Beware of the fluffy white flowers that suddenly appear in your late summer garden. They belong to an aggressive North American wildflower known as white snakeroot and tall boneset (Ageratina altissima).
The flower provides much needed late season nectar for a variety of bees, moths and flies. And several different caterpillars feast upon its leaves.
Unfortunately, it does reseed readily and spreads by rhizomes making it difficult to contain.
White snakeroot contains tremetol that causes “trembles" and even death when consumed in sufficient quantities by animals. The bitter tasting foliage deters most mammals, including people, from eating dangerous quantities.
But in the early 19th century colonists unknowingly allowed cows to feed upon this plant. Thousands of people and calves that drank the tremetol-tainted milk became ill or died. Current milk processing has all but eradicated this problem.
A bit more information: For help identifying wildflowers see the National Wildlife Federation’s Field Guide to Wildflowers of North America by David M. Brandenburg. There are thousands of thumbnail images and he has included common weeds to help you differentiate between the wildflowers and weeds.