Description
Like many of the Senecio genus S. congestus can be an annual or biennial and perhaps rarely perennial, depending on the conditions. A villous broad leafed plant with a single hollow stem and favoring mud flats like Marsh Groundsel (Senecio hydrophilus) but not alike in that Marsh Ragwort cannot tolerate alkaline sites nor standing water.
In the early stages of growth, the leaves, stem, and flower heads are all covered with translucent hairs, producing a "greenhouse effect" close to the surface of the plant, essentially extending the growing season by a few vital days by allowing the sun to warm the tissues, and preventing the heat from escaping.
Leaves and stems: An erect plant standing 6 to 60 inches (15 to 150 cm) tall, S. congestus varies as much in stature as it does in the distribution and the persistence of its tomentum (the closely matted or fine hairs on plant leaves). Sparse to dense villous stems are more hollow towards the base; hairs that are white, light yellowish, or reddish brown. Basal and cauline leaf edges mostly toothless or with a few coarse teeth that sometimes wither before flowering. The leaves all basically similar in shape, oblong with the lower ones often spatulate, 1.6 to 8 inches (4 to 20 cm) long, 0.2 to 2.5 in (0.5 to 6 cm) wide, with the basal leaves occasionally larger, glabrous or villous in patches, rounded at the tips, toothed, often deciduous with clasping bases.
Flowers: "Congested" clusters of several to many pale yellow flower heads that sometimes appear tubular and incompletely opened. a branched inflorescence, with 13 to more than 21 flower heads per stalk. Each flower head is ¼ to ½ inch (6 to 13 mm) across and 0.16 to 0.4 inch (0.4 to 10 mm) in length, with small but obvious rays in a corolla laminae surrounded by (usually) 21 green or yellowish green, pink tipped bracts which can be scarious toward the tips.
Seeds: Typical of the Senecio genus, S. congestus produces strongly accrescent one-seeded, one-celled, 1.5 to 2.5 millimetres long dry achenes on very fine & numerous, white or dirty white, fluffy, pappus bristles. The seeds have been shown to survive in the soil for more than a year but less than 5 years, the maximum longevity unknown.
Roots: Fibrous and without a tap root.
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