White europeans weren't the only ones who invaded the Americas. Many european plants, birds, and animals have come along, too. And the plants, which are often relatively innocuous in their home grounds, find themselves without any of their usual enemies or predators, and guess what? They flourish.
Italian thistle is one such invader. Despite its lovely flowers, it is classed as a noxious weed because it is dangerous to livestock, and because without serious effort on the part of the landowner, it will spread unchecked and take over otherwise productive pastureland.
Other nasties from abroad include yellow star thistle, which hasn't started to bloom yet. It will rate a page of its own when it appears later in the season. (It's noxious because, among other things, it's quite poisonous to horses).