Hoary cress, 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 rootstock. It grows erect from ten to eighteen inches high and has a gray-white color.
The alternate leaves clasp the stem and are oval or oblong with toothed oral most smooth margins. The leaves are often covered with very fine white hairs. Each leaf is 1/2 to two 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 seedpod contains oval, finely pitted, red-brown seeds each about 1/12 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.
Hoary cress can often be confused with field pennycress. They are both members of the mustard family, and both have white flowers of similar shape. However, field pennycress comes up earlier, has darker green leaves with fewer hairs and has whorled flowers (come up the stem like a spiral staircase). They are both weeds, but field pennycress is not on the noxious weed list, and differs from hoary cress in many ways.
For more info and how to control, check out the La Plata County Weeds Dept. Fact Sheet