Yarrow is a plant whose habits are rather variable. One or several stiff stems may grow from the root. They are usually between one and two feet tall but are occasionally shorter, and may be smooth or rough-textured. The leaves are larger at the base and progressively smaller toward the top of the stalks and are arranged alternately. They clasp the stems at their bases and are delicate and finely divided, resembling feathers more than leaves Yarrow blooms from June to September in the eastern part of the United States. Flowers are in flat-topped clusters at the ends of the stems. The individual "flowers" are very small, with fine white "petals" and a yellowish center. This "flower" is two separate, distinct male and female flowers. The female flowers are in the yellow center surrounded by five white "petals," each one of which is a male flower. Yarrow is a perennial, reproducing by seeds and from underground runners. It is native and widespread in the United States and grows throughout Europe and Asia as well. There is a variety of Yarrow that has beautiful purple flowers. It is grown in gardens and is called Ornamental Yarrow. This color is sometimes (but rarely) found in wild plants as well.
Yarrow herb - Achillea millefolium - A City Herbal by .., http://www.ashtreepublishing.com/Book_City_Herbal_Yarrow.htm (accessed January 19, 2017).