Bump, I'd like to know this too, I've been watching my Zen Garden for a bit, and none of them are spewing out coins, also none of them are indicating when they need water, fertilizer or they have bugs on them. They're just sitting there like idiots, with the snail moving in circles... >_>
Playing: CS 1.6, TF 2, HL2, Plants VS. Zombies http://www.youtube.com/watch?v=p4cRzC6w3YM
Seems the last 'growth' always needs music or the bug spray, at which point they occasionally spit out coins. I haven't seen any grow beyond that, I usually sell them at that point. Doesn't seem to be worth keeping them unless I'm missing something.
Each plant that I have seen has 3 sizes (if I remember right) and then 1 final feeding taking either bugspray or music that will make it happy for a day.
When happy it has a golden glow under it and will spit out a coin every roughly 30 seconds.
The most basic plants sell for 3k coins while ones you get as drops can sell for much more. The Puffshroom I have can be sold for 10k and the three headed peashooter for 8500. When my three headed one hit full size I got 2 diamonds.
One more day (another 3 plants) and I will be feeding my snail some chocolate to see how many coins I can get in a hour of it collecting for me.
About the question whether to sell or not I only sold mine when I was halfway to getting my mushroom garden. I may change my mind based on how much I can get with my snail collecting non stop for a hour.
BTW I found that while the plants from the other gardens cannot grow or be fed in the main garden they can be moved to it and keep producing coins once made happy.