The primary difference between Black-eyed Susans and Brown-eyed Susans lies in their botanical classification. While both belong to the Asteraceae (daisy or sunflower) family, they are distinct species within the same genus, Rudbeckia. The most common "Black-eyed Susan" is scientifically known as Rudbeckia hirta. This species is a short-lived perennial or biennial, often behaving as an annual. In contrast, the "Brown-eyed Susan" typically refers to Rudbeckia triloba. This distinction is crucial, as it explains the variations in their physical characteristics, growth habits, and life cycles.
The common names are derived from the appearance of the central disc florets, but the plants differ in several other morphological aspects.
Flowers: Both species feature the classic daisy-like form with yellow-orange ray florets (the "petals") surrounding a central cone. However, the cone of Rudbeckia hirta (Black-eyed Susan) is typically a dark brown, almost black, prominent, and dome-shaped. The cone of Rudbeckia triloba (Brown-eyed Susan) is a smaller, more rounded, and a softer reddish-brown to purple-brown, giving it a "brown-eyed" appearance. The ray florets of R. hirta are often a bright, solid yellow, while those of R. triloba can be a more golden-yellow and sometimes droop slightly.
Foliage and Stature: This is a key identifying feature. Rudbeckia hirta has lance-shaped or oblong leaves that are coarse and hairy (the species name "hirta" means hairy). The plant forms a clump and typically grows 1 to 3 feet tall. Rudbeckia triloba has a different leaf structure, as indicated by its name "triloba," meaning three-lobed. Its lower leaves are often deeply divided into three lobes, while upper leaves may be simpler and more oval. The plant is more branched and airy, often growing taller, reaching 2 to 5 feet, and can have a somewhat weedy, sprawling habit.
The life cycle and growth behavior of these two Rudbeckia species are markedly different. Rudbeckia hirta is generally a short-lived perennial. In many gardens, it may complete its life cycle in two years (biennial) or even a single season (annual), especially if deadheaded. It spreads primarily by self-seeding, forming robust clumps.
In contrast, Rudbeckia triloba is a true short-lived perennial but often acts as a vigorous self-seeding biennial. It is known for its prolific seeding, which can cause it to naturalize readily in meadows and open areas. Its branching habit gives it a lighter, more informal presence in the landscape compared to the bolder clumps of R. hirta.
Both species are native to North America and are valued for their adaptability and ease of care in sunny locations with well-drained soil. However, there are subtle differences in their preferences and hardiness. Rudbeckia hirta is extremely drought-tolerant and thrives in hot, dry conditions. It is hardy in USDA zones 3-9, making it adaptable to a wide range of climates.
Rudbeckia triloba is also tough but tends to prefer consistently moist soil, though it will tolerate some drought once established. It is often found in moister prairies and along woodland edges in its native habitat. It is hardy in zones 4-8. While both can grow in partial shade, R. triloba may perform better with a bit more moisture and can handle slightly shadier conditions than R. hirta, which blooms best in full sun.
