A beautiful floral herb, this weed is often overlooked or mistakened for another more common species, Queen Anne's lace (or wild carrot). With thick stems, multiple small, narrow leaves, and bundles of small white or purple flowers, you won't overlook this one anymore if you know what it looks like.
Yarrow is an incredibly versatile herb. Historically, it's been used to treat cystitis (bladder infection), digestive sluggishness, as an astringent (to decrease bleeding), as a hypotensive (to decrease blood pressure), and as a diaphoretic (to increase sweating).
In a practical sense, Yarrow is most commonly used at the onset of colds, for chronic diarrhea, epitaxis (bleeding nose), hemorrhoids, and constipation. Its typical preparation include infusion (an extended steeping) and tincture (extraction in alcohol).
As a minimum, the next time you see this beautiful herb, stop and smell its flowers...you won't be disappointed!