I've spent the morning clearing weeds out of the back yard, which was almost entirely neglected during our year in England. One of the common weeds I've been pulling out is white snakeroot (Ageratina altissima), a shade-loving weed that also seems to be quite common in the undergrowth of the oak savanna restorations in the Arboretum, where this photograph was taken. White snakeroot is notorious in American history as the plant that killed Abraham Lincoln's mother, Nancy Hanks Lincoln. The plant contains a strong toxin known as tremetol, which causes muscular debility, trembling, vomiting, and, in many cases, death. When the plant is eaten by dairy cows, the poison passes into their milk, which causes "milk sickness" in those who drink it. It was this milk sickness that killed Abraham Lincoln's mother. She died on October 6, 1818, when Abe was about nine and a half years old.
A medical correspondent, reporting about the prevalence of the sickness in northern Kentucky in 1822, wrote that it was probably caused by "some poisonous vegetable" that tainted the milk of cows who ate it. It wasn't for several decades that white snakeroot was firmly established as the cause of the sickness. White snakeroot continues to be a problem for livestock grazing on its stems and leaves. In 2004, for example, three horses in the New Ulm, Minnesota, area were reported to have died of snakeroot poisoning.