Pennycress is a plant in the genus Thlaspi. Several species within this genus are referred to as pennycress, and the common names for pennycress species often intersect, sometimes making it difficult to determine which species is under discussion. Common pennycress, field pennycress, and Alpine pennycress are some examples of commonly used terms for several species in this plant genus.

Many people regard pennycress as a weed, because it spreads rapidly and it tends to look scraggly and unsightly. It also poses a danger to livestock, since it is toxic to animals like cattle and sheep, especially in large amounts. As a result, people often work to eradicate pennycress, although this can be challenging in areas where the plant is widespread, as the pennycress may reseed itself, courtesy of other plants in the community.

This plant is native to Eurasia, where it adapted to sunny environments and poor soil conditions. It is typically an annual, producing leaves which start as a low rosette close to the ground before developing tall stalks covered in toothed leaves and topped with clusters of white flowers. Pennycress is in the mustard family, and the flowers and stalks superficially resemble those of its radish and broccoli relatives.

The “penny” in pennycress is a reference to the shape of the plant's seedpods. The pods look a great deal like coins, and are often around the size of a penny, although they lack the coppery color many people associate with one cent pieces. When the pods burst, they can disseminate seeds across a considerable distance, and each plant produces numerous pods, which explains how pennycress spreads so rapidly.

Although pennycress is treated like a weed, it actually does perform some useful functions. The plant appears to be capable of sequestering some environmental toxins, such as cadmium. Because of this, pennycress is sometimes used in phytoremediation, a practice in which plants are used to pull toxins out of the soil for easy disposal. Pennycress used in phytoremediation is particularly toxic, thanks to the accumulated heavy metals and other toxins, and it is typically disposed of with care.

The best way to deal with pennycress in the garden is to keep pulling it up, and to plant dense groundcovers which will prevent the pennycress from establishing itself. Encouraging neighbors to control their pennycress can also help to prevent the spread of the plant; for neighbors who are not interested in gardening, volunteering a few hours a month to remove pennycress may be required.