The Daylily (Hemerocallis genus) is an exceptionally popular perennial, celebrated by gardeners for its vibrant, trumpet-shaped flowers and renowned hardiness. These plants are native to Asia and are known for their ability to thrive in a wide range of climates and soil conditions, often requiring minimal care once established. Their vigor and ability to persist frequently leads to questions about their potential to spread unchecked in a garden setting. This confusion hinges on the difference between a plant that is merely assertive in the garden and one that poses a genuine threat to natural ecosystems. This article clarifies the status of daylilies and details the one species that causes environmental concern.
Aggressive Gardeners vs. Ecological Invaders
The question of daylily spread requires a clear distinction between the terms “aggressive” and “invasive” in a botanical context. An aggressive plant spreads rapidly and enthusiastically within a cultivated space, often outcompeting its immediate neighbors for resources like water and light. This behavior is generally a horticultural nuisance that requires regular maintenance, such as dividing the plant clump every few years to keep it in check. Most modern daylily hybrids, like the popular ‘Stella D’oro,’ fall into this aggressive category, forming dense, self-propagating clumps.
An invasive plant, conversely, is defined by two specific criteria: it must be non-native to the ecosystem under consideration, and its introduction must cause environmental or economic harm. These species escape cultivation and establish themselves in natural areas, where they displace native flora, reduce biodiversity, and alter the ecological function of the habitat. While all invasive plants are aggressive, modern hybrid daylilies do not possess the characteristics to spread beyond the garden and cause widespread environmental damage.
The Specific Concern: Hemerocallis fulva
The species that gives daylilies their reputation for being invasive is Hemerocallis fulva, commonly known as the Tawny, Orange, or Common Daylily. This plant, introduced to North America from Asia in the late 19th century as an ornamental, has proven to be a true ecological threat in many regions. It is the classic orange daylily often seen growing in large, uncontrolled patches along roadsides, ditches, and abandoned homesteads.
The spread of H. fulva is rooted in its unique underground structure. It spreads primarily through a network of thick, fleshy, tuberous roots and rhizomes, not just by seed. These underground storage organs allow the plant to store massive amounts of energy, creating dense mats that effectively smother native groundcover and prevent the establishment of other species.
Once established in natural areas like meadows, floodplains, and forest edges, H. fulva is exceptionally difficult to remove. Its rapid, vegetative reproduction through rhizomes and tubers means that even a small fragment left behind can regenerate an entirely new plant. The species is listed as invasive or noxious in certain areas, particularly across the Northeast and Midwest United States, where it actively displaces native flora.
Managing and Removing Unwanted Daylily Clumps
Controlling overly aggressive daylilies, whether hybrid clumps or established stands of H. fulva, requires persistence and a focused approach on the root system. The most effective method for controlling small to moderate clumps is manual removal, which must be extremely thorough. Gardeners need to dig up the entire plant clump, taking care to sift through the soil and remove all fleshy tubers and rhizomes. Any root fragments left in the ground will likely sprout new plants, requiring the process to be repeated as new growth appears over several weeks.
To contain aggressive hybrids from spreading into lawn areas or neighboring plants, a physical root barrier can be installed around the planting area. This barrier should be made of a durable material and extend at least a few inches below the soil surface to block underground growth.
For very large, established stands of Tawny Daylily where manual digging is impractical, chemical control may be the only option. Systemic, non-selective herbicides can be applied carefully to the foliage, which is then transported to the extensive root system. This method should be a last resort due to the potential for harm to surrounding plants.