Daylilies (Hemerocallis spp.) are a common sight in gardens across the United States, prized for their vibrant, trumpet-shaped flowers and remarkable resilience. The question of their invasiveness, however, requires a nuanced answer that distinguishes between the common hybrid cultivars and a specific, aggressive species.
The vast majority of daylilies found in American gardens are complex hybrids. From a botanical perspective, these cultivated varieties are largely sterile triploids; they produce very few, if any, viable seeds. Their primary method of spread is through vegetative means, specifically the slow, gradual expansion of their tuberous roots and rhizomes. A single clump will get larger and denser each year, but it does not typically spread far from the original planting. These plants lack the biological mechanisms for rapid, long-distance dispersal that characterize truly invasive species. They are not considered invasive and are generally well-contained within a garden setting.
The significant exception is the old-fashioned orange daylily, Hemerocallis fulva, often called the 'ditch lily' or tawny daylily. This specific species is a vigorous, diploid plant that reproduces both vegetatively and, crucially, by seed. Its biology makes it far more aggressive. It forms dense, impenetrable clumps through its robust rhizome system. Furthermore, unlike most hybrids, H. fulva can produce viable seeds, though its primary colonization method remains vegetative. It is frequently found naturalized along roadsides, in ditches, and in abandoned fields, where it can outcompete native vegetation and form monocultures.
While H. fulva exhibits invasive *behavior* in many parts of the country, its official status is complex. It is not federally listed as a noxious weed by the USDA. However, several states, including Pennsylvania, Virginia, and Wisconsin, have recognized its potential for ecological disruption and have listed it on their invasive plant advisories or watchlists. The primary ecological concern is its ability to form dense stands that displace native herbaceous plants, reduce local biodiversity, and alter the habitat structure for insects and other wildlife. It is not typically a threat to healthy, intact forest ecosystems but thrives in disturbed areas and edges.
Several plant traits contribute to the potential invasiveness of H. fulva. Its rhizomatous root system allows for efficient nutrient storage and the generation of new shoots, enabling a single plant to cover a large area over time. The plant is also exceptionally adaptable, tolerating a wide range of soil conditions, pH levels, drought, and pollution. It is a highly successful competitor for space, sunlight, and soil resources. While it does not spread via runners like some grasses, the mechanical movement of soil containing even small fragments of its hardy rhizomes can lead to new infestations.
