They look like little trash bags! I've used them for fertilizer, but I'm not sure that's very helpful. Come to think of it, my plants did start getting bug infestations with the option to spray for bugs after that plant arrived. The spray takes care of it, but I think maybe I will get rid of the trash plant, or at least move it away from the others...!
Karen
Plants naturally get bugs, with or without fertilizing with trash.
The trash plants by themselves actually are quite clean and tidy, oddly. The only thing that happens is that being near them is absolutely disgusting to the sim, 'very uncomfortable' emotion, and then after tending them, they rush off to empty the trash in the house. Heh. But the bugs in the garden seems to just happen about the time gardening got high enough to spray for bugs.
I've not used them for fertilizer yet, not even for themselves. The 'ground' under the plant seems to be replaced from the trash texture after the plant either matures or evolves, to normal dirt.