagation to water pots from below.

  21. James says:

    Hey there!

    I was wondering, do you have a recommended method for larger plants, such as full sized fruiting tomato plants? I love the idea of constant water and nutrients, but I’m afraid of water-logging the plants, or not getting enough water to them. I’d be using 10-15 gallon pots most likely.

    What about for the fabric pots, like Smart Pots? I’m very interested in those but I’m not sure they could properly be used with this sort of method.

    Thanks for your time!

    1. Angelo (admin) says:

      For large plants such as tomatoes, you can purchase or build your own self watering pots, which have a water reservoir underneath, the water is wicked up into the soil above it to provide constant moisture. I’ll have to write an article on how to build those too!

    2. Angelo says:

      In UK hozelock do a plastic reservoir system that has spikes with cloth wicks that puncture the grow bag from underneath. The reservoir holds enough water for a week or more depending on the size of the tomato plants.

      1. Angelo (admin) says:

        Sounds like quite an innovative idea! I haven’t seen anything like that here in Australia, I’d do a product review and test it out if they were willing to send me a sample 🙂

  22. I’d like some guidance about how to use the fabric pots. I tried following the Larry Hall videos, but I didn’t see any watering instructions. My plants took off initially, but I ended up drowning them. It’s interesting that an over-watered tomato plant looks the same as an under-watered plant.

    My setup is a tomato plant in a walmart bag in a plastic tub that’s a bit bigger than the bag.

  23. Ananda Munasinghe says:

    I am trying an inverted bottle of water as in a chicken drinker as a water dispenser.

    I believe this method will allow the right amount of water to keep the water level in the mat. The rate of water oozing out from the terracotta pot might be too slow. However practically it might be sufficient. I am also trying a plastic sheet placed in a one inch deep and ten foot long sand filled groove in the garden with the four edges of the plastic sheet folded up to hold water as a long capillary mat. The inverted bottle of water is retained to dispense the water required to keep the level of water in the groove. This arrangement can cater for a larger number of pots.

  24. Katherine Tan says:

    May I ask if mosquito larvae can form on the capillary matting? I’ve tried this method by using different kinds of material, since I couldn’t find capillary matting in our country. And I noticed that small pools of water form in some parts of the capillary mattings that I’ve used. Will this attract mosquitos? Thank you very much for your help.

    1. Angelo (admin) says:

      Water can’t pool in capillary matting, it’s porous and wicks the water very slowly!
      Try using old hessian sacks or newspaper for capillary matting material.

  25. jlegere says:

    could you tell me how well this will work in very hot dry conditions? full sun in temps over 100 and low relative humidity. this is a condition I found myself in last year and I lost most of my plants.

    1. Angelo (admin) says:

      Yes it will work in hot conditions, if the weather is really extreme, cover the plants with 50% shade cloth as well.

  26. jlegere says:

    Do you think that a layer of coco fiber would work as a capillary mat?

    1. Angelo (admin) says:

      If it absorbs and wicks water, it will work. Please feel free to experiment and let us know how it goes!

  27. Mike says:

    Forgive me for being stupid but if you seal the hole in the terracotta pot, how does its water get out into the matting!?!?

    1. Angelo (admin) says:

      It’s a valid question – all unglazed terracotta is porous and if you make a container that holds water out of it, it weeps water slowly through the walls of the container.

  28. Vijay Kumar says:

    polyester bathroom door mats or scrubbing pads can be used as capillary materials

  29. Ramesh says:

    Hi how does water from the sealed pot be available to the capillary matting?

    1. Angelo (admin) says:

      Terracotta clay pots are porous and water will very slowly wick through them.

  30. mark butcher says:

    The medium to large clothes that retain a great deal of water (Used to dry off a vehicle after washing) make good wicking. In fact after reading this sites info I plan to experiment and see if a few clothes will make both the planters wicking in reverse; In past I just cut a two to three inch strip of it and placed it into the bottom a planter. There long enough to reach up the interior of the pot and into a water container such as a two liter bottle. Slit the upper two liter bottle from the treads straight down just a few inches. Pack in the towel with a tube to refill the two litter. Push together, duct tape it where it’s cut, and replace the cap. If the plant is already established just lightly push the cut cloth strip around the inner edges of the plants container. Tie one strip to the next if length is needed. Go to the beach and no worries if it’s dry times. Enough water will flow for even daily moist soil. ( I think this material would work for same or very similar set up as you show here)!!

    Thank you, Mark B.

  31. beatrix kiddo says:

    I just wrote a huge long thing and it got lost
    Tl:dr
    Would a capillary mat just be helpful in general, in most pots (flower or veg? You can always pick up a pot and put it in a tray. Would it cause root rot? I thought it might be a good application and thanks for this!

    1. Angelo (admin) says:

      It’s best used for small pots and punnets because the supply of water would probably be insufficient for larger plants and their roots would grow through the capillary matting and get stuck there.

  32. Marcia says:

    I’m setting up a self-watering system in a 2’x4′ lined cedar planter for our paved condo courtyard. The bottom is perf pipe packed in vermiculite or peatmoss, with a vertical filler tube and a small overflow tube exiting the planter from high inside the perf pipe. The plants, some fairly large, will be in Growbags sitting on top of the pipe/vermiculite combination. I know of similar systems that work well with potting soil directly on the pipe/vermiculite base, but none using Growbags.

    I’m wondering if I will need to add wicking directly into a slit in the growbags, or if they will themselves wick water sufficiently well into the potting soil inside the bags.

    I expect to put the system into several larger custom cedar planters as well so want to get it right!

  33. Heather Watson says:

    Have just tried a similar capillary tray out in 50 lt frosted containers as my seedling hot house. I didn’t use a mat. I used river sand with a small pot and upturned bottle. This has worked really well. If fact a lot better than I expected. I did have roots going down in to the river sand but easily came free and easy to transplant seedlings. The root didn’t become tangled in a mat. I also used recycled toilet rolls or paper seedling pots and neither got too wet. I will be using this method for self watering trays from now on.

    1. Angelo (admin) says:

      Hi Heather, thanks for your suggestion, sounds like an excellent technique, glad to hear it’s working well. Please be aware that sand drains fast, and needs a lot more moisture to wick effectively compared to capillary matting, and therefore may not be ideal with seedlings or plants that dislike high moisture.

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