I finally ended up getting begonias and dahlias from seed packs, but I had to buy a whole bunch of them before I finally got what I was looking for - expensive and frustrating.
I bought a bunch of rare packs (1k simoleons each) to try and get an orchid, and I've only ever gotten pomegranates or magic beans from rare seed packets. So then I tried by grafting and the plant would not sprout an orchid - I waited 3 sim weeks for an orchid, and it wouldn't happen so I finally wound up fishing it out of debug. Didn't want to cheat, but I felt I'd waited long enough / bought enough rare packs to justify it.