Garlic mustard has become Portland’s poster child for plants that overwhelm the landscape by seeding: a single plant can make hundreds of small seeds. Like many weeds, dense patches form along roads, streams and other disturbed areas. Unlike most other species, though, garlic mustard moves from disturbed areas into healthy forest. There it forms dense patches which dominate and displace native wildflowers, tree seedlings, and other native plant species of intact forests. The reduced plant diversity that comes with garlic mustard monoculture means less resources for wildlife, and, ultimately, no new trees.
In addition, the roots of garlic mustard are thought to produce a toxin that kills soil fungi many plants depend on. The seeds are about the size of a grain of mustard and can move around easily. Combine that with these seeds surviving several years in the soil and you have a plant that’s difficult to manage.