Description: This native annual plant is
½–3' tall, branching sparingly. The central stem is glabrous and
occasionally angular. The opposite leaves are up to 5" long and 1" across,
although usually smaller in size. They are linear-oblong to lanceolate-ovate,
serrated along the margins, and hairless. These leaves are never
lobed nor
pinnately compound. At the base, the pairs of opposite leaves clasp the stem
and nearly surround it (i.e., they're connate), or they are sessile. The upper
stems terminate in flowerheads about ½–1½" across. These
flowerheads have a tendency to nod downward with age and the central head of
disk florets becomes larger and more rounded. Each disk floret is yellow with 5
tiny lobes. There are about 8 ray florets surrounding the disk florets, but
sometimes the flowerheads are without ray florets. The ray florets are variable
in length, depending on the local ecotype. At the base of each flowerhead,
there are both inner and outer bracts. The inner bracts are pale yellow,
membranous along the margins, and rather broad, tapering to blunt tips. The
outer bracts are green and oblong-linear. These latter bracts are about as long
or longer than the ray florets, but they have a tendency to curl backward with
age. There are about 6 outer bracts per flowerhead. The blooming period occurs
from late summer to early fall and lasts about 1-2 months for a colony of
plants. Each achene is oblong, although broader and somewhat truncated at its
apex, where there are usually 4 barbed awns. The root system is shallow and
branches frequently. This plant often forms colonies and spreads by reseeding
itself; sometimes the lower portion of a stem will form rootlets at the leaf
nodes when it lies against moist soil. Nodding Bur-Marigold is rather variable
across its range.
Cultivation: The
preference is full or partial sun, wet conditions, and mucky soil. Occasional
flooding is readily tolerated, although this species is not an emergent
aquatic. Sometimes the leaves succumb to powdery mildew during the fall. This
plant is less tolerant of dry conditions than other Bidens
spp.
Range & Habitat:
Nodding Bur-Marigold is common in central and northern Illinois, but occasional
to absent in southern Illinois (see
Distribution
Map). Habitats include swamps, bogs, seeps, marshes, edges of rivers and
ponds, soggy meadows in floodplain areas, and ditches along roads and
railroads. Nodding Bur-Marigold is often found in degraded wetlands, although
it also occurs in higher quality wetlands.
Faunal Associations: Plants with showier flowers
attract an abundance of bees, wasps, butterflies, skippers, moths, and various
kinds of flies. Bee visitors include honey bees, bumblebees, Miner bees, large
Leaf-Cutting bees, and Plasterer bees. These insects suck nectar from the
flowers; bees also collect pollen. The caterpillars of the butterfly
Nathalis iole (Dainty Sulfur) feed on the foliage. The caterpillars of
several species of moths also feed on Bidens spp., including Epiblema
otiosana (Bidens Borer Moth), Cirrhophanus triangulifer (Goldenrod
Stowaway), and Platysenta mobilis (Mobile Groundling). To a limited
extent, the seeds of Bidens spp. are eaten by various kinds of birds,
including the Mallard Duck, Swamp Sparrow, Purple Finch, and Common Redpoll
(the latter during the winter). The foliage is occasionally eaten by the
Cottontail Rabbit. The seeds of Bidens spp. are notorious for their
ability to cling to the fur of animals or the clothing of humans, by which
means they are distributed far and wide.
Photographic Location: At the edge of a river in a
partially shaded area at Crystal Lake Park in Urbana, Illinois. The leaf in the
lower photograph is broader than average.
Comments: The attractiveness of the flowers is
variable, depending upon the length of the ray florets; these florets may or
may not be present. It is fairly easy to distinguish Nodding Bur-Marigold from
other Bidens spp. because of its undivided leaves and achenes with 4
awns (rarely fewer). Plants with flowerheads that are large and showy can
resemble Bidens laevis (Smooth Bur-Marigold), but this latter species
doesn't occur in Illinois. The ray florets of Smooth Bur-Marigold are at least
as long as the outer floral bracts and it usually has showier flowers. When the
flowerheads of Nodding Bur-Marigold lack ray florets, this species can be
confused with Bidens connata (Purple-Stemmed Beggar's Tick) and
Bidens comosa (Swamp Beggar's Tick). Nodding Bur-Marigold differs from
these latter species by its leaves, which are sessile or clasp and nearly
surround the stem. Purple-Stemmed Beggar's Tick and Swamp Beggar's Tick have
leaves with distinct petioles that are winged or unwinged. The flowerheads of
Nodding Bur-Marigold nod downward with age, while the flowerheads of the latter
two species usually remain more erect.