Invasive Plants in Minnesota

Hoary Alyssum is and introduced erect annual to short-lived perennial forb, growing on stiff leafy stems from 10 to 28 inches  high. Stems are hairy and have branching in the upper part.

The leaves are alternate and covered with a grayish down. There is a clump of basal leaves, which may or may not be present at flowering time, that are narrowly elliptical, tapering at the base to a stalk and with a more rounded tip. Stem leaves are more linear with pointed to rounded tips and a base that abuts the stem but not clasping. Stem leaves decrease in size toward the top of the stem.

The inflorescence is a rounded cluster of stalked flowers atop the stem. These clusters elongate considerably into a spike as the plant produces fruit and additional flowers continue to open in the upper part of the spike.

The flowers are 4-parted with a yellowish-green hairy calyx that has 4 sepals with long pointed teeth. The 4 white petals of the corolla are deeply divided at the apex creating a 'mouse-ear' appearance. Flower stalks are hairy. The stamens are tetradynamous, meaning that they are in three pairs with two pair in the center being longer than a shorter pair at the side. The shorter ones have a pair of nectar glands at their base. The pistil is a light green with a rounded stigma.

Fruit: Mature flowers produce a hairy, plump flattened elliptical seed pod with the style persistent at the tip. The seeds are flattened, rounded, 1 to 2.3 mm in diameter with a narrow wing on the margin.

Invasive: See notes at bottom of page.

Habitat: Hoary Alyssum adapts readily to waste places, disturbed soils along paths, roadsides, railroads, etc. where the soil is poor, dry to mesic and sunny - or in gardens in fertile soil where there is a free spot. It grows from a taproot. Seeds germinating in the fall produce a rosette of basal leaves that sends up the flower stalk the following year. Early seeds can produce flowering plants the same season. The quantity of germinating seeds crowd out other plants.

Names: The genus Berteroa, is an honorary for the Italian physician Carlo Guisepppe Bertero (1789-1831) who botanized in the West Indies. The species incana is from the Latin for 'hoary' or 'gray' referring to the general appearance caused by the down on the leaves. The author names for the plant classification are: First to classify in 1753 assigning the name Alyssum incanum was - '(L.)' which refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His work was amended in 1821 with the change of name by ‘DC’ who is Augustin Pyramus de Candolle (1778-1841), Swiss botanist, who influenced Charles Darwin. He studied plants, began a systematic catalogue and has 2 genera named for him.

Comparisons: Although the "mouse-ear" style flower is seen on other plants, when in flower, with its hairy leaves and stem, Hoary Alyssum is unlikely to be confused with other plants in Minnesota. Elsewhere, there are two other mustard family plants that look similar except that in flower, the petals are yellow.  These are Aurinia saxatilis, Basket of Gold; and Alyssum simplex, Wild Alyssum. Diagram below.