It's amazing to me how different plants can grow in various areas of US. I've heard and read people saying how invasive horehound has been for them. For me, horehound has been very hard to get growing from seed and keep alive. I've had the same one small plant for years now! I even had to move it to keep it alive.
After four years of coddling my one little horehound plant that I love for making tea, (it's excellent for curing bronchitis and other upper respiratory problems) this summer my hubby took the weed whacker to it... it hasn't come back.
Although, it didn't seem to keep the grasshoppers out of my garden!