Rudbeckia, commonly known as Black-eyed Susan, is a popular garden plant celebrated for its bright, long-lasting flowers. The genus includes species that function as annuals, biennials, and true perennials. Whether these plants spread depends entirely on the specific species planted and the mechanism of expansion it employs. A Rudbeckia’s ability to multiply can range from being easily contained to aggressively colonizing a space.
The Dual Mechanisms of Rudbeckia Spread
The expansion of a Black-eyed Susan population occurs through two primary biological methods: sexual reproduction via seeds and asexual reproduction through underground stems called rhizomes. Understanding both mechanisms is important for managing the plant’s presence in a cultivated setting.
Many Rudbeckia varieties are prolific self-seeders, producing numerous seeds that drop to the ground and germinate, leading to new “volunteer” plants the following season. This method is common in annuals or short-lived perennials, such as Rudbeckia hirta. The seeds, housed in the spent flower head, are often scattered randomly by wind or birds, causing new plants to appear unpredictably across the landscape.
Perennial species, such as Rudbeckia fulgida, primarily spread through short, creeping underground rhizomes. These horizontal stems extend outward from the main plant crown, establishing new roots and shoots. This causes the original clump to gradually thicken and expand its diameter.
This rhizomatous growth is a physical expansion of the same plant, resulting in a denser patch of foliage and flowers. While slower and more predictable than self-seeding, some varieties can be vigorous enough to crowd out neighboring plants if not managed periodically.
Species Differences in Garden Behavior
The behavior of Rudbeckia is strongly tied to its life cycle. The most common garden species, Rudbeckia hirta, is typically an annual or biennial, meaning the parent plant dies after flowering. This species relies entirely on prolific self-seeding to return each year, creating new plants that appear randomly across a wide area, which can make the population seem aggressive.
In contrast, popular perennial varieties, such as Rudbeckia fulgida ‘Goldsturm,’ exhibit clumping behavior. They form dense masses that spread slowly via short, compact rhizomes, resulting in a predictable expansion of the plant’s footprint. These perennials require division every few years to maintain vigor and prevent the center of the clump from dying out.
Practical Strategies for Managing Growth
Controlling the spread of Rudbeckia depends on targeting the specific reproductive mechanism used. For species that primarily spread by seed, such as R. hirta, the most effective strategy is deadheading. This involves removing spent flower heads before the seeds fully mature and drop, preventing the formation of unwanted volunteer plants the following season.
If the goal is to encourage a naturalized look or to feed local wildlife, some seed heads can be left standing through autumn and winter. For rhizomatous perennial types, like R. fulgida, growth is managed by physical division of the clump.
Division is best performed in early spring or late fall by digging up the entire root mass and using a sharp spade to divide it into smaller, healthier sections. Dividing clumps every three to five years manages their size and helps rejuvenate the plant, preventing the center from weakening or dying. Unwanted sprouts emerging from the edges can also be removed by hand-pulling to maintain a tidy border.