As you play the game, you will receive plants as random drops from zombies.
The store ONLY stocks marigolds and when you are collecting gold from your plants, the marigold drops gold far less often than plants earned from random drops. That said, make sure you buy your three marigolds each day to fill up your garden.
Make sure you have space for at least one plant left in your garden. If your garden is completely full, you won't be able to add plants that drop.
Delete marigolds BEFORE deleting special plants received from drops since dropped plants pay out gold at a faster rate than marigolds. Marigolds just make a nice filler until you get enough drops to make the big dough.