Bidens discoidea (Torr. & A. Gray) Britton — Overview

Swamp Beggar's Ticks learn more about names for this taxon

Comprehensive Description

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This herbaceous plant is a summer annual about 1-2½' tall that branches occasionally to frequently. The terete to slightly angular stems are light green, purplish green, or purple, and they are glabrous. Pairs of opposite trifoliate leaves occur along these stems, although some of the uppermost leaves are usually simple and alternate. The trifoliate leaves are up to 4" long and 3" across in outline, while their petioles are up to 1½" long. The petioles are light green to purplish green and mostly glabrous, except for some sparse pubescence along the narrow grooves of their upper sides. The terminal leaflets are about 1½-2" long and ½-¾" across; they are narrowly lanceolate to lanceolate in shape and coarsely dentate along their margins. The petiolules (basal stalklets) of terminal leaflets are about ¾" long and slender. The lateral leaflets are about 1-1½" long and ½-¾" across; they are lanceolate in shape and coarsely dentate along their margins. The petiolules of lateral leaflets are very short (up to 3 mm. long). The upper surfaces of leaflets are medium green and glabrous, while their lower surfaces are pale green and glabrous (or nearly so). During autumn, the leaflets often turn purple in response to cold weather. Upper stems terminate in either individual or small clusters of 2-3 flowerheads. The peduncles of these flowerheads are relatively short (up to 12 mm. or ½" long). Each flowerhead is about 3-6 mm. across (excluding the outer bracts), consisting of 10-20 disk florets and no ray florets. The corollas of disk florets are yellow, short-tubular in shape, and 5-lobed; they are about 3 mm. long. Originating from the base of each flowerhead, there are 2-5 leafy outer bracts. The outer bracts are medium green, narrowly oblanceolate or elliptic in shape, and glabrous (or nearly so); they are up to 1½" (4 cm.) long and 8 mm. across (usually about one-half the maximum size, but highly variable). Around the base of each flowerhead, there are several inner bracts. These inner bracts are 4-6 mm. long, light green or yellowish green, oblong-ovate in shape, and appressed together in a slightly overlapping single series. The blooming occurs from late summer into the fall, lasting about 1½-2 months. In the absence of cross-pollination, the disk florets are self-fertile. Afterwards, the disk florets are replaced by achenes with pairs of awns. The achenes (excluding their awns) are about 4 mm. long, 1-1.5 mm. across, and flattened-oblanceoloid in shape; they are truncate at their apices, while their outer sides have faint midribs. The barbed awns of these achenes (2 awns per achene) are about 1 mm. in length. Mature achenes are greenish brown to dark brown. The root system is shallow and fibrous. This plant reproduces primarily by reseeding itself. The lower nodes of its stems can develop rootlets when they contact moist ground. Occasionally, colonies of plants of variable size will develop at favorable sites.


Creative Commons Attribution Non Commercial 3.0 (CC BY-NC 3.0)

© John Hilty

Source: Illinois Wildflowers

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