Earlier this week, there was an article in the New York Times about research on how the infestation of garlic mustard may be having a destructive effect on forests. Essentially, there is a system of fungi (mycorrhizal fungi) which delivers nutrients to most plants. Mustard plants don't need the fungi for nutrients and produce an antifungal chemical. Mycorrhizal fungi have had time to get used to, and learn to live with, most mustard plants. But the spread of garlic mustard has been so fast that the fungi hasn't been able to adapt, so it is killing the fungi and distrupting the growth of new trees. This is pretty ironic, because it seems to prefer the shade.
Once introduced to an area, garlic mustard outcompetes native plants by aggressively monopolizing light, moisture, nutrients, soil and space. Wildlife species that depend on these early plants for their foliage, pollen, nectar, fruits, seeds and roots, are deprived of these essential food sources when garlic mustard replaces them.
It's a very wily plant. It isn't very strong, 70% of the first year plants die, it is easily removed by the roots, and while it will keep coming back if mowed down, if you keep mowing it, the root will die. However, once let loose, the seeds stay viable for 5 years and I've noticed that deer won't eat it (choosing my strawberry plants instead). The stuff spreads like wildfire in a dry prairie.
Study the pictures. Know your enemy. Then pull it up by it's roots.