Aster family (Asteraceae)
Description: This perennial wildflower is about 1-2' tall. It is unbranched, except near the apex, where the flowerheads occur. The central stem is pale green and more or less covered with white cobwebby hairs. The alternate leaves are up to 6" long and 1" across, becoming slightly smaller as they ascend the stems. Each fern-like leaf is pale to medium green, elliptic in outline, and widest in the middle – however, its structure is either simple-pinnate or double-pinnate and its overlapping leaflets are either simple-pinnate or pinnatifid. The leaves and sometimes their leaflets (when they are simple-pinnate) are upward-angled along their rachises (central stalks), while pinnatifid leaflets and subleaflets are either curled, crinkled, or flat. Like the stems, the leaves and their subdivisions often have fine cobwebby hairs. The leaves are sessile.
The upper stems produce flat-headed panicles (compound corymbs) of small flowerheads. Each flowerhead is about ¼" across, consisting of 5 ray florets (their petaloid rays are white, rarely rose or other pastel colors) and a similar number of disk florets that have cream or pale yellow corollas. The petaloid rays are often slightly notched at their tips. The floral bracts (phyllaries) are pale green and lanceolate-oblong; they often have cobwebby hairs. All parts of this plant exude a distinctive aroma that is somewhat soapy and astringent. The blooming period occurs from early to mid-summer and lasts about a month. Each floret is replaced by an achene that is oblong and somewhat flattened; it lacks a tuft of hairs. The root system produces abundant rhizomes, often forming clonal colonies of plants.
Cultivation: The preference is full or partial sun, mesic to dry conditions, and a somewhat heavy clay-loam soil.
Range & Habitat: Yarrow is a common plant that has naturalized in all counties of Illinois (see Distribution Map). The variety of Yarrow that occurs in Illinois is probably native to Eurasia, although there is a variety of this plant that is native to western North America. This latter variety tends to be smaller in size and its foliage is more heavily covered with woolly hairs. Habitats include mesic to dry prairies, pastures, fallow fields, grassy waste areas, and edges of paths, yards, or hedges. Disturbed areas are preferred; Yarrow persists in native habitats (e.g., prairies) to a limited extent. Yarrow is often cultivated in flower and herbal gardens, from where it occasionally escapes.
Faunal Associations: The nectar of the flowers attracts many kinds of insects, especially flies and wasps. Among the flies are such visitors as bee flies, Syrphid flies (including drone flies), thick-headed flies, Tachinid flies, flesh flies, Anthomyiid flies, and others. Halictid and other short-tongued bees occasionally visitor the flowers, where they suck nectar and collect pollen. Many species of grasshoppers feed on Yarrow (see Grasshopper Table), as do several aphids, a seed bug, a flower thrips, leaf beetles, and caterpillars of some moths (see Insect Table). Sometimes Mordella spp. (Tumbling Flower Beetles) are found on the flowerheads. Because the foliage of Yarrow has a bitter and biting taste, it is rarely consumed by most mammalian herbivores. However, sheep will eat it when the opportunity arises.
Photographic Location: Judge Webber Park and Meadowbrook Park in Urbana, Illinois.
Comments: Among members of the Aster family, the fern-like foliage of Yarrow is rather unusual and it has a distinctive odor. Other members of the Aster family with this kind of foliage include Anthemis spp. (Mayweed), Matricaria spp. (Chamomile), and Tanacetum vulgare (Tansy). Unlike Yarrow, species of Mayweed and Chamomile produce daisy-like flowerheads with long petaloid rays. Tansy is a larger plant with medium to dark green foliage. While its flowerheads have a similar size and structure as compared to those of Yarrow, they are bright yellow and their petaloid rays are even smaller in size or absent.