Common yarrow

Common yarrow is a creeping perennial with finely divided, feathery, fern-like leaves that release a strong, distinctive aroma when crushed. The alternate leaves are soft and intricately dissected and range from one to several inches long. Where it is allowed to grow tall it produces flat-topped clusters of small daisy-like flowers, usually white but sometimes pink to pale purple; in closely mown turf it rarely flowers and instead adapts to the cutting height, forming a low, dense, gray-green mat. The combination of soft, ferny, aromatic foliage and a spreading, mat-forming habit distinguishes yarrow from other lawn weeds.

Common yarrow forms dense, spreading mats of feathery foliage that crowd out turfgrass and create patches with a markedly different texture and gray-green color, disrupting lawn uniformity. Because the rhizomatous patches stay green and competitive during drought when surrounding turf goes dormant and brown, the weed becomes increasingly dominant under summer stress, and the extensive underground rhizome network makes the colonies expand outward and resist removal.

Common yarrow is a perennial that reproduces both by seed and, importantly, by an extensive network of horizontal underground stems (rhizomes) bearing fibrous roots. The rhizomes spread laterally and send up new shoots at intervals, forming dense, expanding patches or colonies, and roots can penetrate well into the soil profile. This vegetative spread, combined with seed production from flowering plants, makes yarrow persistent and difficult to eradicate, since fragments and rhizomes left in the soil can regenerate new plants.

Yarrow is highly tolerant of drought and poor, infertile soils, and it commonly invades thin, low-maintenance, and stressed turf where the desirable stand is weak. It adapts readily to low mowing heights by forming a flat mat, so close mowing does not eliminate it. It is especially conspicuous during hot, dry periods, when drought-stressed turf turns brown while the deep-rooted yarrow patches stay green, allowing the weed to expand into the weakened canopy through its spreading rhizomes.