Once you’ve planted your seeds indoors, they need just the right amount of water to germinate and mature. Capillary mats, also called self watering trays, are a great watering option. Without a steady supply of water, the results can be disastrous. Give your pots too much or too little water and seeds can fail to germinate, plants can die, or they can fail to reach their full potential. For first-time gardeners, the tendency is to overwater and drown plants, or alternately, forget to water altogether.
Benefits of Seed Capillary Mats
Capillary mats are a surefire way of providing a steady, adequate supply of water to germinating seeds and seedlings. A capillary mat is a mat made of felt (or other absorbent fabric) upon which potted seedlings sit. The mat absorbs water from a reservoir and the pots are able to draw a steady supply of water from it, absorbing the exact amount they need, when they need it.
Essentially, it is a wicking process, whereby the mat wicks water and then the soil in the pot wicks the water from the mat. Capillary mats are traditionally used in greenhouses or with hydroponic systems, but they are now widely used by home gardeners as well.
Benefits of capillary mats are numerous. The obvious benefit is convenience — all you have to do is fill the water reservoir once or twice a week and the plants get exactly what they need, when they need it. According to a University of Arizona study, capillary mats also save on water — up to 71 percent less in summer and 62 percent less in winter — compared to hand watering. The researchers also found that some plant species using capillary mats grew more robustly, producing a bigger canopy. Using capillary mats tends to make plants grow deeper roots, because they have to reach to absorb water from down below.
How to Make a Self Watering Tray: Supplies Needed
You can purchase capillary mats from your local garden supply store or you can make one yourself, using a few on-hand supplies and a little bit of know-how.
Items Needed
- Plastic tray
- 1-inch-thick piece of Styrofoam
- Scissors or utility knife
- Wool blanket, sweater or scarf
- Nails or thumbtacks
- Pots or seed trays with drainage holes in the bottom
- Water
Building the Self Watering Tray
- Cut Styrofoam to fit inside plastic tray, with about 1 inch on all sides to spare.
- Cut wool to fit Styrofoam but leave the wool long enough on two ends to hang down under so it can absorb the water.
- Use nails or tacks to fasten the wool to the Styrofoam.
- Place wool-covered Styrofoam in the tray, wool side up.
- Fill the tray with water, making sure that the blanket is saturated without being totally submersed in the water. The wool will absorb the water as needed, provided it is hanging down on the ends and touching the water.
- Place pots and/or seed trays on top of wool-covered Styrofoam and let them absorb the proper amount of water.
Tips for Using Seed Capillary Mats
- You can use capillary mats effectively with peat pots, soil blocks or pots and trays with holes in the bottom. You can even use a variety of containers on top of one capillary mat.
- Monitor your plants to gauge how often you have to fill the tray. It could be anywhere from several days to one week between waterings.
- Make sure that at no point the plant pots are submerged in standing water, as this will cause mold and rot, and could weaken or kill the plant.
- We have recommended wool as matting material, but in fact any absorbent material will work, including cotton and felt purchased from your local fabric store. If you are growing plants for food, be sure to select a nontoxic material and avoid strong detergent or fabric softener residue.
- Always monitor for any signs of mold or bacteria, as this can cause growth problems or even kill your young plants.
Want to learn more making seed capillary mats and using self watering trays?
Capillary Mats for Maintenance of Plants in the Retail Nursery from University of Arizona
Guidelines for Watering Indoor Plants from Colorado State University
Great information! A couple of questions – what purpose does the Styrofoam serve?
Could you use coconut choir mats in place of the wool?
Thanks!
The styrofoam floats on the water to hold trays above the water so as to come into contact with only the water wicked to the wool or felt.
your instructions said to use a tray with drain holes. put styrofoam inside tray and fill with water.how is the tray going to hold water if it has drain holes in it ??
The holes are in a tray of seedlings that are laid on the wet mat. Wet mat goes on a platform of 1″ thick styrofoam.
A very interesting post – thanks!
I have been experimenting with DIY capillary trays – 10″ x 20″ plastic trays, and also 13″ x 9″ metal cake pans (from Walmart – sold in two’s, cost $3 and supplied with transparent dome lids), getting ready for the new season.
I have found that microfiber cloths, cut to size, or just separate pieces overlapping, also works really well for wicking across the plinth.
As I haven’t used this system for seed germination before, I’m wondering if a heat mat is practical to use under the main tray? Would heat transfer effectively through the water & ‘styrene to the pots?
thank you very much for sharing.
it would be very helpful to have some pictures in the instruction. I am very new in gardening and really interested in this method of watering.
This was genius, thanks! Could you use this method with 5″ biodegradable fabric grow pots?
You said not to use toxic fabric for the capillary mat. I have some fabric felt leftover would that be toxic? If so, what types of fabric or key words should I use when looking? I am growing microgreens so fabric would be used for around 2 weeks then dried out and cycled back in or thrown away depending.
This is exactly what I’ve been trying to find, thank you!! My only concern … styrofoam contains toxins, won’t it leach these into the water/wicking mat …?
I have lavender seeds in peat pots can I put them directly on the capillary mat instead of putting them in the container which sits on the mat
I haven’t grown lavender, but some of my veggie and flower seeds are in peat pots, set directly on top of capillary mats. Seems to work just fine for me.
Do capillary mats grow mold? Do they have to be cleaned with bleach periodically? Does it matter which way the fibers in the capillary mats are spun? Which way is best, a Z twist or an S twist? Can you put soil on top of a capillary mat and grow vegetables in that soil? I would like to try it in a raised garden bed. At the bottom of the bed I would have a reservoir with cement blocks in it. Then I would drape the capillary cloth over the cement blocks and put my soil directly on top of the capillary cloth. I know it sounds rather far out, but there has to be some way to achieve wicking action in a raised garden bed.
Wicking beds are a unique and increasingly popular way to grow vegetables. They are self-contained raised beds with built-in reservoirs that supply water from the bottom up – changing how, and how much, you water your beds.
Google “Wicking garden raised beds” There is lots of information
I think styrofoam causes birth defects in animals. Love your ideas, but looking for food safe plastics…
How about a block of wood to cover with matt and set the seedling trays on?
If anyone has a non-styrofoam means of hanging the wool from in the wicking tray, please share. I don’t like the idea of chemicals leaching into the water from the styrofoam and then wicking up into my plant roots.
There are many things that could replace the Styrofoam. It depends on the size of your seedling cells. If you are using the entire 10×20 tray, you could use something like a plastic cutting board. Or a casserole dish, glass or the metal disposable type, old tv dinner or takeout containers, plastic food containers are often rectangular.
If it is a cutting board and it’s too low, prop it up with something you have laying around. Marbles, rocks, bubble wrap, old Tupperware, bottle caps, yogurt cups upside down etc. Make and even layer and put your cutting board on top. Then your wool or fabric on top. I would like to try burlap from coffee shops.
Or egg cartons, unfolded so they are flat. The biodegradable ones will wick water but may break down faster and allow mold/fungus growth. Plastic egg cartons would be preferred.
You just want it to be flat but also somewhat level. Use what you have on hand and what you feel comfortable with. I hope these ideas help someone ?
I just thought of something. How about a piece of natural, thin wood and placing some little legs on it so the water sits below and wool still hangs over the edge. I think I’ll try it unless anyone can think of something other than non treated wood.
If you have not received an answer to your question yet,I offer this:
My guess is that after a time, the wood will warp and or get moldy messing up your whole project.
Instead of styrofoam how about a bunch of pebbles or baby food jars with a slab of glass on top.
Like wood idea, but wood does rot and could grow molds. Most would be harmless, but a few molds can be bad news for you and/or your plants.