About Bidens amplissima Greene
Bidens amplissima Greene resembles small sunflowers. Each flower head has many small disk florets at its center, with 6 to 11 yellow ray florets arranged around the outer margin. Its stems and inflorescences are often covered in hairs. The species produces wedge-shaped achenes that carry 2 to 4 backward-facing barbed awns. Its leaf margins are coarsely toothed; leaves are typically widest at the base and narrow toward the tip, and when a petiole is present, it often has winged margins. Bidens amplissima leaves vary considerably in shape. Most published descriptions of this species note that plants usually have at least some three-lobed, or tripartite, leaves, but unlobed or two-lobed individuals have been observed frequently. Authors have pointed out that B. amplissima individuals often lack the tripartite leaves that are commonly used to identify the species. Many botanists have misidentified these unlobed individuals as Bidens cernua, a common species that is widespread across most of North America and Europe. After this observation was made, researchers examined herbarium material to look for potential misidentified specimens. This examination uncovered 5 specimens from Washington State that were originally labeled as B. cernua, and were later confirmed to be B. amplissima. These specimens include historic collections from as far south as Seattle, and more recent collections from near Bellingham, Washington. Before this discovery, B. amplissima was thought to be endemic to southwestern British Columbia. Bidens amplissima has a restricted global range. Over 85% of all known populations are located in southwestern British Columbia, with the remaining populations found in northwestern Washington. It is uncertain how many previously documented populations are still currently extant. When populations were last systematically surveyed in 2012, 21 out of 59 previously documented populations in British Columbia were confirmed to be extirpated. Between 2022 and 2024, 21 extant populations of B. amplissima were confirmed, including 6 newly discovered populations that had not been previously recorded in British Columbia. Bidens amplissima is an annual wetland plant that grows across a notable range of environmental conditions. It can be found along the edges of freshwater ponds, and also grows in saline conditions in estuaries. The species is generally restricted to a narrow band of habitat around the margins of ponds, lakes, and streams. It often grows in sites where waterfowl are common, and has a clear preference for silty alluvial soils.
