The primary difference lies in fundamental taxonomy. Rudbeckia hirta is a distinct species within the genus Rudbeckia. When gardeners refer to "Black-eyed Susan," they are often using a common name that can apply to several species, but R. hirta is the most widely recognized and classic bearer of the name. Other popular varieties belong to different, though closely related, species. For example, the well-known cultivar 'Goldsturm' is a selection of Rudbeckia fulgida. Another common garden plant is Rudbeckia triloba. While all are Rudbeckia, they are separate species with inherent biological differences that influence their appearance and behavior in the garden.
This is a critical distinction from a horticultural perspective. Rudbeckia hirta is typically a short-lived perennial, often behaving as a biennial or even an annual depending on the climate. It completes its life cycle relatively quickly, flowering profusely in its first or second year but often dying out after 2-3 years. In contrast, many other Black-eyed Susan varieties are true, long-lived perennials. Rudbeckia fulgida and its cultivars, for instance, form robust, spreading clumps that persist for many years, reliably returning each season. Rudbeckia maxima (Great Coneflower) is another long-lived perennial species. This difference in life cycle means R. hirta may require more frequent replanting or may self-seed to maintain a presence, while others provide a more permanent garden structure.
Close examination of the leaves and stems reveals clear distinctions. Rudbeckia hirta is characterized by its specific epithet "hirta," which means hairy. Its stems and leaves are covered in dense, bristly hairs (trichomes), giving the plant a rough, sandpapery texture. The leaves are typically lance-shaped to ovate, with minimal lobing. Conversely, Rudbeckia fulgida has glossier, more elongated leaves with a less rough texture. Rudbeckia triloba is named for its foliage, which often (but not always) has a three-lobed pattern on the lower leaves. Rudbeckia laciniata (Cutleaf Coneflower) has deeply dissected, pinnately lobed leaves that are vastly different from the simple leaves of R. hirta.
While all Black-eyed Susans feature the classic daisy-like form with a central cone (the disc florets) surrounded by ray florets ("petals"), the details vary. The flowers of Rudbeckia hirta are generally large, often 3-4 inches in diameter, with bright golden-yellow ray florets that tend to be slightly drooping. The central cone is typically dark brown to black and prominently domed. In comparison, Rudbeckia fulgida var. sullivantii 'Goldsturm' has slightly smaller flowers (2-3 inches) that are held more upright, and the central cone is a darker, almost black-brown. Rudbeckia maxima has a uniquely large, protruding cylindrical brown cone that can be 2-3 inches tall, with drooping yellow ray florets, making its flower architecture quite distinct.
The overall shape and spreading habit of the plants differ significantly. Rudbeckia hirta often grows as a more upright, singular clump with multiple flowering stems. It may self-seed readily, creating a casual, naturalized look. In contrast, Rudbeckia fulgida is a strongly rhizomatous perennial. It spreads steadily by underground stems (rhizomes) to form dense, expanding colonies that are excellent for ground cover. This makes it more aggressive but also very effective for filling space. Rudbeckia triloba has a bushier, more branching habit and can become quite tall and wide, producing a profusion of smaller flowers, giving it a different architectural presence in the border.
