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Thlaspi arvense

field penny-cress

WaterLow

Overview

Thlaspi arvense is an erect annual reaching 4-24 inches (10-60 cm) tall, with a smooth, hairless, branching stem rising from a basal rosette. Stem leaves are alternate, oblong to lance-shaped, 0.5-3 inches (1.3-7.5 cm) long, with arrow-shaped clasping bases and toothed margins. Small white four-petaled flowers, each about 0.1 inch (3 mm) across, are crowded in elongating clusters from April through July. The flowers give way to flat, round, broadly winged seed pods 0.4-0.8 inch (1-2 cm) wide, deeply notched at the top, each holding several dark, concentrically ridged seeds. The pods turn papery and straw-colored at maturity, and the whole plant gives off a sharp garlic-like odor when bruised. A single plant produces up to several thousand seeds, which stay viable in soil for years. Crushed foliage and seed can taint the milk and meat of grazing animals that eat large amounts. Plants finish their cycle within a few months and die after seed set.

Native Range

Native to temperate Eurasia and naturalized across North America, South America, and other temperate regions. Common in disturbed and cultivated ground, including grain fields, gardens, roadsides, and waste places from lowlands to about 6,000 feet (1,830 m).

Suggested Uses

Not cultivated ornamentally; it grows as a weed of fields and disturbed soil. The oily seeds are under study as a winter cover crop and biofuel oilseed under the name pennycress. It is otherwise treated as an agricultural weed.

How to Identify

Recognized by flat, round, broadly winged seed pods 0.4-0.8 inch (1-2 cm) wide with a deep notch at the tip, held in long clusters. The clasping, arrow-shaped leaf bases and garlic-like odor of crushed tissue separate it from other mustards. Seeds are dark brown to black with concentric ridges.

Appearance

Size & Dimensions

Height4" - 2'

Width/Spread2" - 8"

Colors

Flower Colors

white

Foliage Colors

green

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~6 weeks

Spring Summer

Flowers from April through July, and into fall where plants germinate in spring. The small white flowers are mainly self-pollinated and open over 4-6 weeks. Seed pods ripen within a few weeks of flowering, often while later flowers are still opening.

Detailed Descriptions

Foliage Description

green

Growing Conditions

Sun Requirements

SunFull Sun

Requires 6-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.0 - 7.5(Neutral)

357912

Soil Types

clay loam sand silt

Drainagewell drained

Water & Climate

Water Needs

Low

Care & Maintenance

Care Guide

Grows as a fast-cycling annual in full sun on most soils, favoring fertile, disturbed ground. Both fall- and spring-germinating cohorts occur, so plants appear over a long season. No irrigation or feeding is needed. In crops and gardens it is controlled by removing plants before the pods ripen, because soil-stored seed stays viable for many years. The garlic-like compounds make it unpalatable to most grazing animals once mature.

Pruning

Not pruned in the ornamental sense. Removing plants before the pods turn papery limits the large, long-lived seed bank this species builds in soil. Cut plants seldom regrow once flowering has finished.