Invasive Plants in Minnesota

Field Pennycress is an introduced erect annual forb growing from 4 to 20+ inches high on hairless flowering stems, without branching or only a very little near the top. Plants that germinate from seed in the late summer will form a winter rosette and the flowering stem the next spring. Stems are ribbed and sometimes with small wings.

The leaves are of two types. Basal leaves are oblanceolate, that is, taper to a pointed base resembling a stalk, are longer than wide and broadest above the middle. The blade is oblong with the margin smooth or with some marginal wavy indentations; the tip is rounded. Stem leaves are oblong, stalkless and clasping with an auricled base. These auricles have a blunt arrow shaped point at the base. Upper leaves have margins with coarse rounded to pointed teeth. The basal leaves drop away early.

The inflorescence is a cluster (a raceme) of stalked flower buds, somewhat flat-topped, that occurs at the top of the main stem and any side stems rising from the leaf axils. As flowers open from the base of the cluster, the cluster elongates and flower opening proceeds upward.

The flowers are 4-parted with a calyx of 4 green sepals and the corolla has 4 white petals that are 2x as long as the sepals and narrow to a claw-like base. Each flower is only 1/8 inch wide blooming first from the lower section of the raceme, which then elongates as more flowers come into bloom and older flowers produce seed. Flowers are held by short stalks that curve upward slightly. The stamens are slightly tetradynamous, that is, in two sets, 4 around the greenish pistil and two shorter lateral stamens that have a pair of nectar glands at their base. This is typical in the Mustard family. Anthers are yellow; the pistil has a blunt flattened tip to the stigma.

Seed: Mature flowers produce a rounded 2-celled 1/2 inch wide seed pod that is very flat and broadly winged all around with a notch at the apex that is deeper than it is wide. These pods are held erect to the flower raceme at the end of the old flower stalk and when dry are blown off by the wind. The pod splits down the middle releasing seeds from each cell. Individual seeds are slightly flattened ovals with concentric striations, turning from dark reddish brown in color to blackish-brown.

Habitat: Field Pennycress germinates its seed in waste places and disturbed areas. It blooms early, needs moderate moisture and will tolerate partial shade.

Names: The genus Thlaspi is from the Greek word for a cress, coming from the word thalo which means 'to compress' referring to the flattened seeds. The species arvense means 'growing in fields' where this plant might typically be found as the common name implies. 'Pennycress' is therefore derived from the flattened seed shape, as wide as a penny, as is the alternate common name of 'Fanweed' - the seeds on their stalks look like little hand-held fans. The flowers are not aromatic but the crushed foliage will have a rank, garlic like odor, hence "Stinkweed." The author name for the plant classification, 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.

Comparisons: Two plants may confuse with this species: First is Pepper Grass, Lepidium densiflorum, but there while the seed pods are similar, the leaves do not clasp and the flower petals are shorter than the sepals. Second is Shepherd's Purse, Capsella bursa-pastoris. There the seed pods are triangular without wings, but the upper leaves are similar with auricles; the flowers are also similar but with the sepals colored more pinkish at flowering time. Comparison drawings are show below.