I've tried to keep adding more authentic prairie plants, but mostly I have to keep ripping out the yarrow. According to my prairie guide1, common yarrow is Abundant and widespread in fields, pastures, disturbed sites, and along roadsides, as well as in prairies, especially prairies with a history of previous disturbance; found throughout the tallgrass region.
Yarrow is native to Eurasia as well as North America, and some of our populations are introduced weeds.
It certainly acts like a weed. Not only does it keep trying to take over the whole prairie patch [only the goldenrod has the strength to resist it], it sneaks out to infest the yard. In this, it employs two methods. Yarrow spreads vigorously from underground rhizomes, but it also spreads readily through seeds to locations further away from its original colony. Once established in the grass, it is difficult to dig out, though it is susceptible to several herbicides. Although it is normally a tall plant, it readily adapts to the use of the lawnmower by taking up a low profile and forming mats. One method of keeping it from spreading afar is to dead-head the flowers after they have bloomed to prevent the formation of seeds, but this will not stop the slow creep of the rhizomes.
Of course there are many wildflower lovers who will champion the yarrow. It makes a good tall cut flower and is willing to repeat bloom. It is quite attractive to bees and butterflies, often planted in butterfly gardens. Herbalists are fond of its styptic properties. The name "Achillea" supposedly comes from its use by Achilles to staunch the bleeding of his wounds. Some of its other names are Staunchweed, Sanguinary, Woundwort and Nosebleed Plant.
Yarrow is also known as Milfoil from its species name "millefolium," which means thousand-leaved. This refers to the cut, ferny appearance of its foliage. The flowers are a flat cluster with dozens of small disk flowers on several bracts. While the weedy sort are usually white, pink, red and other colors have been cultivated.
Yarrow is highly drought tolerant and prefers poor soils, which is undoubtedly why it thrives in my prairie patch. This trait aids in its invasion of disturbed prairieland as well as vacant lots. It does not care for damp soil and can suffer from mildew. This gives me hope for the lawn, as the part of the yard sloping down from the bank is not well-drained and likely to be inhospitable to the yarrow. It would do a lot better to stay in the prairie patch and leave the grass alone.
1 Ladd, Doug, and Oberle, Frank, Tallgrass Prairie Wildflowers, 2nd edition, (Morris Book Publishing LLC, 2005) pg 164.