Not only is bull thistle an invasive plant, but also, as indicated by the barbs in this picture, it can just plain hurt to run up against this plant! That's enough to put it on my "noxious weeds" list, any day. Some birdwatching enthusiasts do grow the plant, however, due to its ability to attract goldfinches.
From the above picture of bull thistle, you may be able to tell that it is related to some of the perennials grown in the landscape. For example, it bears a strong resemblance to a yellow Centaurea sometimes referred to as "globe knapweed" (for a picture of spotted knapweed, see the photo on the prior page).
Those of you familiar with the flora of overgrown pastures may know bull thistle as a common inhabitant of such areas. But this Old-World native can also take advantage of disturbed ground along, e.g., roadsides. No shy wallflower, you'll know this robust noxious weed when you see it: plants can reach a height of 7 feet under ideal conditions!
Bull thistle is a biennial. It develops rosettes the first year, succeeded by flower stalks the following year. Plants are covered with spines, making the sort of spontaneous removal that you may practice with other noxious weeds a bad idea for bull thistle. No, this is the type of plant you come prepared to remove, wearing heavy gloves and long sleeves/pants and wielding a shovel. Dig bull thistle out by the root. Don't let it go to seed.
The reason why letting it go to seed is such a bad idea is that the seeds of bull thistles, like those of dandelions and common milkweed, are easily transported across the air. Each seed is attached to thistledown, so it can travel far away from the mother plant when the wind blows.