Florida waterways, already infested with invasive snakes and fish, are now under siege by a plant foe, the Carolina willow, researchers say. The willow plants are starting to strangle portions of the St. Johns River, they said, forming impenetrable thickets that prevent boating and eliminating duck habitat. They also use tremendous amounts of water, leaving less available for wildlife and people. Scientists at the University of Central Florida in Orlando say the slender trees, once used by Native Americans for medicinal purposes, may be thriving because of water-management projects initiated in the 1950s. Canals built to control runoff and provide water for agriculture had an unintended consequence -- stable water levels -- that allowed Carolina willow to spread and thrive, they said. University biologists are helping scientists at the St. Johns River Water Management District develop new ways to reduce willow cover and slow down the expansion in the river, Florida\'s longest. \"It\'s important that these trees be controlled to maintain water quality and availability, conserve wildlife and continue enjoying recreational activities in the river,\" UCF Professor John Fauth said.