I've had this issue twice now where a Zyra plant has ended up on my bar. It takes a spot on the bar permanently after this happens.
The plant can't be sold (worth 0 gold)
The plant can be moved onto the board, but only in the first two rows. Once the round is over, it returns immediately to the bar.
Once a plant takes a spot on the board, that spot is lost for the rest of the game, nothing else can be moved into that spot on the bar. I tried to put the plant on the board, and then fill my bar with other characters, but I could not buy more than 8 (There are 9 spots on the bar for characters)
I'll try to capture a video of this if I can.