We call it tickweed. I asked the same question once. We had never had it before and suddenly one year it appeared. Those little seeds are an effective method of spreading and a real stinker to get rid of once the children are coated in them.
@Matu - Thanks for that. We cut wide swaths through it to avoid touching it with our jackets. Why can't it come in mid summer when you're wearing a t-shirt and shorts?
I'm checking the other threads on this plant.
Anyone else have a name for this plant? Can't find anything relating to tickweed, so I think that's a no-go. Someone in another thread said 'toxic water hemlock' (Cicuta) but that's not it.