Common yarrow (Achillea millefolium)
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Common yarrow is a perennial broadleaf plant with an extensive system of underground creeping stems. This species is comprised of a complex of highly variable biotypes and is found throughout California, except in deserts, to 11,500 feet (3500 m). It inhabits turf, landscaped areas, and pastures. In mowed situations, such as turf, flowers may not be observed. Common yarrow occurs in cool climates and in areas of frequent irrigation.
Usually common yarrow is considered a desirable member of natural communities. Some cultivars are grown as ornamentals in landscapes. However, because of its creeping habit and ability to establish seedlings, it can be weedy in landscaped areas and turf.
Common yarrow is found in many plant communities such as those in coastal bluffs, sand dune, salt marshes, grasslands, semi-deserts, and alpine areas.
Cotyledons (seed leaves) are oblong to egg shaped, about 1/8 to 1/6 of an inch (3–4 mm) long and roughly 1/12 of an inch (2 mm) wide. The first true leaves are divided with sharply pointed teeth. Leaves are alternate to one another along the stem. The first leaf is deeply cut into linear segments. Common yarrow seedlings look like those of wild carrot, Daucus carota. At the young plant stage when they form into rosettes, differences in leaf hairs can be used to easily distinguish the two plants (see “Young plant”).
Until the flower stem develops in late spring or summer, plants exist as rosettes. Leaves of young plants have a fernlike or feathery quality. Although similar as seedlings, common yarrow rosette leaf hairs lay mostly flat against the leaf surface, whereas wild carrot rosette leaves are either hairless or short and bristly.
Mature plants grow low to the ground in clumps. The flowering stem is usually about 3-1/3 feet (1 m) tall. Leaves are finely cut into linear segments giving them a feathery appearance and are hairless or have soft hairs. Common yarrow can be distinguished from other weeds that have finely cut leaves by its creeping belowground horizontal stems. Rosette leaves and leaves low on the stem are about 4 to 8 inches (10–20 cm) long, while leaves above are smaller, stalkless, and clasp the stem.
Bloom takes place from March through September. Small white or yellow flowers cluster into dense, flat-topped flower heads at the ends of stems.
Fruits are roughly 1/12 of an inch (2 mm) long, narrowly oblong to lance shaped, flattened, ribbed longitudinally and do not open at maturity. Each fruit (achene) contains one seed.
Common yarrow reproduces by seed and by sprouting from underground horizontal, creeping stems (rhizomes).