Seneca snakeroot (or Senega snakeroot) is distributed across much of Canada and the central and eastern parts of the USA. It is a low-growing perennial species, ranging from 10-50cm in height at maturity. Some of its preferred habitats include prairie grasslands and dry, open woods. I observed plants in both the Mars Hill Wildlife Management Area (growing in oak savannah) and the Tolstoi Tall Grass Prairie Preserve (remnant prairie grassland) in Manitoba last week, though today's photograph was taken a couple years ago in the Quyon Alvar of Québec.
In Manitoba, it is a common species, so much so that three-quarters of the world's wild-harvested supply is from Manitoba's Interlake region. However, in Québec, it makes an appearance in the "List of Plant Species Likely to be Designated as Threatened or Vulnerable" (PDF), with 38 known occurrences in the province as of December 2004. In British Columbia, Polygala senega is a red-listed (endangered) species. From what little I can find online, perhaps it is extirpated in the province, as the most recent collection of a specimen was in 1958 from its single known locality near Pouce Coupé. The BC Conservation Data Centre ranks it as "historical".
Agriculture and Agri-food Canada's Canadian Medicinal Crops site explains why Polygala senega is harvested (in an excellent fact sheet on the species): "Seneca snakeroot was utilized by the Seneca Indians in treatment of rattlesnake bite. Canadian botanist Frère Marie-Victorin suggested that the resemblance of the knotty root crown to a rattlesnake's tail may have contributed to its use by the Seneca as an antidote...Seneca snakeroot was sent to Europe in the early 1700s and held a regular place in European drug stores during the 1800s for use in treatment of pneumonia...The root is ground into powder and used in various patent medicines, particularly in cough medicines, as a stimulant expectorant. It is present in some prescription drugs used in the treatment of bronchitis and asthma..."
Lastly, a note of interest to pass along, particularly to local readers: we've launched a new provincially-accredited Horticulture Training Program here at UBC Botanical Garden. For those who can make it in person, there are two upcoming information sessions: June 26 at 7pm and July 8 at 3pm (registration for autumn 2012 closes July 15). See the link for additional details.