Evening Primrose (Oenothera species) is a common sight in many gardens, often prized for its ability to thrive in poor soils and attract nocturnal pollinators. However, this popular plant frequently spreads rapidly and uncontrollably, leading to confusion about its classification. This prompts the question of whether it is truly an invasive species or simply an aggressive one. The nature of its growth is rooted in its reproductive strategies and the specific variety planted.
Understanding Aggressive Growth Terminology
The distinction between an aggressive plant and an invasive plant is important in gardening and ecological science. An aggressive plant is typically defined as one that spreads rapidly within a controlled environment, such as a garden bed, often outcompeting nearby plants for resources and space. This plant may be a nuisance to the gardener, but its spread is largely confined to the cultivated area.
In contrast, an invasive species is a non-native plant introduced to an ecosystem that causes, or is likely to cause, environmental or economic harm. For a plant to be legally categorized as invasive, it must meet two criteria: it must be non-native to the area under consideration, and its introduction must threaten the local ecosystem or economy. Since many Oenothera species, including the common Evening Primrose (Oenothera biennis), are native to North America, they cannot be classified as ecologically invasive in those regions. Evening Primrose is overwhelmingly considered an aggressive or weedy plant, rather than a true invasive species.
The Mechanisms of Spread
The plant’s reputation for aggressive spreading is due to a combination of two highly effective biological mechanisms: prolific seed production and, in some varieties, extensive root systems.
The common Evening Primrose, Oenothera biennis, is a biennial that relies almost entirely on self-seeding for colony expansion. This species produces numerous seed capsules, with a single plant capable of generating between 20,000 and 120,000 seeds. These tiny seeds are easily dispersed by wind, water, and even on the fur of animals, allowing them to colonize new areas quickly. Furthermore, the seeds can remain viable in the soil for a long period, with some reports suggesting they can persist in the soil seed bank for more than 70 years.
This aggressive behavior is further complicated by the growth habits of perennial species, such as the Showy Evening Primrose (Oenothera speciosa). Unlike the biennial varieties that possess a deep, singular taproot, these perennial types spread aggressively via underground stems called rhizomes or runners. These rhizomes creep horizontally, sending up new shoots and forming dense colonies that rapidly colonize large patches of garden space. This difference in root structure is essential, as the rhizomatous varieties are often the source of the most frustrating, hard-to-control spread.
Controlling Evening Primrose in the Garden
Preventing Seed Spread
The single most effective preventative measure for all varieties is diligent deadheading, which is the removal of spent flowers before they can form mature seed capsules. Since the biennial varieties rely heavily on self-seeding, preventing this seed set stops the next generation from emerging.
Physical Removal
Physical removal is the most reliable method for established plants, but it must be thorough to prevent regrowth. When dealing with biennial species like Oenothera biennis, the entire deep taproot must be extracted. For the perennial, rhizomatous types, every piece of the underground runner must be removed, as even a small fragment can sprout into a new, independent plant.
Containment Strategies
Gardeners can also employ active containment strategies to limit the spread of rhizomatous varieties from the start. Planting these aggressive types in containers, even if the containers are buried, effectively restricts the lateral movement of the rhizomes. Installing a physical root barrier, such as deep plastic edging or corrugated fiberglass, can also interrupt the horizontal spread of the underground stems.