What is the point of a Plants VS Zombies Zen Garden?

In my zen garden ive got heaps of those marigolds and i won a couple of plants which are in there as well ( a doom shroom and a watermelon flinger thing) but they dont grow and they dont give me money so i dont see the point ... also the marigolds in crazy daves shop are allll sold out so i cant buy any more! help anyone?