Even though yarrow has fine, delicate, fern-like leaves, it is a tough, durable perennial. It spreads by strong rhizomes and is an exceptionally aggressive weed in turf. Yarrow forms dense patches, hugs the ground and avoids the mower. When crushed, it has a pungent odor and bitter taste. The finely divided leaves are covered with soft hairs. Yarrow can reach a height of one to two feet at maturity. Flowers are formed in clusters at the tips of branching stems. They are composed of five tiny white ray petals with pale yellow disc flowers in the center.