Milk Parsley is a plant in the family Apiaceae

Apiaceae

The Apiaceae , commonly known as carrot or parsley family, is a group of mostly aromatic plants with hollow stems. The family is large, with more than 3,700 species spread across 434 genera, it is the sixteenth largest family of flowering plants...

. Also known as Marsh Hog's Fennel.

It resides on wetland

Wetland

A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....

s, mires etc., shallow riverbanks, occasionally on ditches and other smaller water features. It's relatively shadow-tolerant. It requires seasonal submerging of the site to compete with other plants. It's the main foodplant for Old World Swallowtail.