Thistle

Here is a large, mature thistle, still in the early growing season.  I imagine that if left unchecked it will be at least 6 or 7 feet tall by Summer.  It is worth noting that thistle, as with many weeds, is a perennial, so it will just get bigger and larger.  Thistle is a prairie plant so likely the roots are giant 10 foot monster roots.

Thistle

There are a few kinds of thistle that are native to our area, but, I think, most of the thistle we see is an invasive sort. Not that it matters -- few Chicago gardeners want any thistle, native or not. Perennial, so must be dug up completely or sprayed. Obviously be careful.