Convallaria majalis, or Lily of the Valley, is a perennial beloved for its fragrant, bell-shaped white flowers and its ability to flourish in shady areas. Its dense foliage forms a low-growing carpet, making it a popular choice for groundcover in woodland settings. However, the plant is notorious among gardeners for its vigorous spreading habit, leading to confusion about whether it is simply “aggressive” or truly “invasive.”
Defining Aggressive Growth vs. True Invasiveness
The difference between an aggressive plant and an invasive plant involves both scope and legal definition. An aggressive plant spreads rapidly and vigorously within a cultivated garden, often out-competing and displacing other ornamental plants. Lily of the Valley is an aggressive plant because its dense root system allows it to quickly dominate a flower bed or border.
A truly invasive species meets a specific two-part criteria defined by government agencies. It must be non-native to the ecosystem and its introduction must cause, or be likely to cause, environmental or economic harm. This harm involves escaping cultivation, spreading into natural areas, and displacing native flora. Although Convallaria majalis is non-native to North America, it is rarely listed as a regulated noxious weed, meaning its primary threat is to the garden, not the wilderness.
The Mechanism of Spread: Pips and Rhizomes
The plant’s aggressive behavior is due to its specialized underground structure, which allows it to reproduce and spread horizontally. Lily of the Valley spreads primarily through thick, creeping underground stems known as rhizomes. These rhizomes form a dense, interconnected mat just beneath the soil surface, enabling rapid colonization.
New shoots, which gardeners call “pips,” emerge from the growing tips of these rhizomes as pointed buds containing the plant’s future leaves and flowers. Because the plant relies on this vegetative reproduction rather than seeds, it quickly establishes dense, unbroken colonies, choking out neighboring plants. Even small, broken fragments of the rhizome network can sprout into a new plant, making manual removal challenging.
Strategies for Containment and Removal
Controlling or removing Lily of the Valley requires patience and a multi-pronged approach targeting its rhizome system. The most effective non-chemical method is persistent physical removal, which means digging up the entire clump and focusing on removing all root fragments. Since leaving even tiny pieces of rhizome leads to regrowth, repeated digging over several seasons may be required to exhaust the underground network.
To contain its spread proactively, gardeners can install physical root barriers made of plastic or metal sheeting sunk at least 10 inches deep. A less labor-intensive method is soil solarization or smothering, involving covering the patch with thick black plastic, cardboard, or heavy mulch for an entire growing season. This starves the plant of light and heat, eventually killing the rhizomes. For large infestations, a non-selective systemic herbicide can be applied to the foliage in early spring, allowing the plant to draw the chemical down to the root system.