Tuesday, May 14, 2013

Watering seedlings with a wicking mat

The great seedling experiment is yielding some interesting results, and today I thought I would share the outcome of the wicking mat watering technique.

I have found seedling to be very sensitive to watering, particularly a missed watering over summer. On really hot days even a daily water seems not enough, and if you are not there to provide it then it’s all over red rover. Seedlings don't have to be completely shrivelled to become useless either. My understanding is that thirst can cause them to close their stomata, the tiny holes in the leaves and stems used for breathing. Recovery can sometimes take weeks, or sometimes never, stunting the plants growth. So I wanted to find a low tech self-watering solution, and the one I have experimented with is a wicking mat.

The principle behind a wicking mat is similar to cleaning up a spill with a dish cloth. A porous mat that draws water up out of a reservoir is used to distribute water to the base of a set of seedling pots sitting on it. The soil in the pots then soaks the water up from the wet mat.

I made this one with a plastic tray for the water reservoir, a couple of bits of wood to hold the mat and pots above the water level, and for the mat I just used a dish cloth. I sat the bits of wood in the tray, laid the dish cloth over the wood with enough overhang to touch the bottom of the tray, and sat the pots on top. I also drilled a couple of holes in the sides of the tray to prevent it over filling with water during a rain.

Quite an ingenious little idea I thought, so simple it was bound to work, right?

For the first few weeks I watched in great excitement as not only did the wicking work, but the seeds germinated and their growth was ahead of the rest. This was it! - the ultimate self-watering system had been found. I was only going to have to water every 5 days, and the seedling were never going to have to be put in the bathtub again on weekends away.

Wicking Mat
Watering Can
But as the old saying goes "Don't count your chickens before they hatch". Sadly as the weeks went on their growth seemed to come to stop and the other, non-wicking mat seedlings, raced on past. And the beetroot seedlings have even now died. What was interesting is that the wicking was still working - the soil was still moist. I suspect however, too moist. My understanding is that plants also respire through their roots, drawing oxygen from the air spaces within the soil. Over watering can thus cause the plant to drown, which is what I think has happened here.

Based on the results of the experiment thus far it looks like the good old fashioned watering can is still the winning method. But probably due to a desire to see the wicking mat method work, the question "did I do something wrong, or is there another way?" keeps rattling around in my head.


  1. If you take some of those seedlings out I think you will find the roots have rotted off. I found this was happening to seedlings that I had left in basins (cat litter trays) of water when we have gone away...well mainly the ones I forgot to empty out after we came back.
    You would need to let them dry out regularly perhaps each weekend.
    This still could be useful for leaving seedlings during hot weather if you needed to go away for a sort time instead of the bathtub.

    Oh well was a good experiment thanks for sharing your results.

    1. I have thought about letting them dry out, except I think it would be error prone. During hot days I might let them dry out too long, and would 2 days of drying out be the best or just 1, or depending on the weather 3? And if it rains for too many days in a row I would have to remove them from the mats. It's just too tricky.


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