Form:
	perennial vines, grow up to 8 feet in length by late summer
Origin:
	Europe
How it got here:
	uncertain
How it spreads:
	large numbers of wind borne seeds
Threat:
	Black and Pale swallow-worts invade fields and field edges. Black swallow-wort also invades woodlands. Close relatives of milkweed, these species may interfere with monarch butterfly reproduction. The two species are difficult to tell apart except by their flowers.