This is part of the "23 yus" series (pool #1738 and some others)
The plot as a whole is that both thanks to the humans that set up the "garden" and various real life experience, these yukkuri that would otherwise be literal garbage manage to make a relatively decent society.
We've already discussed all of this on Discord. xD Ringyou wanted to do this one with no dialogue for (apparently) their other language crowds (especially English)
Also, the gardens weren't there at first. The humans must have come back at some point and decided to make use of this lot. It was an open lot that they dropped a bunch of yukkuri into and wanted to see how well they'd survive. This is some kind of post-yukkuri armageddon where they basically stopped selling Reimu and Marisa types as pets.