Do you deadhead yarrow? If you do, do you just tear of the tops, or do you cut them all the way down the stem? Or do you simply save seeds somehow?
Yes, you do deadhead yarrow (Achillea millefolium) as some of the smaller plants tend to die back to the ground. If you deadhead early enough in the summer it will rebloom. I cut it with sequiteurs just below the flower as it is faster. If you try tearing the heads off you take your chances with ripping the whole plant out.
Given it's tendencies to seed even in the cracks between tight set flagstone I have never wanted to collect seed but you could do so by deadheading and shaking the dried flower into an envelope.