Ageratum conyzoides (tropic ageratum or billygoat weed) is a South American species that is now a common weed in nurseries and landscapes in the gulf coast states, and distributed as far north as Massachusetts. It is related to the cultivated garden ageratum, and may have served as a parent of the hybrids grown in gardens.  But this species seeds itself prolifically. Nursery crops grown in gulf coast states are often contaminated with this pretty invader.  Control this potentially invasive species before it has a chance to spread. She seed are short-lived (12 months or less).  So preventing seed production will stop the weed before it can get established.  Remove plants before they flower.  Research conducted at the University of Florida suggests that preemergence herbicides containing oxyfluorfen or flumioxazin have been effective.