Crown Vetch is a low, sprawling legume with ferny, pinnate leaves and rounded clusters of pink-and-white pea flowers that bloom from late spring through summer. It was planted deliberately and widely across North America from the 1950s through the 1990s — highway departments and mine-reclamation crews valued it for exactly the traits that now make it a problem: a deep, spreading rhizome network and nitrogen-fixing roots that stabilize bare slopes fast and improve poor soil as they go.
That same root system is why it's so hard to be rid of. Mowing alone doesn't touch it — cut stems simply resprout from the rhizomes — and a single planting can smother an entire embankment or field edge into a dense, single-species mat within a few years, crowding out everything else. The most effective control combines repeated cutting through the growing season to exhaust the roots with a systemic herbicide (glyphosate or triclopyr) applied to regrowth in late summer or fall, when the plant is drawing reserves down into its roots; expect the effort to take more than one season.
The foliage contains nitro-compounds that are toxic to horses and other non-ruminant grazers in quantity, causing weakness and depression — a real concern if it's growing in or near a pasture, though it poses little risk to cats and dogs, which rarely eat it.