From the perspective of the plant itself, the question of whether daylilies (Hemerocallis spp.) are invasive is a story of biological strategy and adaptation. The answer is not a simple yes or no, as it depends heavily on the specific type of daylily and the local environment. Understanding their plant biology is key to managing their spread effectively.
Many common daylily varieties, particularly the old-fashioned tawny orange daylily (Hemerocallis fulva), are exceptionally vigorous and can become invasive in a garden setting. From the plant's point of view, this is a tremendous success story. These plants have evolved multiple, highly effective strategies for propagation and survival. Their primary method is through robust, tuberous roots that form dense, spreading clumps. Each season, the plant's energy is directed not only into flowering but also into expanding its root system, sending out rhizomes that can travel several inches underground to establish new shoots. This vegetative reproduction allows a single plant to colonize a large area over a few years, outcompeting less aggressive neighbors for water, nutrients, and space.
Beyond root expansion, many common daylilies are also prolific seed producers. While some modern hybrids are sterile, species like Hemerocallis fulva are triploid, meaning they can produce seeds asexually (through apomixis) without pollination. This guarantees successful seed set. These seeds are then dispersed by wind, water, or wildlife, potentially establishing new colonies far from the original plant. Furthermore, some types, known as "stoloniferous" daylilies, produce above-ground runners (stolons) that creep along the soil surface, quickly forming new plantlets. This combination of underground rhizomes, prolific seeding, and above-ground runners makes these plants formidable spreaders from a purely botanical perspective.
It is crucial to distinguish these aggressive spreaders from the vast majority of modern hybrid daylilies. From the plant's perspective, these cultivars have been selectively bred by humans for specific traits, often at the expense of their natural reproductive vigor. Most popular garden hybrids are "clump-forming." Their growth habit is focused on enlarging the central crown rather than spreading via far-reaching rhizomes. More importantly, many are sterile or produce very few viable seeds. Their biological imperative to reproduce has been intentionally curtailed through breeding, making them well-behaved, non-invasive garden residents that will stay put for years.
In regions where common daylilies have escaped cultivation, they can become ecological problems, forming dense monocultures that shade out and displace native herbaceous plants. From the plant's perspective, its success lies in several key advantages: high drought tolerance, adaptability to various soil types, and a general lack of serious pests or diseases. The dense mat of roots and foliage creates a physical barrier that prevents other plants from establishing. This competitive edge, combined with its efficient reproductive systems, allows it to dominate an ecosystem, reducing local biodiversity.
To control the spread of invasive daylilies, one must work with their biology, not against it. The most effective method is physical removal, which involves digging up the entire clump, including all tuberous roots. Any small root fragments left in the soil can regenerate, as the plant's survival instinct is to regrow from any available tissue. For ongoing management in a mixed border, planting clump-forming daylilies within a sunken bottomless container or a sturdy root barrier can physically restrict the horizontal growth of the rhizomes, channeling the plant's energy upward instead of outward. Regularly deadheading spent flowers before they form seed pods directly interrupts the plant's seed-based reproductive cycle, preventing unwanted self-sowing.
