Thlaspi arvense
field pennycress [Blooms: Apr-Jun]
Field pennycress (Thlaspi arvense) flowers and fruits. Left: a mature field pennycress with flowers still in bloom at the tip of the inflorescence, but mature seedpods are apparent below. The insert in the lower left corner shows a magnified view of a single seedpod. Bottom right: a mass of flowers at the tip of the inflorescence. A few flowers have already been fertilized, and a developing seedpod (arrow) can be seen between the stamens with the stigma still apparent at the tip of the seedpod. Top right: a closer view of field pennycress; again, a few flowers already have developing seed pods (arrows) tipped with a remnant stigma.
Field pennycress’s central stem and the upper side stems produce racemes of flowers at their tips. The flowers bloom at the tip of the raceme, which continues to elongate to 3-8″ long and to produce new buds; the prominent, disk-shaped fruits develop lower on the raceme. Individual flowers are about 1/8-1/4″ across on a 1/2″ pedicel; there are four green sepals with whitish edges, four white, oval petals, six yellowish-green stamens (three on each side of the flattened ovary), and a single short style. The fruit is a 1/2″ wide flattened, winged disc with a distinct U-shaped notch at the apex; the fruits are borne on the flower pedicels, which spread perpendicular to the stem. Each seedpod has two compartments, each compartment containing several dark brown seeds. Other mustards have long, cylindrical seedpods; the nearly-circular seedpod of field pennycress is unique among Jackson Park mustards. Young field pennycress plants are edible but become very bitter at flowering. Cows that feed on field pennycress produce “tainted” milk that tastes like garlic.
Field pennycress (Thlaspi arvense) stems and leaves. (1) The center of the image shows a clear view of the petiolate basal leaves (arrow) of the plant with two stems visible, one on either side. (2) A lower stem leaf showing the “ears” that make the stem leaves appear to clasp the stem. This leaf is somewhat unusual in its lack of marginal sculpture. (3) A single, mature stem leaf showing the upper side of the leaf (3a) and the underside of the same leaf (3b). Again, note the “ears” on the base of the leaf. (4) A small upper stem leaf showing the upper side of the leaf (4a) and the underside of the same leaf (4b).
Field pennycress is an exotic annual that is native to Eurasia; it grows 1-2.5 feet tall, sometimes in dense, single-species stands. As an annual, it is less predictable in its appearance in a given site year-to-year than most Jackson Park wildflowers (the majority — 58% — of which are perennials). Field pennycress may be a winter annual, a summer annual, or a winter biennial. Winter annuals produce a basal rosette 6″ across of egg-shaped leaves with long, winged petioles. Summer annuals or winter biennials produce a central stem and several side stems that are hairless and ribbed; some of the ribs may be winged. The stem leaves are alternate, hairless, and four times longer than wide, with a leaf blade that is lance-shaped or egg-shaped, with slightly wavy margins bearing a few blunt teeth. The lower stem leaves may have short petioles or may be sessile; the upper stem leaves clasp the stem with ear-like lobes and taper to a blunt point.
A field pennycress (Thlaspi arvense) growing in the northwest corner of Columbia Basin. This plant has yet to produce any fruit, but the basal leaves (arrows) are easily seen.
Field pennycress is an exotic annual that is native to Eurasia; it grows 1-2.5 feet tall, sometimes in dense, single-species stands. As an annual, it is less predictable in its appearance in a given site year-to-year than most Jackson Park wildflowers (the majority (58%) of which are perennials). Field pennycress may be a winter annual, a summer annual, or a winter biennial. Winter annuals produce a basal rosette 6″ across of egg-shaped leaves with long, winged petioles. Summer annuals or winter biennials produce a central stem and several side stems that are hairless and ribbed; some of the ribs may be winged. The stem leaves are alternate, hairless, and four times longer than wide, with a leaf blade that is lance-shaped or egg-shaped, with slightly wavy margins bearing a few blunt teeth. The lower stem leaves may have short petioles or may be sessile; the upper stem leaves clasp the stem with ear-like lobes and taper to a blunt point. The central stem and the upper side stems produce racemes of flowers at their tips. The flowers bloom at the tip of the raceme, which continues to elongate to 3-8″ long and to produce new buds; the prominent, disk-shaped fruits develop lower on the raceme. Individual flowers are about 1/8-1/4″ across on a 1/2″ pedicel; there are four green sepals with whitish edges, four white, oval petals, six yellowish-green stamens (three on each side of the flattened ovary), and a single short style. The fruit is a 1/2″ wide flattened, winged disc with a distinct U-shaped notch at the apex; the fruits are borne on the flower pedicels, which spread perpendicular to the stem. Each seedpod has two compartments, each compartment containing several dark brown seeds. Other mustards have long, cylindrical seedpods; the nearly-circular seedpod of field pennycress is unique among Jackson Park mustards. Young field pennycress plants are edible but become very bitter at flowering. Cows that feed on field pennycress produce “tainted” milk that tastes like garlic. A recent article by Gautam et al. (2026. Nature Plants. 12: 74–87) describes the domestication of field pennycress into a plant that can produce a crop of oilseed suitable for use in renewable fuels or food, allowing Midwestern farmers to produce three crops in two seasons while enhancing soil health.
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![Evening primrose — clusters of yellow flowers occur at the apex of the plant. The flowers are 1-2" across, with four 1/2-1" long green sepals strongly reflected over a long (3/4-2"), green, cylindrical floral tube (the ovary is located just above the leaf axil), four heart-shaped yellow petals, eight stamens, and a single style with an X-shaped stigma. The sepals typically remain connected (in pairs) along one edge until the flower opens. The seedpods develop within the ovary (the base of the elongate floral tube); they are hairy, urn-shaped with rounded edges, and have four or eight small lobes on the apex. The seeds are claimed to remain viable in the soil for up to 70 years. The flowers open at dusk and close early in the day. — — — There are (at least) three evening primroses in Jackson Park — common evening primrose (Oenothera biennis), northern evening primrose (Oenothera parviflora), and hairy evening primrose (Oenothera villosa). They are easily confused with each other. (Judging by the conflicts between the accounts on Illinois Wildflowers [https://www.illinoiswildflowers.info/], Minnesota Wildflowers [https://www.minnesotawildflowers.info/], and Missouri Plants [http://www.missouriplants.com/], even the experts are confused.) All three species have yellow flowers with an X-shaped stigma, reflected sepals, petals with a small notch in their apex, stems that may be red or green, and lance-shaped leaves with short, widely-spaced teeth. Northern evening primrose is distinguished by (1) the presence of a knob or ridge at the tip of each sepal which the other two species lack (2) a seed capsule with four robust, flaring lobes with rounded tips at the apex of the seed capsule, and (3) a stigma that is very robust and almost fills the opening of the flower. Northern evening primrose has green to yellowish-green sepals. Hairy evening primrose (often but not always) has (1) striped or reddish-tinged sepals (check the buds where this character is often easier to see), (2) flowers that turn orange as they begin to senesce (although the reliability of this character is uncertain), and (3) always has glandular hairs covering the ovary that have "pimple-like" bright-red bases; these red pimples may also occur on the stems. All three species are somewhat similar to prairie sundrop (O. pilosella); the latter has very hairy stems and leaves, and translucent lines that radiate from the base of the petals in the flowers.](https://jacksonparkwildflowers.info/wp-content/uploads/2025/03/Oenothera_biennis-1t-300x300.jpg)
![Northern evening primrose flowers are 3/4-1.5" across, have four yellow, heart-shaped petals, eight yellow stamens, and a cross-shaped stigma on a modest style; the stamens are as long or longer than the petals and the stigma is nearly fills the floral opening. At the base of the flower are four sepals 1/2-1" long, that bend back along the 3/4-1.5" long floral tube as the flower develops with pairs of sepals attached along one edge until the flower opens. Only northern evening primrose with a distinct ridge or knob near the tips of the sepals. The seed capsule develops from the ovary; it is 3/4-1.5" long, cylindrical but tapering slightly towards the tip, with four robust, flaring lobes at the tip. — — — There are (at least) three evening primroses in Jackson Park — common evening primrose (Oenothera biennis), northern evening primrose (Oenothera parviflora), and hairy evening primrose (Oenothera villosa). They are easily confused with each other. (Judging by the conflicts between the accounts on Illinois Wildflowers [https://www.illinoiswildflowers.info/], Minnesota Wildflowers [https://www.minnesotawildflowers.info/], and Missouri Plants [http://www.missouriplants.com/], even the experts are confused.) All three species have yellow flowers with an X-shaped stigma, reflected sepals, petals with a small notch in their apex, stems that may be red or green, and lance-shaped leaves with short, widely-spaced teeth. Northern evening primrose is distinguished by (1) the presence of a knob or ridge at the tip of each sepal which the other two species lack (2) a seed capsule with four robust, flaring lobes with rounded tips at the apex of the seed capsule, and (3) a stigma that is very robust and almost fills the opening of the flower. Northern evening primrose has green to yellowish-green sepals and is the only one of the three evening primroses to (sometimes) have reddish color on some or all of the midvein of the leaves. Hairy evening primrose (often but not always) has (1) striped or reddish-tinged sepals (check the buds where this character is often easier to see), (2) flowers that turn orange as they begin to senesce (although the reliability of this character is uncertain), and (3) always has glandular hairs covering the ovary that have "pimple-like" bright-red bases; these red pimples may also occur on the stems. All three species are somewhat similar to prairie sundrop (O. pilosella); the latter has very hairy stems and leaves, and translucent lines that radiate from the base of the petals in the flowers.](https://jacksonparkwildflowers.info/wp-content/uploads/2025/03/Oenothera_parviflora-1t-300x300.jpg)
![Hairy evening primrose produves an elongating spike of flowers arises from the apex of the stem; flowers bloom near the tip while older flowers and seed capsules develop below. Individual flowers are 3/4-1.5" across; they sit on a long floral tube with the ovary at its base (attached to a leaf axil). The flowers have four yellow, heart-shaped petals, eight yellow stamens, and a cross-shaped stigma on a modest style; at the base of the flower are four sepals 3/8-3/4" long, reflected back along the floral tube as the flower develops with pairs of sepals attached along one edge until the flower opens. The sepals are often red-striped or red-tinged. Note the long, hairy floral tube below the sepals, the reddish stem, and glandular hairs with red bases on the ovary. The seed capsule develops from the ovary; it is up to 1.75" long, cylindrical but tapering slightly towards the tip, with four tiny lobes at the tip. — — — There are (at least) three evening primroses in Jackson Park — common evening primrose (Oenothera biennis), northern evening primrose (Oenothera parviflora), and hairy evening primrose (Oenothera villosa). They are easily confused with each other. (Judging by the conflicts between the accounts on Illinois Wildflowers [https://www.illinoiswildflowers.info/], Minnesota Wildflowers [https://www.minnesotawildflowers.info/], and Missouri Plants [http://www.missouriplants.com/], even the experts are confused.) All three species have yellow flowers with an X-shaped stigma, reflected sepals, petals with a small notch in their apex, stems that may be red or green, and lance-shaped leaves with short, widely-spaced teeth. Northern evening primrose is distinguished by (1) the presence of a knob or ridge at the tip of each sepal which the other two species lack (2) a seed capsule with four robust, flaring lobes with rounded tips at the apex of the seed capsule, and (3) a stigma that is very robust and almost fills the opening of the flower. Northern evening primrose has green to yellowish-green sepals. Hairy evening primrose (often but not always) has (1) striped or reddish-tinged sepals (check the buds where this character is often easier to see), (2) flowers that turn orange as they begin to senesce (although the reliability of this character is uncertain), and (3) always has glandular hairs covering the ovary that have "pimple-like" bright-red bases; these red pimples may also occur on the stems. All three species are somewhat similar to prairie sundrop (O. pilosella); the latter has very hairy stems and leaves, and translucent lines that radiate from the base of the petals in the flowers.](https://jacksonparkwildflowers.info/wp-content/uploads/2025/03/Oenothera_villosa-1t-300x300.jpg)
