Identification

Hoary alyssum is a member of the mustard family (Brassicaceae). It has a slender tap root capable of deep soil penetration. The basal rosette has oval-lanceolate leaves broadest at the tip and 1.2 to 2 inches (3-5 centimeters) long including the long stalk (see Figure 2).

Figure 2. The basal rosette of hoary alyssum with long-stalked, oval-lanceolate leaves.

Flower stems are erect or semi-erect growing seven to 30 inches (20-70 centimeters) tall and usually branched at the top (see Figure 3). Multiple stems are common. Stem leaves are alternately arranged, similar to rosette leaves in shape but smaller with less of a stalk, or stalkless. The margins of all leaves are entire with no teeth or lobes.

Figure 3. The leaves, inflorescence, flower, and silicles of hoary alyssum.

The small flowers (3 millimeters across) have long pedicels and are clustered at the stem tips in racemes. The four ovate sepals are green with white margins. There are four snowy-white petals narrow at the base and spreading at the tip with a deep cleft or notch (see Figure 4).

Figure 4. Hoary alyssum flowers.

There are two short outer stamens and four long inner stamens, four nectaries, and one pistil with a long style. The seed pods (silicles) are ellipsoid to ovoid, five to eight millimeters long, slightly inflated with the persistent style at the tip. They are appressed to the stem on the long pedicel. The seed pod has two chambers divided by a translucent septum or membranous partition which remains on the pedicel after seeds disperse. Each seed pod contains four to 12 dark reddish-brown, small (1-1.5 millimeters) lens-shaped seeds with narrow wings on the margins. The leaves, stems, sepals, and seed pods are covered with star-shaped (stellate) hairs that give the plant a grayish appearance from which the common name descriptor, hoary, is derived.

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Last Modified: 11/18/2008