Field Pennycress: Thlaspi arvense
||Weed Description: A winter
annual (or very seldom a summer annual) that resembles some of the pepperweeds when
mature. Field pennycress is primarily a weed of winter small grains, nurseries, and
horticultural crops that is found throughout the United States.
Cotyledons are oval to oblong in outline and occur on long (5-7 mm) petioles.
First true leaves also occur on distinct petioles and develop into a basal rosette.
Young leaves are without hairs, round to oval in outline, with a distinct white
midvein and wavy margin.
Individual flowers are very small (4-6 mm) and occur in clusters at the ends of the
'bottle-brush' stems. Flowers consist of 4 white petals and 4 green sepals.
||Leaves: Leaves initially
develop into a basal rosette and are without hairs, oval in outline, with a wavy margin.
Leaves along the flowering stem are generally much different than those of the
basal rosette. Leaves along the flowering stem are more lanceolate in outline,
without petioles (sessile), usually have toothed margins, and have pointed lobes
that clasp the stem at the base of the leaf.
A taproot and smaller fibrous root system.
|Stems: Erect, ranging from 4 to
24 inches in height, usually branching in the upper portions only. Stems are without
hairs and the leaves usually fall off the stems as the plants reach maturity.
||Fruit: Many winged silicles
give mature plants the distinctive 'bottle-brush' appearance, similar to many of the
pepperweeds. Each silicle is circular in outline, approximately 1/2 inch in
diameter, relatively flat, and distinctly winged along the outer margins. Silicles
divide in half and may contain as many as 16 seed per silicle.
Identifying Characteristics: The oval, hairless leaves of the
basal rosette and leaves with pointed lobes that clasp the flowering stem are both
characteristics that help in the identification of field pennycress. Additionally,
the bottle-brush appearance of the seedhead helps to distinguish this weed from many other
winter annuals. Thoroughwort or Perfoliate
Pennycress (Thlaspi perfoliatum) is similar but has fruit that are more
distinctly notched and heart-shaped in appearance. Field
Pepperweed (Lepidium campestre) is also similar to field pennycress but has
leaves with short hairs and more rounded fruit than those of field pennycress.