Right, so flowering plant at the end is what we're defining, and OLEANDER fits nicely. Now look at "SisteR EDNA ELOped" — hidden inside there, running backwards, is our answer, and since this is a down clue, "heading north" is the signal to read that chunk in reverse. Take R-E-D-N-A-E-L-O and flip it round, and you get OLEANDER sitting right there in the letters.