Coneflowers are a diverse group of flowering plants in the Asteraceae family, all of which feature a dome-shaped central cone that serves as a seed head. The plants are separated into three genera — Echinacea, Rudbeckia and Ratibida. There is, however, some crossover between the genera — several species are included in two categories. Pale purple coneflower, for example, is known as both Rudbeckia pallida and Echinacea pallida.
According to the U.S. Department of Agriculture, the Asteraceae family includes 477 genera. The original name of the family was Compositae, and the two terms are used interchangeably. Asteraceae is commonly known as the sunflower, aster or daisy family. The inflorescence (flowering part) of Asteraceae plants is somewhat deceptive. While the head appears to be a single flower, it is made up of many small flowers (florets). In the center of most Asteraceae are rows of tubular florets that do not appear to be flowers in the typical sense. In the outer ring is a border of ray florets, each with a single strap-like petal. Taken as a whole, they appear to be a series of ray-like petals on a single flower.
There are nine species within the genus Echinacea, including E. angustifolia (blacksamson), E. atrorubens (Topeka purple), E. laevigata (smooth purple), E. pallida (pale purple), E. paradoxa (Bush’s purple), E. purpurea (eastern purple), E. sanguine (sanguine purple), E. simulate (wavyleaf purple) and E. tennesseensis (Tennessee purple).
The name Echinacea reflects the Greek word for hedgehog (echinos), which the bristly cone is said to resemble. All Echinacea species are perennials. The petals range in color from white to pink to deep purple, with one yellow variety (E. paradoxa). The flower heads reach a diameter of 4 inches, and plants of some species grow to 4 feet tall.
Cultivars of E. purpurea have been developed for ornamental use. Three of the species, E. angustifolia, E. pallida and E. purpurea, have been investigated for their potential for medicinal purposes.
The genus Ratibida, commonly known as the prairie coneflower, includes four species — R. columnifera (upright prairie), R. peduncularis (naked Mexicanhat), R. pinnata (pinnate prairie) and R. tagetes (green prairie). Prairie coneflowers are heat-tolerant wildflowers that grow readily in dry locations. The petals curve downward, and the cone is taller and narrower than that of Echinacea. Ratibida blooms from June to September, displaying petals in yellow, brownish-red or a combination of the two colors.
The largest genus of the three, Rudbeckia (black-eyed Susan) is comprised of more than 20 species, including annual, biennial and perennial varieties. The cones are dark brown or black and the color of the petals ranges from yellow to gold. Black-eyed Susan species are heat- and drought-tolerant and are equally at home in meadows and urban flower borders, blooming from mid-summer to fall. Some species grow as tall as 10 feet, making them suitable as a backdrop for other flowers.