Weed of the Month: Field Pennycress

Published: April 19, 2016

Field pennycress (Thlaspi arvense), also known as stinkweed, fanweed, frenchweed, or mithridate mustard, is a summer or winter annual plant native to Eurasia. A member of the mustard family (Brassicaceae), field pennycress could have been introduced into the United States as early as the 1700s¹ and now it is found in almost every state, from Florida and Texas to as far north as Alaska.

Infestations of field pennycress occur in disturbed nonagricultural areas and agricultural lands (pastures and croplands) over a wide range of soil types and environmental conditions (Figures 1 and 2).

Figure 1: Field Pennycress can invade nonagricultural areas, such as this gravel road.

Figure 2: Field Pennycress can also invade agricultural lands, such as this plant growing in a harvested soybean field.

Field pennycress is a broadleaf, herbaceous plant that has a high degree of genetic variation¹. Seedlings develop into compact, basal rosettes (Figure 3) that possess slender taproot systems. Stems originating from the basal rosette are hairless and usually simple but can be freely branched, especially towards the top. When growing in unfavorable conditions, the stem remains unbranched and may reach only a few inches in height, but in fertile soils and with little competition, the main stem may grow vertically up to 32 inches and many lateral branches can arise from the basal nodes² (Figure 4).

Figure 3: Initially Field Pennycress grows as a basal rosette. The plant can overwinter at the rosette stage.

Figure 4: In fertile soils and with little competition, Field Pennycress will grow with 1 main stem and many lateral branches arising from near the base of the plant.

The leaves of the basal rosettes are simple, alternate, hairless, and have wavy margins (Figure 5). However the leaves along the flowering stem are much different. They attach directly to the stem, without petioles, have toothed margins with pointed lobes that clasp the stem (Figure 6). Basal leaves senesce and fall off at maturity. During the summer, late-flowering field pennycress plants can be distinguished from early-flowering plants by differences in leaf shape. Leaves of the late-flowering plants have longer petioles and deeper serrations than those of the early-flowering cohorts³.

Figure 5: The leaves of the basal rosette of Field Pennycress are simple, alternate, hairless, and have wavy margins.

Figure 6: Field pennycress flowers are white and produced in clusters at the top of the stem.  Each flower is attached on a short stalk or peduncle.  The stem leaves have toothed margins with pointed lobes, lack petioles, and clasp the stem.

Field pennycress flowers are produced in clusters at the top of the stems (Figure 6). Individual flowers are about 1/8 inch wide, have four green sepals and four white petals, and are attached to slender, 1-inch long, pedicels or short stalks (Figure 6). Each flower produces a single seedpod called a silicle or seed capsule, which is bright green, about ½ inch long and ½ inch wide, and nearly circular. The seedpods have two cells, each containing seeds surrounded by a wide membranous wing (Figure 7). They are round and flat, about 0.4 inches in diameter, broadly winged and with a notch at the tip. Each plant can produce as many as 20,000 seeds, which can remain viable in the soil for as long as 20 years⁴ and are easily dispersed by wind and water. It is suggested that seeds produced by winter annual populations are initially non-dormant in autumn, but become dormant in winter while those produced by summer annual populations are dormant in autumn and become non-dormant during winter¹.

Figure 7: The seedpods of Field Pennycress have 2 cells that contain seeds and are surrounded by a wide, membranous wing.

Figure 8: Heavy infestation of Field Pennycress taking over a pasture area.

Field pennycress overwinters as seeds or vegetative rosettes⁴. In Missouri, field pennycress seedlings can emerge in late February and adult plants start blooming by middle March, depending on the accumulated temperatures towards the end of winter; first mature seed are produced by end of April or Middle May.

Infestations of field pennycress occur in small or large patches, either as individual or clusters of plants and in mix with other weeds. Systematic observations in Missouri indicate that in heavy infestations, up to 100 plants/square foot can occur (Figure 8). In soybean, heavy infestations of field pennycress are problematic because pennycress is reported as a suitable host of the soybean cyst nematode⁵.

Field pennycress plants may be confused with those of pepper grass (Lepidium densiflorum), but the leaves of pepper grass do not clasp the stem and the flower petals are shorter than the sepals.  In field pennycress, this arrangement of the petals and sepals is the opposite. In addition, the leaves of field pennycress, when crushed, have a rank, garlic-like odor, hence the alternate name "Stinkweed."

Field pennycress can be easily controlled mechanically with tillage or with herbicides. Herbicide treatments will provide best control when plants are in the early stages of development, preferably in the rosette stage when growth is active and before plants start shedding seed. Standard burndown herbicide combinations containing glyphosate or paraquat plus 2,4-D, dicamba, or Sharpen will provide excellent control of field pennycress.

This article was a collaboration between Kevin Bradley and Mandy Bish of the University of Missouri, and Blake Andersen and Oscar Perez-Hernandez out of the University of Central Missouri.

¹Holm L, Doll J, Holm E, Pancho J, and Herberger J. 1997. World weeds: Natural histories and distribution. John Wiley & Sons, Inc. New York, NY.

²Weed Science Society of America. http://wssa.net/wssa/weed/intriguing-world-of-weeds/#x (then select Pennycress, Field from list). Retrieved on March 15,2016.

³Best KF and McIntyre GI. 1975. The biology of Canadian weeds. 9. Thlaspi arvense L. Can. J. Plant Sci. 55:279-292.

⁴Chepil WS. 1946. Germination of weed seeds. 1. Longevity, periodicity of germination and vitality of seeds in cultivated soil. Sci. Agri. 26:307-346.

⁵Venkatesh S, Harrison K, and Riedel RM. 2000. Weed hosts of soybean cyst nematode (Heterodera glycines) in Ohio. Weed technology 14:156-160.