"The thistle is a common plant, typically characterized by prickly, greenish leaves with a purple tuft in the center. There are several varieties of thistle, such as the yellow thistle. The yellow variety is more rare and not typically seen in most spots in the United States, whereas the purple variety is known as a noxious weed considered an invasive species. The yellow thistle plant is the opposite and is even considered to be endangered in some states." (source)
This text is written as if there only are two species (here called varieties, also a mistake) of thistles in the US, which is wrong. There are many species of purple-flowered thistles, and several kinds of yellow-flowered thistles too. Some of the purple-flowered ones are invasive, others are not. The text oversimplifies and is wrong on several accounts. Not all thistles are in the same genus either. Since the author never mentions a scientific name, it is hard for the reader to figure out exactly what he/she is talking about.
|Screenshot from eHow article on thistle plants.
(c) eHow, fair use