PITTSBURGH—Ronna Scoratow has had a cat, poodles and parakeets, but no companion has proved more durable than her 42-year-old philodendron.

"This is my girl!" she announced as visitors arrived one recent morning to see the 7-foot-tall potted plant, known as a lacy tree philodendron, that dominates her living room. There is only one shadow in this love story: "This plant will definitely outlive me," said Ms. Scoratow, a landlady who is 63 years old and in good health. "After I pass, I don't want her...