Nodding Bur Marigold is a low erect annual plant (up to 3 feet high but usually much less) of wet places, branching occasionally, but usually only within the inflorescence. Stems can be glabrous or have short stiff hairs. The central stem is angled (grooved).
Leaves are long and lance shaped with pointed tips, hairless, opposite, stalkless, with coarse teeth. The leaves clasp the stem, and as they are opposite, often appear to surround the stem.
Flowers occur in loose stalked clusters branching from the upper leaf axils near the top of the stem. The flower head is 1/2 to 1 1/2 inches wide and nods after or during flowering. The stalked head has two types of flowers. The outer circumference consists of about 8 yellow ray florets with rays of variable length. These are neuter or sometimes have styles, but are sterile. Rays have a point or notch at the tips. Some heads may lack ray florets entirely. The ray florets surround 40 to 100+ central disc florets with tubular orange-yellow corollas that have a five-lipped throat. Each disc floret has 5 stamens with darker anthers that tightly surround a single style. These are bisexual and fertile. The outside of the flower head has two sets of floral bracts, an inner layer of phyllaries, 6 to 8+, that are a pale yellow color and thin as to be somewhat translucent. The outer bracts (calyculi) are dark green, 5 to 8+ in number, and as long or longer than the yellow rays. These spread and curl backward as the flower head matures.
The seeds (cypselae) are blackish-brown, somewhat flattened to angled, and have 2 to 4+ barbed awns which stick to clothing and animal fur - hence the word 'bur' in the common names. Thoreau refers to the Bidens seeds as “shaped somewhat like a little flattish brown quiver, with from two to six downwardly barbed arrows projecting from it.” See his comments below.
Habitat: Nodding Bur Marigold grows from a shallow root system in wet but not flooded mediums. Being an annual, if sufficient seed sets, a sustaining colony is formed, unless subsequent flooding occurs. Depending on the amount of sun and the growing conditions and location, there will be variability in the plants, particularly in the flower. The seeds can tolerate some standing water early in the growing season, but the area must be free of standing water by mid summer for the seeds to sprout, which if they do, even late in the season, flowers will be produced as the plant is a fast grower.
Names: The genus name, Bidens, refers to the 2 "teeth" or the bristles on the cypsela, of the original species. It is from the Latin bis meaning 'two' and dens, meaning 'tooth, i.e. two-toothed. The species name, cernua is also Latin for 'nodding' or 'drooping'. The author name for the plant classification - 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Above: Flowers appear in small clusters arising from the leaf axils. Note the long green bracts behind the flower head composed of about eight yellow ray florets and central disc florets. Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: Two rows of bracts form, the inner phyllaries are yellowish-green and thin; the outer bracts (known as 'calyculi') are green, much longer usually and reflex toward maturity.
Below: 1st photo - The yellow rays have a point or a notch at the tips. 2nd photo - The opposite leaves are sessile and appear to clasp the stem. 3rd photo - The stem is angled and can have stiff white hair as shown .
Below: The seeds of Nodding Bur Marigold are blackish-brown, somewhat flattened to angled, and have 2 to 4+ barbed awns which stick to clothing and animal fur.
Notes: Eloise Butler had catalogued this plant in her plant index as present in the Garden area. It has been listed on every Garden census. The plant is native to most of Minnesota except some counties in the south-central part of the state. Widely distributed across Canada and the U. S. absent only in Florida, Mississippi and South Carolina.
There are eight species of Bidens found in Minnesota, all named with 'Beggar-ticks' as part of the common name. Four species are found in the Garden: B. cernua, Nodding Beggar-ticks (Bur Marigold); B. connata, Swamp Beggar-ticks; B. frondosa, Devil's Beggar-ticks; and B. tripartita, Three-lobe Beggar-ticks.
Thoreau wrote in his journals about the seeds "If in October you have occasion to pass through or along some half-dried pool, these seeds will often adhere to your clothes in surprising numbers. It is as if you had unconsciously made your way through the ranks of some countless but invisible lilliputian army, which in their anger had discharged all their arrows and darts at you, though none of them reached higher than your legs."
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"