I made the mistake of planting two 25 cent seed packets of cosmos in my garden and promptly forgot about them. I neglected to stake them and they grew horizontally, about 4-5' long. When I cut them back after flowering, I had to use an AXE, some stems were 8 inches in diameter. I did not dig them up because they were labeled annuals. Please tell me these things aren't going to come back next year.
Certified GKH Gardening Expert
While most Cosmos are annuals, there are perennial varieties. However, unless yours dropped seeds, it is unlikely to return. If it does, you can promptly pull it up and discard it.