I noticed that apple and cherry will not allow me to graft. I suspect that might make pomegranate or something else. Additionally, the daisy + strawberry combination will not work. I had noticed at times that certain plants wouldn't let me graft others - these must be rares that are NOT in the world, yet the game is bugged and not allowing the graft to go through. It acts like it's successful in that the Sim does the animation, but the plant doesn't change and you still have the cuttings. There may be a few more combinations like this , but it's obvious I will not be getting a pomegranate through any other means than fishing. I do have a trash plant growing though, thanks guys.
Edit: Ok. Tulip and Chrysanthemum doesn't work either. I am thinking this combination might produce Orchid? Regardless, it's obvious we are not able to make new plants through grafting and have to find them in the wild for now. I will get around to making a table of perfect plants' values very soon and perhaps time how long it takes for the different types to grow (bush, flower, tree)