The best way I have found to do this, is like Pam said. I plant my plants, after I harvest them, I dispose of the old ones, then I take the fruit or veggies and plant them and then sell what is left in my inventory or put them somewhere else and then keep repeating the process until I have my perfect plants. I have done the perfect garden a few times and it is pretty neat, I don't think I have even used fertilizer that much and have not had any trouble getting perfect plants.