Thlaspi alliaceum L.

Roadside Pennycress


CC = *
CW = 5
MOC = 4

© DETenaglia

Family - Brassicaceae

Habit - Taprooted annual forb, often glaucous, smelling strongly of garlic when bruised.

Stems - Erect, to 60 cm, pubescent near the base, glabrous and glaucous above, sometimes multiple from the base.

Thlaspi_alliaceum_lower_stem.jpg Lower stem.

© DETenaglia

Thlaspi_alliaceum_leaves2.jpg Middle stem and leaves.

© SRTurner

Leaves - Alternate, sessile, auriculate, clasping, simple, 1-5 cm long, narrowly elliptic to oblong or ovate, toothed, glabrous. Auricles pointed. Basal leaves mostly absent at flowering time.

Thlaspi_alliaceum_leaf1.jpg Leaf adaxial.

© SRTurner

Thlaspi_alliaceum_leaf2.jpg Leaf abaxial.

© SRTurner

Thlaspi_alliaceum_leaves.jpg Pressed leaves.

© DETenaglia

Inflorescence - Terminal and axillary racemes, compact in flower, greatly elongating in fruit. Pedicels to 5 mm long in flower, quickly elongating and spreading in fruit to 1.3 cm long, glabrous.


© DETenaglia

Flowers - Sepals 4, 1-2 mm long. Petals 4, 2.5-4.0 mm long. Styles 0.2-0.3 mm long. Stamens 6, erect. Filaments greenish, to 1.2 mm long, glabrous. Anthers yellow. Ovary superior, green, rotund, 1 mm in diameter, glabrous. Style short, thick, 0.2 mm long.

Thlaspi_alliaceum_calyx2.jpg Calyces.

© SRTurner

Thlaspi_alliaceum_flowers2.jpg Corollas.

© SRTurner

Fruits - Silicles 5-10 mm long, obovate, inflated, narrowly winged below, 2-valved, green, glabrous, with a shallow notch at the apex. Seeds mostly 3-5 per locule, 2-4 mm long, broadly ellipsoid, the surface with a netlike or honeycomb-like pattern of ridges and pits, dark brown to black.

Thlaspi_alliaceum_fruits.jpg Infructescences.

© SRTurner

Thlaspi_alliaceum_fruit2.jpg Fruits.

© SRTurner

Flowering - April - May.

Habitat - Wet fields, roadsides, crop fields, disturbed flood plains.

Origin - Native to Eurasia.

Lookalikes - Thlaspi arvense.

Other info. - This species is rare in Missouri, thus far reported from only four counties. The plant was first cataloged in North America in the early 1960s in North Carolina. Since then it has spread throughout parts of the eastern continental U.S., although it is apparently still widely scattered. At present Missouri represents the far western extent of the plant's spread. It is possible that some populations have been overlooked due to the plant's resemblance to the extremely common T. arvense. The latter species differs by having fruit which are strongly flattened and thin, rather than inflated. T. alliaceum also has long fruiting inflorescences a stronger allium odor when crushed. Fields with large populations of this plant present are redolent of garlic.

Photographs taken at the Dr. Frederick Marshall Conservation Area, Platte County, MO., 5-6-01, and in Reidsville, NC., 3-23-03 (DETenaglia); also near Morse Mill Park, Jefferson County, MO, 4-20-2015, and at Columbia Bottoms Conservation Area, St. Charles County, MO, 4-24-2016 (SRTurner).