Annuals, 1-13+ cm. Stems 1, de­cum­bent to erect (straight), gla­brous. Leaves ± fleshy, margins entire, eciliate, apices obtuse, faces glabrous; basal deciduous at bolting, petiolate, blades oblanceolate 44-50 × 5-7 mm [cult.], bases attenuate; proximal cauline and some distal withering by flowering; cauline sessile, blades linear-lanceolate to spatulate, 15-55 × 1-4 mm, reduced distally, bases cuneate; distal sessile, blades linear-lanceolate to spatulate. Heads (disciform) in ± densely paniculiform arrays. Peduncles glabrous, bracts linear. Involucres campanulate, 5-8+mm. Phyllaries in 3-4 series, lanceolate, subequal, bases scarious, margins eciliate, green zones foliaceous (outer), apices obtuse, faces glabrous. Ray florets 0. Pistillate florets 14-47+ in 2-3+ series; laminae 0 (corolla tubes 2-2.4 mm, shorter than style branches). Disc florets 5-8+; corollas yellow, 3.1-3.9 mm, lobes deltate to triangular, 0.2-0.3 mm. Cypselae purple, obovoid, ± compressed, 2.8-2.9 mm, 2-4-nerved, faces strigillose; pappi white to yellowish white, 4.8-5.2 mm. 2n = 14.

Flowering Aug-Sep. Brackish sand and mud, intertidal salt marshes, moist dune slacks; of conservation concern; 0 m; N.B., P.E.I., Que.

Symphyotrichum laurentianum is known only from the southern shores of the Gulf of St. Lawrence.