The hollow stem grows to a height of 60–170 cm (24–67 in), branching to umbels of small white flowers. Flowering time is mid spring to early summer.
The tripinnate leaves are 15–30 cm (5.9–11.8 in) long and have a triangular form. The leaflets are ovate and subdivided.
Cow parsley grows in sunny to semi-shaded locations in meadows and at the edges of hedgerows and woodland. It is a particularly common sight by the roadside. It is sufficiently common and fast-growing to be considered a nuisance weed in gardens. Cow parsley’s ability to grow rapidly through rhizomes and to produce large quantities of seeds in a single growing season has made it an invasive species in many areas of the United States. Vermont has listed cow parsley on its “Watch List” of invasive species, while Massachusetts has banned the sale of the plant.[citation needed] It is classed as a Class B Noxious Weed in the State of Washington since 1989,[4] where its sale is also banned. In Iceland, cow parsley has been classified as an alien invasive species.[5]