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This year I am starting seedlings indoors in soil blocks for the first time. My first attempt is going well, but I can't find a good method for watering.

I have two 10in x 20in trays in a 22in x 22in grow tent. It's cozy in there.

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So far I have tried misting them daily and bottom watering. The misting delays how often I need to fully water, but it hasn't been enough by itself.

My trouble with bottom watering is how awkward the process is. I awkwardly remove the tray from the tent and fill it with water. 20 minutes later, I very awkwardly drain the tray through a combination of gentle tilting/pouring and a few minutes of bailing it out with a shot glass to get the rest. I then awkwardly return the tray to the tent.

There must be a better way.

I would like to water them easily, ideally without removing the trays from the tent. What are the practicable methods for a home gardener to bottom water soil blocks?

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    Totally non-ironic question: What about bottom-watering with significantly less water, so that there’s no need to pour/scoop out the excess? Am I missing something?
    – Stephie
    Mar 12 at 6:02
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    What would the downside be of letting a little water sit in the bottom of the trays? Presumably this will pick up over time - which is what I would have imagined you would want anyway?
    – davidgo
    Mar 12 at 20:17
  • @Stephie I think you hit the nail on the head. I had not seen the moisture make its way even halfway up the blocks after 90 minutes in shallow water and decided to fill the trays to the brim and then drain. After your comment, I took a single block out and put it in a ramekin of shallow water and recorded a time lapse. The water did make it to the top over 7 hours.
    – MackM
    Mar 13 at 17:36
  • @davidgo I was avoiding standing water because I was concerned about fungus and insect pests, but after your and Stephie's comments and my experiment, I went ahead and put about 2cm of water in the trays. 4 hours later they were still standing in it, and I went to bed fretting. By this morning the trays were completely dry! I just had a commitment problem.
    – MackM
    Mar 13 at 17:40

2 Answers 2

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Let me see if I can think though a way to make capiliary matting (which is the default answer to bottom watering a bunch of plants with minimal effort) with the setup up you have.

Find somehting to serve as an elevated bottom in your trays (old cracked trays that leak would work, or something roughly the right size that's water-resistant. Elevate that above the bottom of the water-holding tray by 3/4" or 2cm or so. Cut it a bit narrower than your bottom tray is wide.

Cut capilliary matting as long as, but wider than, your tray, so that it can sit on the elevated bottom and hang down into the water-holding tray on the bottom.

Put your plants on top of the matting. Maintain the water level below the elevated bottom.

If you prefer, you can cut the elevated bottom shorter, rather than narrower, and hang the matting off the end, rather than over the side.

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Water wicking or tray dredging may be a way to go. I have done some agricultural greenhouse work with seedlings in a combination of pots and trays, and oftentimes we will layer a shallow tray of seedlings on top of a deeper, empty tray. The shallow trays have drainage slits. We would water once or twice a week, and the seedlings seemed to manage well. I can make and upload a diagram if that would be helpful.

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  • Thank you, I think I understand. I already have my trays doubled up because they are so flimsy, so this could be an option for me. Do you drain the bottom trays? I'm fighting gnats and fungus in the grow tent, which is my hesitation to leave water in there at all.
    – MackM
    Mar 15 at 16:51
  • We typically don't drain the bottom trays, simply because it's absorbed or evaporated pretty quickly for the crop/conditions we use. However, draining on a schedule might work really well for you!
    – OllieVet
    Mar 21 at 15:06

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