@kris I did kind of a hack of this for some of my indoor planters a couple years ago and got great results.
Your idea is smart for, like, new plantings. Great illustration, too!
My take on this same concept was to get a yogurt container and poke holes in the bottom of it. Then I transplanted a really "rooted" house plant into it, taking care to ensure the roots could actually grow through the punched holes. You can even pull some roots through if they're well established.
Then put that yogurt container inside a non-plastic container with lots of water, pea-gravel, 1/4" minus lava rock, and some organic material.
Opaque glass or ceramic works well for the non-plastic container, but I would recommend using something that light can't penetrate, since most roots prefer darker non-light exposed.
The dark container slows evaporation, and the roots really like being able to find water in little cracks and spaces in the gravel.
@indie sounds like a really effective setup! I also have a couple yogurt cups sitting here, I was just going to use them 'as-is' but they could definitely make small hydroponics!
I will consider how to black out the walls -- since it's potting soil in the top and water in the bottom I can't do it with filler material alone. Seems against the point to use spray paint... A construction paper insert around the resovoir wall could work
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