It may have pretty purple flowers, but Eichhornia crassipes can be a green menace. Introduced to Africa from the neotropics, this invasive weed is choking Lake Victoria, the world’s second largest lake. Water hyacinth has carpeted vast stretches of the lake, fouling fishing nets and blocking harbors. Ari Daniel Shapiro teams with reporter in the field, Gastive Oyani, to speak with local fishermen and botanist Helida Oyieke. They learn how the lake and the lives of the people who depend on it are responding to this weedy challenge.