Flat pea arrives from Great Britain and Europe, ranging east to Siberia, and has naturalized across Canada and the northern United States where it colonizes disturbed ground, embankments, and waste areas with minimal help. The narrow green leaves, winged stems, and clusters of purple-pink flowers in early summer have a certain rough charm, but this plant earns its place primarily through utility: it fixes nitrogen, tolerates poor and acidic soils, withstands drought once established, and will scramble over and stabilize an eroding slope while other plants struggle to gain a foothold.

With support it can reach 7 feet; without it, it mounds to about 30 inches and spreads by rhizomes to form a mat. That spreading habit is the main thing to manage. It can be weedy and will outcompete more delicate plants if not contained. It is best planted with a companion grass such as tall fescue to provide ground cover during the two to three years it takes to establish. Seed must be pressed into soil to germinate; broadcast sowing does not work. The plant contains compounds toxic in quantity to both humans and livestock, so it belongs in practical landscape applications rather than kitchen gardens.