The confusion surrounding the spread of “lilies” is common in gardening due to their reputation for rapid growth. While many gardeners worry about a plant taking over a flowerbed, the answer to whether lilies are truly invasive is complex. It depends heavily on the specific plant and requires clarifying the difference between a garden pest and a true ecological threat.
Defining Aggressive Growth and True Invasiveness
A fundamental distinction exists between a plant that is merely aggressive and one that is truly invasive. An aggressive plant spreads rapidly within a cultivated garden space, often through runners, rhizomes, or prolific self-seeding, making it a nuisance for the gardener. This growth is generally contained to the immediate landscape and presents a management problem rather than an ecological one. A plant is defined as truly invasive only when it is non-native to a specific ecosystem and causes or is likely to cause environmental or economic harm. This harm occurs when the plant outcompetes native flora for resources, reducing biodiversity and altering the ecosystem’s integrity. Non-native status and documented ecological harm are the two conditions that must be met for a plant to be officially classified as invasive in a particular region.
The Botanical Identity Crisis of the “Lily”
The primary source of confusion regarding a lily’s spread stems from a botanical identity crisis, as many plants with “lily” in their common name are not true lilies. True lilies belong exclusively to the genus Lilium, which are herbaceous flowering plants that grow from scaly underground bulbs. These Lilium species, such as the Easter lily or the Turk’s cap lily, are generally not considered invasive. However, some can be aggressive garden spreaders through the production of small bulb offsets or rhizomes. The plants that cause the most worry are often not true lilies at all, such as the Daylily (Hemerocallis), Calla Lily (Zantedeschia), and Peace Lily (Spathiphyllum). These plants share the common name but belong to different families and genera, like the Daylily being in the family Asphodelaceae, not the true lily family, Liliaceae.
Specific Species Known for Overgrowth
The most notorious species for aggressive spread is the Orange Daylily, or Ditch Lily (Hemerocallis fulva). Although widely grown as an ornamental, this non-native species introduced from Asia has escaped cultivation across much of the United States and Canada. The Orange Daylily is listed as invasive in many mid-Atlantic states because it forms dense patches that displace native vegetation in meadows, floodplains, and along roadsides. This species spreads primarily through thick, tuberous roots and rhizomes, which are subterranean stems that send up new shoots. Unlike many modern hybrid daylilies, which are often clump-forming, Hemerocallis fulva is highly effective at vegetative increase, allowing it to rapidly colonize new areas. True Lilium species, while sometimes aggressive, typically spread more slowly from bulb offsets. Another aggressive spreader is Lily of the Valley (Convallaria majalis), which is not a true lily but spreads relentlessly via rhizomes in the garden, sometimes escaping into natural areas.
Controlling Lilies in the Garden
Managing aggressive lily-like plants requires different strategies depending on their underground structure. For rhizomatous spreaders like the Orange Daylily, mechanical removal is the most effective method, but it demands meticulous attention. All parts of the thick, tuberous roots and rhizomes must be removed, as the plant easily regenerates from small pieces left behind. Digging out the entire root system is the goal, often requiring the use of a shovel to loosen the soil around the plant.
Another method for controlling large infestations is solarization, which involves cutting the foliage low and covering the area with thick black or clear plastic for an entire growing season. This process cooks the rhizomes by depriving them of light and trapping heat. For true lilies that spread via bulbs, dividing the clump every few years and removing the offsets is often sufficient. Preventing the spread of all aggressive varieties can also be achieved by planting them inside a bottomless container, which acts as a physical barrier to underground runners.