Hello moussy,
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I have never used a capillary mat. I start my seedlings in 3-inch square pots and, except for an initial top watering, I water from below by adding water to the trays the pots are in. The water enters the medium (I use ProMix BX) from the holes in the bottom of the pot and capillary action carries it upward into the medium. If I watered from above, I would risk washing the seeds too deep into the medium. And it is handy to just pour some water into the tray the pots are sitting in every two or three days.
ZM
OK here's what I do - it is not really what capillary mats are intended to do but...I start most of my seedlings in soil "blocks" - no pots - on top of a section of capillary mat which is cut to the size of my seed tray. (the trays are standard 12x24-type trays). The trays are ridged so that the soil sits a little up off the lowest part of the tray.
Then I water into the tray enough to moisten the capillary mat, and the soil blocks take up the water via the mat. I find that I can put a good amount of water into the trays (lots of it sits in the troughs as reserve), maintain a good level of moisture without having the blocks sitting in water in the beginning. This works well for small-ish seeds - I put the bigger ones (squash, melons) into 3- or 4-inch pots and they are less fussy about moisture levels in my experience.
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I've never used a capillary mat, I don't see a need to. I operate similar to Zen-Man -- I have the seedlings in small pots in leak-proof trays, and pour water into the trays. I wait a little while until the seedling pots have absorbed all the water, and then drain the excess from the tray.
IME there is no need to water germinating seeds as long as the germinating medium is adequately and thoroughly moistened prior to sowing and the germinating trays are covered with plastic. I use old food service clear clamshells and other clear-lidded containers of that ilk. As soon as the majority of seeds sprout I uncover. Keeping an eye on watering at this stage is critical -- they dry they die; when the seedlings put on more growth they're more tolerant to short bouts of soil on the drier side.
ETA: To clarify, I generally germinate in trays filled with bulk media or Jiffy pots, then transplant to individual pots once they have true leaves.
This business of applying water to germinating seeds is unfamiliar to me. I germinate in 3-inch pots. When the seeds are planted, in pre-moistened soil, I briefly spray the top of the soil with water, to avoid disturbing the seeds, and then throw a plastic bag over each pot. I think mxk3 made that good point. That locks the water in. Nothing ever dries out. No over-or under-watering. If it takes a few weeks for germination, no problem. I do absolutely nothing to those pots until the seeds germinate, and roots are descending in the soil. Then I remove the plastic bags and water from the top. That's the low-maintenance way to germinate seeds in pots. Works every time. Wish I could do that when I plant out seeds in the soil bed. As in, throw a plastic bag over my whole garden bed!
I should say that if the soil is appropriately friable, there is no such thing as over-watering. The soil should drain properly no matter how much water you apply.
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