Capsella bursa-pastoris is the kind of plant that grows everywhere and is noticed by almost no one. A native of Europe and Asia, it has spread so thoroughly along roads, field margins, and garden beds that it now inhabits most of the temperate world. The name comes from the tiny, triangular, heart-shaped seedpods — said to resemble the leather purses carried by medieval shepherds. Each pod holds around twenty seeds, and the plant produces them in generous quantity, germinating in both spring and autumn with a flexibility that explains its staying power.

It is not merely a nuisance. Before flowering, the young leaves are edible raw, added to salads or cooked as a green; the seeds can be ground as flavoring; the root has been used as a ginger substitute. The plant has a curious relationship with mosquito larvae: the seeds emit a gummy substance that traps them, then a toxic compound that suppresses larval development. In salty or marshy soils it absorbs salt and sweetens the ground for plants that follow. Growing between six inches and two feet depending on conditions, it blooms in loose white clusters in almost any season, utterly indifferent to whether it was invited.