Hoary Cress (White top) (Cardaria draba L. Desv.), a member of the Mustard family, was probably introduced from Europe in alfalfa seed. It is a creeping perennial which reproduces by seed and creeping roots. The extensive root system spreads horizontally and vertically with frequent shoots arising from the root stock. It grows erect from 10 to 18 inches high and has a gray-white color.
The alternate leaves clasp the stem and are oval or oblong with toothed or almost smooth margins. The leaves are often covered with very fine white hairs. Each leaf is 1/2 to 2 inches long with blunt ends. The flowers are white, 1/8 inch across, and numerous in compact flattop clusters which give the plant its name. Each heart-shaped seed pod contains two oval, finely pitted, red-brown seeds each about 1/2 inch long.
Hoary cress is one of the earliest perennial weeds to emerge in the spring. Flowers are produced in late April and May. It grows in waste places, cultivated fields and pastures, and is capable of vigorous growth on the irrigated, alkaline soils of the West. It is a major problem in certain pastures and cultivated areas in Colorado from 3,500 to 8,500 feet.