I'd say it was the caring for the cute little orphans which was the out of character bit. She works better IMHO, as someone who really can't see the value of a human over that of a plant and treats the world accordingly. We chop up plants to feed to people, she does the reverse and can't see why people view that as any different.
It was "The Poison Tomorrow" which introduced the notion that Ivy is sterile, since her body breaks down all poisons and toxins, and would treat foreign genetic material, and certainly a pregnancy, as hostile and break them down. I still recall the spit take I did when they used the same notion in BTAS: House and Garden (Not going into so much detail, but certainly having her explain that she can't have children because of her "condition")