Scientific Name: Thlaspi arvense
Description. Pennycress is a small mustard, no more than 10 inches tall. Small white flowers form clusters along branches. The plant makes flat seed pods that can be carried by the wind. This plant is a nonnative mustard, but it does not pose a threat to native wildflower communities. It has historically been used in roadside revegetation projects. It invades disturbed areas and will form a monoculture, but only for a year or two. It’s successional. It is quickly outcompeted by other species after the first couple years. While this plant has no long term effects on native plant communities, it is bad for the native cabbage butterfly. This small white butterfly recognizes the mustard compounds in the plant as a hostplant and it will lay eggs on this plant. However, the caterpillars do not have the ability to eat this plant and they all die. If you want your garden to be a nice place for caterpillars to live, you could remove pennycress.
This plant is successional and does not pose a threat to native plant communities in the Rocky Mountains. There is no need to control it. However, if you want to get rid of the plant for other reasons, it is easily removed by hand-pulling.