Persona 5 - Plant died
1.) You can buy plant nutrients of varying qualities from flower stores. You have one plant store available in the Underground Mall, and you'll unlock at least a couple more as the game goes on.
2.) You gain kindness when you add the nutrients to your plant. The more expensive the nutrients, the bigger the kindness boost. All of the nutrients "support" your plant for equal amounts of time and the plant can never officially die, so you're best off always giving it the most expensive nutrients available as soon as possible.