Aster family (Asteraceae)
Description: Butterweed is a winter annual or biennial that initially forms a low rosette of basal leaves. During the spring, this plant bolts to become 1-3' tall; it is unbranched, except for short flowering stems in the axils of some upper leaves along the central stem. The hollow central stem is stout, light green or reddish green, and glabrous; it has conspicuous longitudinal veins. The alternate leaves are up to 10" long and 2½" across, becoming smaller as they ascend the central stem and any lateral stems. These leaves are either simple-pinnate or deeply pinnatifid, usually with a larger terminal leaflet (or lobe) and smaller lateral leaflets (or lobes). These leaflets or lobes are highly irregular in shape, although the lateral leaflets or lobes tend to be obovate in shape, while the terminal leaflet or lobe tends to be orbicular in shape. In addition to the irregular leaflets or lobes, the leaf margins are coarsely dentate, providing the leaves with a ragged appearance. Leaf surfaces are mostly pale green and glabrous, although the leaf midribs are often reddish green. The central stem and any lateral stems terminate in flat-headed panicles of flowerheads. These flowerheads are bunched together initially, but they become more spread out with age, forming an inflorescence up to 6" across.
Each flowerhead is about ½" across, consisting of 5-15 ray florets that surround numerous disk florets in the center. The petaloid rays of the flowerhead are yellow and narrowly oblong in shape, while the tiny tubular corollas of the disk florets are golden yellow. Around the base of each flowerhead, there are floral bracts (phyllaries) in a single series. These bracts are light green, linear in shape, and glabrous. The blooming period occurs from mid-spring to early summer, lasting about 1-2 months. The pleasant floral scent of the flowerheads resembles the fragrance of buttercups. After the blooming period, the flowerheads are replaced by achenes with small tufts of white hair. These achenes are distributed by the wind and probably water. By mid- to late summer, Butterweed dies down. The root system is shallow and fibrous.
Cultivation: The preference is full to partial sun, wet to moist conditions, and a rich loamy soil. This plant is easy to grow, and it can volunteer in unexpected places. Most growth and development occurs during the moist cool weather of spring. This plant will survive temporary flooding. It is somewhat weedy.
Range & Habitat: Butterweed is native to southern Illinois, but after several decades it has migrated northward, becoming established in central Illinois. As a result, this plant is common in southern and central Illinois, but it is still uncommon or absent in the northern section of the state (see Distribution Map). Habitats include wet to moist areas in open floodplain forests, soggy meadows along rivers, prairie swales, low areas along ponds and streams, swamps and seeps, ditches, agricultural fields, grassy areas that are not regularly mowed, and waste areas. Sometimes, this plant can form large colonies in disturbed areas, especially in agricultural fields during a wet spring (which delays the planting of crops). Butterweed is more common in wetland areas than either prairies or woodlands.
Faunal Associations: Primarily small bees and various flies visit the flowers for nectar or pollen. Less common visitors include small butterflies and skippers. An Andrenid bee, Andrena gardineri, is a specialist pollinator, or oligolege, of native Senecio spp. The larvae of a moth, Phyllocnistis insignis, are serpentine miners of the leaves. Other insect feeders include a seed bug, Neacoryphus bicrucis, and an aphid, Aphis lugentis. Like other native Senecio spp., the foliage of Butterweed probably contains pyrrolizidine alkaloids, which are highly toxic to the mammalian liver. While White-Tail Deer seem to avoid this plant, while domestic livestock sometimes consume it, thereby poisoning themselves. The Cottontail Rabbit usually avoids this plant, although it may nibble on the lower leaves occasionally during the spring.
Photographic Location: The photographs were taken of a plant growing in the lawn of the webmaster's apartment complex in Urbana, Illinois.
Comments: Butterweed is the weediest native Senecio spp. in Illinois. It is more tall and coarse in appearance than these other species, which are short-lived perennials. In particular, the hollow central stem of Butterweed stands out as being more stout. Because Yellow Rocket (Barbarea vulgaris) is sometimes abundant in agricultural fields during the spring, it is possible to confuse this plant with Butterweed, especially when such plants are viewed from a distance. The inflorescences of Butterweed are more flat-topped and its flowerheads are a deeper shade of yellow, while the inflorescences of Yellow Rocket have a more vertical structure and its flowers are pale yellow. These two plants are not closely related, as Yellow Rocket is a member of the Mustard family (Brassicaceae).