Considering how long I’ve been at this flower blog, it is a wonder that I haven’t posted a portrait of yarrow yet! It is so ubiquitous that it practically goes unnoticed in summer fields and roadsides. And yet, there is something very special about this simple white flower.
Achillea millefolium
Of course you know by now that plants in the Composite Family typically have two kinds of flowers: ray flowers and disk flowers. Usually the disk flowers form a flat or rounded center, and the ray flowers encircle the disk. A daisy is the classic example of a composite: it has yellow disk flowers and white ray flowers.
Yarrow is a composite too, but instead of having just one flower head per stem, this plants bears dozens of small white (sometimes pink) flower heads in a flat-topped cluster at the top of each stem. Each single “flower head” usually has 5 white rays. Look closely at the slides below to see that each ray has 3 little notches. Yarrow’s small, aromatic leaves are fern-like and much dissected. In fact, they are often described as feathery.
Yarrow varies in height from 12-36 inches tall, and is often found mixed with tall grasses and other wild, summer wildflowers like queen anne’s lace and flowering spurge. Bloom time is June-August.
Yarrow is a real rockstar in the world of herbal medicine. The plant contains a very large number of active chemicals that have been proven effective in treating everything from bleeding to fever to inflammation to toothache pain.
According to Wikipedia, common names for this historically important herb include nosebleed plant (stops bleeding), old man’s pepper (used as snuff), devil’s nettle (used in potions and spells), sanguinary (attendant at bloodshed), milfoil, soldier’s woundwort or herbal militaris (useful in times of war), thousand-leaf (describes the many-segmented leaves), and thousand-seal. Next time you see it in your travels, chew a leaf or two to see what the plant has to offer. Expect your mouth to get numb!
VIRGINIA WILDFLOWERS 
I’m more familiar with the yellow yarrow — is it the same species or perhaps just an ornamental variety?
As far as I know the brightly colored yarrows are cultivars.
The leaves contain numbing agents? How cool!