Hmm, did you manage getting all four notes? And trusting the ghost before heading to the garden?
That must be it, I can't find the 4th one
I see then, you need to plant the petunias back and when you're asked whether you'd try or get help, get help and then head to the library for the Idiot's Guide to Gardening then head back to the Garden. Once you finished planting the petunias, the fourth note will be there :D