Description from Flora of China
Clypeola alyssoides Linnaeus, Sp. Pl. 2: 652. 1753; Alyssum calycinum Linnaeus; A. campestre (Linnaeus) Linnaeus; Psilonema alyssoides (Linnaeus) Heideman; P. calycinum (Linnaeus) C. A. Meyer.
Herbs annual, 5-35(-50) cm tall, canescent with appressed, sessile, 6-10-rayed stellate trichomes, these mixed with simple and forked ones on pedicels and sepals. Stems erect, ascending, or decumbent, 1 to several from the base. Cauline leaves subsessile or lowermost shortly petiolate; leaf blade narrowly oblanceolate or linear, sometimes spatulate or obovate, 3-4(-4.5) cm × (0.5-)1-3.5(-5) mm, base attenuate or cuneate, apex obtuse or acute. Fruiting pedicels divaricate or ascending, 2-5(-6) mm, slender, straight, covered with stellate trichomes mixed with fewer, simple and forked ones. Sepals oblong, (1.5-)2-3 × 0.7-1.1 mm, persistent, pubescent as pedicels. Petals white or pale yellow, linear to linear-oblanceolate, rarely obovate, 2-3(-4) × 0.3-0.7(-1) mm, glabrous or sparsely stellate outside, often persistent, apex emarginate. Filaments 1-1.5 mm, slender, neither appendaged nor toothed; anthers ovate, 0.15-0.2 mm. Ovules 2 per locule. Fruit orbicular, (2-)3-4(-5) mm in diam., apex emarginate or truncate; valves not veined, uniformly inflated at center, strongly flattened at margins, sparsely stellate; style 0.3-0.6(-1) mm, slender, usually glabrous. Seeds 1 or 2 per locule, oblong or ovate, 1.1-2 × 0.7-1.1 mm, compressed, narrowly margined all around, margin ca. 0.1 mm wide. Fl. and fr. May-Jul. 2n = 32.
Roadsides, waste grounds, disturbed sites; near sea level to 2800 m. Naturalized in Liaoning [Afghanistan, Kazakhstan, Kyrgyzstan, Russia, Tajikistan, Turkmenistan, Uzbekistan; N Africa, SW Asia, Europe; naturalized in North and South America].