ok for the sweet potatoe you did one of the treasure hunts, but im not sure which one it was so i'll do both:
1st treasure hunt: get friendly with tim,bob and your dog; after tim gives you the real treasure map take your dog to the villa anyday except from 11am-2pm and if your dog luvs you enough he will find the sacred land; plant the seeds next to the flower shop and that is one of them the other
2nd treasure hunt: do the same as the 1st but instead of taking your dog go to the villa by yourself and get friednly with dia and maria, some time dia will come to your farm with a bag of seeds she found and plant them next to the flower shop again and thats it.
Hope i helped