Water hemlock or poison hemlock.

[Hermio]

These plants are among the most toxic plants in North America, and they seem to be spreading rapidly. They are typically found along fence lines, at the edge of wooded areas and other open areas where they get plenty of sunlight. They are so toxic that even handling any part of the plant with bare hands can make you sick. You should never weed whack them or bush hog them, as the particulate cloud can make you very ill, sometimes resulting in death. The only safe way to deal with these plants is to spray them with strong herbicide. They are in bloom now and are relatively easy to identify. Search online for pictures. They sometimes are confused with queen Anne's Lace, which is not toxic.

 

[esean]

Poison hemlock has been around here for many decades, mostly in bottomland and damp ditches. I've never heard of anybody getting sick from it. Not disputing its toxicity, just saying that most people know to leave it alone. I've always known not to eat it, but I've handled the leaves without issue. Never thought about the mowing hazard, but it makes sense.