ipomea as in sweet potato vine? that is what you have, an ornamental sweet potato tuber. if it is not rotten, try and pot it up again for this season.

wow -- that explains it, then! And no, it is not rotten. I am excited that I could re-pot it -- and not have to buy another one! (I threw some of the tubers out, but saved a few to show my husband)... Thank you!

yes, i had a few of those last year, too. it freaked me out to see the spuds in my window boxes. even stranger still, i brought one of the window boxes in for the winter. about a week afterwards, we saw a MOUSE in our house, which had (no doubt) been feeding on the tubers while outside, and then made the trip back into our house. eeeeeekkk!

In Florida, the tubers get so large they break containers! I used them the first year I moved here and was thrilled to see they quickly came back the next spring. Less thrilled when I tried to add a couple other plants to the pots because there was no room. By fall they had cracked the clay pot and deformed the plastic one. By the second spring, the plastic pot was ruined too! Where the vines rooted into the ground around the pots I had tubers too.
They are beautiful though. I like the look of the purple, green, and varigated vines together.

Yes, much as I like the Ipomeoa Margeurite vine, I am not going to replant it because it just takes up too much room in my containers. It was such a water-hog over the summer, too! I had to water it constantly it seemed, or everything wilted. It did look fabulous trailing over my steps, though.
This year, I am doing vigna caracalla (climbing shell plant) and passiflora vines in my containers.