Berteroa incana aka Hoary Alison

Berteroa incana, or hoary alyssum, belongs to the mustard family, Brassicaceae. Its native range runs from east-central Europe to northern China. It is not native to Britain or Ireland. It has spread abroad through crop seed mixed with its seeds. In the United States, it occurs outside the southernmost states. It invades fields, grazing land, pastures, and other disturbed ground. Several states list it as a noxious weed.

This fast-growing wildflower reaches 1–3 feet [0.3–0.9 metres] tall. It often lives for two years. It can also grow as an annual or short-lived perennial. Plants have one stem or several branching stems. Dense hairs cover the green stems and leaves. The star-shaped hairs give the leaves a gray-green cast. Young plants form a rosette at ground level. These lower leaves often die when the plant flowers. The stem leaves alternate and have no stalks. Their edges lack teeth, and their tips vary in shape. Lower leaves have long stalks and smooth to slightly wavy edges.

Small white flowers form long clusters called racemes. Each flower has four petals with deep splits. These splits make the flower seem to have eight petals. Six yellow-green stamens sit within each flower. Bloom starts in mid-spring and can last until frost. Green, flat, round seed pods form along the branching stems. Each pod holds many seeds and keeps its style at the tip. Plants can keep making seed through summer. Seed is also the listed means of propagation.

Hoary alyssum grows in full sun and needs good drainage. It favors sandy or rocky ground but also grows in loam. It tolerates poor soil and neutral to alkaline conditions. Its growing sites can become dry at times. Root rot can develop where drainage is poor. The source lists no other major pest or disease problems. Its white blooms and gray leaves are clear features. Yet its spread creates more concern than landscape value. Control takes repeated hand pulling or tilling to exhaust stored seed. NC State reports no successful chemical control.

This plant poses a serious risk to horses in pastures. Poisoning can cause fever, swollen legs, and painful hoof disease called laminitis. Other signs include bloody diarrhea, pregnancy loss, and death. The substance that causes poisoning remains unknown. ASPCA lists it as non-toxic to dogs and cats. That distinction does not make it suitable as horse forage.