Gladys Chappel has waited for more than 50 years for the answer to one question: What is that plant growing outside her back door? She got the seeds from her grandfather and planted them in 1950, never knowing what the 5-foot-tall plant with purple flowers and sticky seed pods was. Last week I posed that question to readers and was overwhelmed with responses.

Some readers guessed purple hyacinth bean or sweet pea, but horticulturists and botanists identified the genus as desmodium. They couldn't pinpoint the species. Experts don't like to identify plants using pictures.

After much debate and scouring the Internet, the best guess is Desmodium viridiflorum.

The genus is often called tick-trefoil, tick clover or beggar lice, and there are dozens of species. It's tough to tell one from the other. Many are grown as cover crops; they take nitrogen from the air and make it available to the roots of plants. Some plants contain allelopathic compounds that kill weeds. Desmodiums are considered weeds in some parts of the country, but don't tell that to Mrs. Chappel.

"I'm so excited that it's finally identified," she said from her Upper St. Clair home.

After all these years of trying to figure out what this living family heirloom was, she had one message for everyone that helped with the identification: "I feel good about that. I'm so thankful."

Doug Oster writes a blog, "Growing With Doug," exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.