White snakeroot (Ageratina altissima) occupies a tragic place in U.S. history, having indirectly caused thousands of deaths during the 19th century. The Plants that Kill book explains.
When Europeans started to settle in the Midwest region of the United States in the 1800s, they and their livestock began to fall ill. The animals developed violent trembling when they were forced to move or became agitated, and the disease became known as trembles. People who drank the milk of affected animals developed so-called milk sickness, and it is estimated that in some areas of Indiana and Ohio 25–50 per cent of the deaths of early settlers were caused by this condition. One casualty in 1818 was Nancy Hanks Lincoln, whose son, nine years old at the time, would become President Abraham Lincoln.
The National Park Service website for the Lincoln Boyhood National Memorial in Indiana adds more context.
The illness was most common in dry years when cows wandered from poor pastures to the woods in search of food. In man, the symptoms are loss of appetite, listlessness, weakness, vague pains, muscle stiffness, vomiting, abdominal discomfort, severe constipation, bad breath, and finally coma. […]
Milk sickness or “trembles” was more prevalent in late summer and early fall, but records show that many cases occurred in the winter and early summer also.
White snakeroot will do you no harm, as long as you don’t consume milk from animals that have grazed on it. That’s good news, because this plant is among the most prevalent late summer wildflowers in shady, woodland habitats, especially in damp areas. If you’ve been on a trail running near an Iowa river or creek lately, you’ve probably seen some blooming. It’s native to almost all of the U.S. and Canada east of the Rocky Mountains.