The flower was not actually made of flames despite looking like it. It was hot though. Yet the heat did not really act as an obstacle to Lex at all, and not just because the universe was being lethargic.
If the flower followed the same principles as the Death Metal Rose, no one would be affected negatively by the rose until they touched it. That was also why Lex anticipated that the Dao radiation from the flower would be self contained until he actually touched it. The radiation that the shell was currently reacting to was from his surroundings.
Once Lex touched the flower, the heat from the extreme yang nature of the flower would incinerate him - or anyone else that touched it. Of course, in this case, the lethargy of the universe meant that the flower would be slow to incinerate him, or that when it began to do so it would be weaker than normal. That would give him sufficient time to absorb it into the necklace.
