Fanwort is currently found in the following waterbodies in New Hampshire: Arlington Mill Reservoir in North Salem, Big Island Pond in Derry, Phillips Pond in Sandown, Robinson Pond and Otternic Pond in Hudson, Lake Massabesic in Auburn, Mine Falls Pond and Nashua River in Nashua.
Fanwort is a submerged bright green aquatic plant. The leaves occur in opposite pairs on the stem, are finely dissected and fan-shaped. Floating, lily-like leaves are found on the water's surface during flower production in August or September. Flowers are small, white and emergent. This plant stands 2-12 feet tall in the water column.
Fanwort can be confused with the native Water Marigold but can easily be distinguished if viewed in cross-section (see below).
Source: Wisconsin Aquatic Invasive Species Early Detector Handbook