Western Yarrow

Western Yarrow
Achillea millefolium
Asteraceae (Sunflower family)

Quick ID
Yarrow forms a spreading carpet of soft, fern-like leaves that grow 3-5" long and have a little silvery tinge to them.  The flower stalks can get to be 3' tall (shorter where it's shaded) and are topped with clusters of creamy white flowers.  Leaves and flowers alike have a distinctive smell, kind of sharp and pungent.

At first glance, the flowers of the native White Spiraea (Spiraea betulifolia) look an awful lot like yarrow, but on closer observation they're pretty different.  Spiraea flowers (on the right in the photo above) have long stamens that waggle out past the petals, and the leaves are broad and toothed.

Tansy, blech!

There are plenty of non-native yarrows grown and sold at nurseries.  These are the yellow and pink-flowered varieties, and they have a strong tendency to be weedy in gardens.  There is also an invasive weed called Common Tansy (Tanacetum vulgare) that's often mistaken for yarrow.  It looks similar, but only in the crudest sense.  Tansy is a pretty plant, but, ahem, a HUGE pain in the ass, devastating to native plant communities, impossible to get rid of, etc etc.  Don't grow it.  Don't pick a bouquet and give it to your honey.  I see you looking at it with those doe eyes.  But it's so yellllowww...!  Quit it.  It's bad.  Tell your neighbors.

Range
Western yarrow is circumboreal, meaning it occurs all throughout the northern (boreal) latitudes of the globe, including every state and province in North America.  You find it especially in the west and central parts of Montana, in habitats ranging from streambanks to open hillsides to wooded forests.

What's in a Name?

Everything about yarrow is steeped in rich history, and its name is no exception.  The common name comes directly from the old Saxon word for the plant, gearwe. The genus Achillea honors the Greek hero Achilles of the Trojan wars, and hints at this plant's long importance as a medicinal herb.  Achilles was taught of yarrow's healing properties by Chiron, his centaur tutor.  Achilles had no need of it himself, of course, having been rendered invulnerable to wounds due to a good dunking in the river Styx as a baby.  The only spot that remained vulnerable was the small place where his mother Thetis pinched his heel as she dipped him in the water, and this is where Apollo shot the arrow that was the death of him.  (Turns out Apollo was also the one who taught Chiron all that good stuff about plant medicines!  Hmmm...)  Anyhow, during his life as a war hero, Achilles is said to have carried the yarrow plant with him into battle to heal his soldiers' wounds.  The fresh leaves are indeed a clotting agent, and can be used to staunch nosebleeds and bloody scrapes.   For this reason, yarrow has also been known in the past as bloodwort, sanguinary, soldier's woundwort, stanchweed and thousand seal.  The name for this blood-clotting alkaloid is achilleine, which is still used in modern medicine to suppress menstruation.
The species name millefolium literally means a thousand leaves, and leads to another common name for yarrow, "milfoil".  Also included in the long list of traditional names is death flower, eerie, bad man's plaything (!), old man's mustard, seven year's love, knyghten, snake's grass and devil's nettle.

Tidbits
Yarrow isn't considered a great grazing plant for domesticated or wild animals.  It's one of those "they'll eat it if they have to" plants, which makes it good for landscaping where deer are a problem.  Milk from cows that graze on yarrow is considered "disagreeable" tasting, and I can tell you from experience that honey from a yarrow patch tastes...really weird.  Very strong.  Disagreeable, you might say.  In fact, the alkaloids, volatile oils and glycosides in yarrow are so apparent, so in your face, that some people just can't stand it.  Late in the summer, when the white flower clusters are starting to brown, the smell coming off a yarrow stand is strong.  "Literally smells like vomit," says a friend of mine.  Whelp, says I.  Smells like yarrow.  You either love it or you hate it.  For me, it's both at the same time.

Those same smelly chemicals are what has made yarrow such an illustrious plant for thousands of years.  The medicinal properties go on and on.  Besides being a blood coagulant, it's also reported to be a good anti-inflammatory chest-rub for colds, induces sweats to break a fever, eases toothaches and earaches, soothes burns, brightens your eyes and repels mosquitoes.  I believe it.  If you dig up a bit of yarrow, you'll see little pink tips on the roots.  Chew on these.  They taste like carrots and make your tongue go all numb and tingly.  There are powerful chemicals at work in this plant.  I've never seen anyone poisoned from it, but it could happen.  Don't say I didn't warn you.  I have seen it be irritating to some people's skin.

Wild Gardening
In many ways, yarrow is a wild gardener's dream.  It's so easy to grow from seed.  Just wait till the flower heads are brown, shake them off into a bag, and make sure they're good and dry so you don't get mold.  The seeds don't even need cold stratification; you can just sprinkle them on any old soil and they'll grow like gangbusters.  They also transplant like nobody's business.  I've literally yanked yarrow out of my yard, thrown it onto a patch of roughed up ground, done a two second "cover up, smoosh down and water," and had a healthy new yarrow patch within a week.
Obviously, this plant is tenacious.  In an irrigated yard, it will take over if you let it.  Maybe that's a good thing!  I'm letting a chunk of my lawn get taken over this summer.  It's nice because you don't have to mow it (it's a wildflower!) but if you do, it's fine.  Just nice soft ferny lawn.  Never have to water or fertilize.  But this tenacity also means that stray yarrow plants are constantly popping up in every other part of my yard.  I'm semi-okay with it, because the foliage is nice and I don't have to feel bad about ruthlessly yanking it out when it's gone too far.  And it will go too far.  So if you want a tidy controlled environment where everybody follows the rules and steps in time, yarrow's probably not for you.  If you want a crazy-easy native plant that needs next to nothing in terms of upkeep, look no further.  In fact, if you don't irrigate at all (and live in a really dry climate like ours) yarrow will be much less of a pain.  So really what you should maybe do is go native, quit watering, and embrace wholeheartedly the plants like these that thrive on neglect.
Also, make sure you like the smell before you plant a bunch.  Some people don't.
The pollinators love it though!  Prolific flowers, nice big landing pad for bees and butterflies, and a long bloom season.  The seed heads look really nice if you don't cut them, too, and add great winter interest to your landscape.  Oh, yarrow.

One More Thing...
I don't suppose I've mentioned, here on Flora montana, my great love of story songs and old folk ballads, but there, I've said it.  Oh I love them, and an old Scottish standard, The Dowie Dens of Yarrow, just happens to be one of my favorites.  Especially Ewan MacColl's version.  So good.