Black-Eyed Susans (Rudbeckia) are popular garden flowers often questioned for their spreading habits. These plants are aggressive in a garden setting, but they are not invasive in the ecological sense. Rudbeckia hirta, the most common species, is native to a wide range of North America. This native status means they support local ecosystems rather than causing the environmental harm associated with true invasive species. Gardeners often perceive them as invasive due to their vigorous growth and tendency to appear unexpectedly throughout the cultivated landscape.
Defining True Invasiveness Versus Aggressiveness
The distinction between an aggressive plant and a formally invasive species lies in the plant’s origin and its impact on the natural environment. An invasive species is defined by ecologists and regulatory bodies as a species that is non-native to the ecosystem under consideration. Its introduction causes or is likely to cause environmental or economic harm. These non-native plants displace local flora, disrupt food webs, and can dramatically alter habitats.
Black-Eyed Susans are native to North America, meaning they cannot meet the non-native criterion required for ecological invasiveness. Rudbeckia hirta is found across nearly all of the contiguous United States and parts of Canada, thriving in prairies, meadows, and along woodland edges. The plant’s vigorous spreading within a garden bed is simply a manifestation of its natural, highly successful reproductive strategy. Gardeners use the term “aggressive” to describe this tendency of a plant to quickly spread or self-seed beyond its intended boundaries, essentially acting as a garden “weed.”
Mechanisms of Garden Spread
The reputation for “aggressiveness” in Rudbeckia is directly linked to its highly effective reproductive methods. The primary mechanism, especially for the common annual or biennial Rudbeckia hirta, is prolific self-seeding. Each spent flower head produces numerous viable seeds that drop to the soil and germinate easily, often resulting in a dense carpet of seedlings the following season.
This vigorous seed production ensures the species’ continuation, especially since R. hirta is a short-lived plant that may only last one or two seasons. Perennial varieties, such as Rudbeckia fulgida ‘Goldsturm’, employ a dual strategy for expansion. While they still self-seed to some degree, their primary method of spread is through the formation of short underground stems called rhizomes.
These rhizomes grow horizontally just beneath the soil surface, sending up new shoots and creating an ever-expanding, dense clump. This root-based spread allows perennial types to gradually take over adjacent planting space. The combination of self-seeding and rhizomatous growth makes perennial species challenging to manage in a mixed border garden.
Controlling Growth in Cultivated Areas
Gardeners can manage the aggressive nature of Black-Eyed Susans using targeted cultural practices. The most effective method for controlling self-seeding is deadheading, which involves removing spent flower heads before the seeds mature and drop. Cutting off the flowers immediately after they fade prevents the plant from depositing seeds into the soil for the next growing season.
For perennial varieties that spread via rhizomes, such as ‘Goldsturm’, the best control strategy is regular division. Digging up and dividing the clumps every few years, typically in early spring, reduces the overall size and vigor of the plant mass. This process prevents the center of the clump from dying out and ensures the plant remains contained. Strategic planting can also help, as placing Black-Eyed Susans in an area where seedlings are easily tolerated or where a border can slow underground runners will minimize maintenance.