Asiatic Lilies (Lilium asiaticum hybrids) are a group of popular, vibrant garden plants known for their upward-facing, colorful blossoms that appear early in the summer season. These lilies are widely cultivated across North America due to their ease of growth and striking appearance, adding a vertical element of color to perennial borders. While they are vigorous growers that can multiply quickly within a garden bed, Asiatic Lilies are generally not considered ecologically invasive, a term with a specific scientific and legal definition. Their robust growth habit is better described as aggressive clumping, which requires occasional management rather than environmental remediation.
The Difference Between Invasive and Aggressive
The confusion between an aggressive garden plant and an invasive species stems from similar language used to describe their spreading habit, but their environmental impact is vastly different. A plant is classified as “invasive” when it is non-native and its introduction causes or is likely to cause economic or environmental harm, or harm to human health. This definition is codified by government agencies and applies to species that escape cultivation, displace native flora, and disrupt the natural balance of an ecosystem. Examples include Kudzu or Purple Loosestrife, which spread far beyond the garden fence into natural areas.
An aggressive plant, in contrast, spreads rapidly and vigorously within a controlled garden setting, often outcompeting its immediate neighbors for resources. Aggressive plants can be native or non-native, and their spread is typically limited to the cultivated area. Asiatic lilies fall into this category because they form dense, overcrowded clumps, creating a maintenance problem for the homeowner. Managing an aggressive plant is generally routine gardening work, unlike controlling an invasive species which requires significant ecological intervention.
How Asiatic Lilies Propagate and Spread
The spreading nature of Asiatic Lilies is focused on the immediate area around the parent plant, driven primarily by asexual reproduction beneath the soil. They spread mainly through the production of underground bulb offsets, or bulblets, which form around the base of the main bulb. Over time, these bulblets mature into new flowering plants, leading to the dense clumping habit gardeners observe. The original bulb naturally splits every few years, resulting in continuous multiplication.
Another method of vegetative increase is bulb scaling, where small bulblets can form at the base of individual scales removed from the main bulb. While lilies can be grown from seed, many modern Asiatic lily hybrids produce seeds that are often sterile or non-viable. This limits their ability to escape the garden and spread into wild areas. The primary mechanism for the plant’s spread remains the gardener’s intentional division and replanting of the underground bulbs.
Practical Management of Clumping Lilies
Managing the growth of Asiatic Lilies is a routine part of garden maintenance necessary to prevent overcrowding and maintain flower quality. When a clump becomes too dense (typically every three to five years), the plants produce smaller blooms and fewer flowers due to competition for nutrients and space. The ideal time for division is in the early autumn, after the foliage has died back and the plant has entered a dormant state. This timing allows the newly separated bulbs to establish roots before winter.
Division Process
To divide the lilies, carefully dig up the entire clump with a garden fork, working a few inches away from the stems to avoid damaging the bulbs. Once lifted, the main bulbs and smaller bulblets can be gently separated by hand, ensuring each new section has a healthy set of roots. These separated bulbs should be immediately replanted in a new location or shared. Give them adequate space, typically 8 to 18 inches apart, to allow for future growth.
Proactive Management
Proper site selection and deadheading are proactive techniques that help control the plant’s vigor. Choosing a location with well-draining soil and sufficient sunlight minimizes stress and the need for frequent division. Deadheading involves removing the spent flower heads immediately after blooming. This prevents the plant from diverting energy into seed production, redirecting it back into the underground bulb to strengthen the plant for the following season.