I've run around to many parts of the Basin, but I have yet to encounter an area where horehound grows. Although I can't harvest them, I am still curious. Anyone know where they are?

Horehound grows in the swamps, but there aren't many swamps and horehound is often harvested.

There's a small swamp south of the Commune Proper in Serenwilde, the major swamp (Balach) which is east over the Inner Sea (fly, swim, or walk, or just run around, although if you run around you have to go through Acknor, which may or may not be a problem for you), and there's one other, but I forget where.

Isn't there a swamp in glomdoring?

And that. A little south and a little west of the Ravenwood. (Once you've circled round out of the grove.)

There's a couple of swampy rooms in Lolliprin too, if you're low enough level to get in there.

horehound is usually overharvested