When I was a new gardener, (some 30 years ago) I remember looking over at my neighbor's wooded lot and loving all of the pretty white flowers that were blooming in her yard in May. Little did I know that I would be spending the rest of my life pulling that 'pretty white flower'. As it turns out, that flower was the non-native, invasive Garlic Mustard.
Like a lot of introduced plants, insects, and animals, the intent was never to harm. The people bringing them over had good intentions, or it was an accident. In the case of garlic mustard, it was brought over by early settlers that used it as a flavoring, a disinfectant, and other medicinal uses. It is also a good source of vitamins A and C.
The problem with some introduced plant species is that they seldom have something else in nature that keeps them in check in their adopted home. Where in its' native range, it has close to 70 insects that depend on it, practically nothing feeds on it here. It also produces chemicals in its' roots (allelochemicals) that can inhibit growth of native plants.
Garlic Mustard is a biennial. It's a short lived plant that spends its first year growing leaves, and will be close to the ground. The 2nd year it will send up a stalk that will produce flowers that will very quickly go to seed. It's important to try and pull it either right before, or during bloom. I always crave Italian food after a day of pulling Garlic Mustard. There is a reason it has that name.
1st year leaves
A patch of Garlic Mustard in bloom.
Pretty, white flowers are suspended high above the ground. You can see how quickly seeds are formed in this picture. The seeds are long, and skinny (on the bottom between the 2 flowers).