This plant appeared in my neighbourhood in Toronto a few years ago. It is one of three invasive species of swallowwort. At first, there was just one plant, a rather handsome plant. We didn’t know what it was and my neighbour let it grow. It produced seeds with fluffs like milkweed seeds from a small, pointed pod. But it’s a very tough vine that weighs down other plants, climbs on trees, and makes it almost impossible to walk through a field (hence its name). If it’s growing in your garden, pull it out! (In the picture: someone in the neighbourhood let it replace their sweet pea vines.)
This links to a picture of swallowwort in action and more pictures of swallowwort from the Ontario Wildflowers site. Here’s a description of a swallowwort invasion. You can visit swallowwort.com for more pictures and information about swallowwort. Parts of the Don Valley now look like the “Before” picture in the Control: Prevention and Reporting section.