The Garden of Eden had its serpent and Virginia—and many other places in North America—have Garlic Mustard.
At first blush Garlic Mustard would seem to have several attributes that would recommend its importation to North America. Gastronomically, it is valued by some as a wild spice; indeed the heavily veined, heart-shaped leaves give off a mild garlic scent when crushed. (The botanical prefix Alliaria means like Allium or the Garlic family.)
The leaves are reputed to make a tasty addition to salads and pasta dishes. In fact, it is believed that this plant was first imported as a table herb. And it has a number of useful applications beyond the kitchen.
In herbal medicine, extracts from the stems have been used in the treatment of gangrene, as well as for skin ulcers and throat infections. It continued to be recommended by practitioners of herbal medicine well into the 20th century.
Ecologically, its expansive root system seemed to make it useful as an effective barrier against erosion, which led to its being planted for that purpose in Canada and the United States in the late nineteenth century. Advocates of the use of Garlic Mustard for this purpose could note that it is not categorized as an invasive plant in Great Britain.
Apparently it was not taken into consideration that this plant in its native habitat, across Europe and the British Isles, has numerous insects and fungi to keep it under control. However, these natural enemies didn't cross the Atlantic with Alliaria petiolata. In addition, this is a very adaptable immigrant plant: marshland, dry fields or deep forest, Garlic Mustard will make itself at home—and it did!.
Growing as high as forty inches, it towers above the lower growing native plants, blocking access to the sun with its thick foliage. Also, it releases poisons (allelopathy) that inhibit the growth of other flora. (Another allelopathic imigrant lily is Mouse-ear Hawkweed.)
Simply put, after the facts are brought together, Alliaria petiolata would seem to be the least likely alien flora to be purposely introduced into North America—but it is here! So severe is its negative impact across North America that it has been declared an "ecological blight" in 34 states and 3 provinces. A number of state university and departments of agricultural have instigated extensive eradication campaigns against this plant.
Unintended consequences, indeed!