This is a common wetlands plant, which blooms from late summer into fall. It is erect, up to 40” tall, with opposite, clasping, lanceolate, and roughly toothed leaves. Flowers are all yellow, with leafy bracts at their base, and 6-8 rays. The combination of rayed flowers, simple leaves, and achenes (seeds) with all four prongs equal (the eponymous beggarticks), is diagnostic within this genus. The species epithet cernua means nodding, and refers to the tendency of the flowerheads to droop.
Hitchcock (2018) lists this plant as introduced from Eurasia, but every other source I could find (including 2 native plant sites and the USDA) lists it as native.And I can believe that is the prevailing opinion, since every introduced species I know of that takes over an area the way Bidens cernua does is listed as a noxious invasive.
It is a welcome site in late summer, lining the drying shorelines of wetlands, bringing vibrant color to a browning landscape. The flowers provide a much needed nectar source to late season pollinators, and there is usually a good diversity of species paying visits to the blooms.
There are several species of Lepidoptera from eastern North America which utilize this species as a larval host, but I can’t seem to find any records from our region. However Calligrapha californica (Tickseed Leaf Beetle), shown above and below, uses the plant as a larval host and adult food source.
Size- Large colonies, up to 40” tall
Habitat- Wetlands and areas which are vernally wet.
Range- Region wide except the most arid parts of the interior, and oddly absent from the Olympic Peninsula.
Blooms- Late summer into fall.