The Rudbeckia are herbaceous plants, with mostly perennial species, but with some annual or biennial species. They grow from 0.5 to 3 m tall, with simple or branched stems. They are mostly fibrous rooted or rhizomatous, and, sometimes,taprooted.
The leaves are alternate, they are entire to deeply lobed, measuring from 5 to 25 cm in length.
The flowers are usally produced in daisy-like inflorescences, arranged with a prominent, cone-shaped array of disk flowers (hence one of the vernacular name : coneflower) surrounded by bright ray flowers. Each inflorescence has up to 20 phyllaries in one, two and eventually 3 series. The ray florets, when present, are not fertile. The disc florets are numerous, bisexual and fertile. Rudbeckia can bloom as early as July in Québec and usually maintain their brilliant colors right through to September and, at times, even to close to the end of October.
Formerly a large number of species have been proposed within the Rudbeckia genus but most are now regarded as synonyms of a more limited list. Botanists now recognize 23 species, all native to North America with several species introduced in Europe. In North America, most species are concentrated east of the Rocky Mountains.
Several of the currently accepted species have several accepted varieties. Some of them, for example the Black-eyed Susan or Blackeyed Susan (Rudbeckia hirta) picture above, are popular garden flowers, distinguished for their long flowering times. There are also many cultivars of these species.
Two species of Rudbeckia are apomictic, Rudbeckia triloba and Rudbeckia laciniata while Rudbeckia hirta is an obligate outbreeder. Little evidence of interspecific hybridization exists even though colonies of two or more species may frequently occur in close proximity to one another.
1. If the leaves are bluish green,
glaucous and the ray florets are bright
yellow it is Rudbeckia laciniata var. laciniata. For the other
three species, the leaves green, not glaucous; ray florets yellow-orange.
2. If the plant is stoloniferous with rosettes forming at stolon apices the species is Rudbeckia fulgida var. speciosa.
3. If the leaves are three lobed the species is obviously Rudbeckia triloba var. triloba.
4. And if the plant does not have the above characters the species is then Rudbeckia hirta var. pulcherrima. This last species is in fact the most widespread and most common in Québec; that is, if in Québec you come across a Rudbeckia, it is likely to be Rudbeckia hirta var. pulcherrima.
Rudbeckia hirta and sometimes other species of the genus are used in experimental studies relating to initiation of flowering and hairy root culture. Most species are rich sources of phytochemicals that may offer potential for pharmaceutical or other uses.
The flowers and stems of Rudbeckia hirta can be used to make dyes .
Some birds and small mammals species occasionally consume Rudbeckia seeds. Rudbeckia species are used as food plants by the larvae of some Lepidoptera species including the Cabbage Moth (Mamestra brassicae) and the Dot Moth (Melanchra persicariae).