The genus Equisetum (horsetails) may be the oldest plant genus on the planet. This means that close relatives of these plants were around when dinosaurs walked the earth. All members of this genus lack green leaves, and instead their stems are green and are the organs that do photosynthesis. The species you see here is the common horsetail, Equisetum arvense. It is related to ferns, so has no flowers or seeds, but rather reproduces by spores that need free water in order to complete the plant life cycle. It would be nice to help this plant along by cutting back the vines and mugwort around it!
Horsetails tend to be found in wet areas. The ones here clearly are growing on a part of the swale where the drainage is poor so it stays wet all the time.
We probably also have a larger and coarser species, Equisetum hyemale, in wet areas around the trail--keep an eye out for it! The picture to the right shows an example of this species.