Yes, Canna lilies spread effectively through an underground network. These perennials, known for their vibrant foliage and showy flowers, grow from a structure that allows them to quickly colonize an area. While often mistakenly called a bulb, understanding how this plant multiplies is key to managing its presence. The speed and extent of their spread depend heavily on growing conditions, which determine whether they remain an accent or become a dense clump.
The Rhizome Engine: How Canna Lilies Spread
The biological engine behind the Canna lily’s expansion is the rhizome, which is a modified, horizontal underground stem. Unlike a true bulb, which is a compact, layered stem structure, the rhizome is a fleshy, ginger-like storage unit that grows laterally beneath the soil surface. This structure is not a root, though adventitious roots and new shoots grow from it at intervals along its length.
The rhizome is covered in small, dormant buds known as “eyes.” Each of these eyes is a growth point capable of producing an entirely new shoot. This process is a form of vegetative reproduction, meaning the new plants are genetically identical clones of the parent plant. As the growing season progresses, the original rhizome continues to elongate and produce more eyes, resulting in an ever-expanding, dense cluster of plants.
The rhizome allows the Canna to colonize a substantial area by rapidly increasing plant density. It also serves as the plant’s food storage system, providing the energy needed for rapid growth and spread. Over time, the original rhizome may become choked out by the new growth, which is a natural part of the clump’s cycle of renewal and expansion.
Growth Rate and Clump Expansion
The rate at which a Canna lily clump expands is highly dependent on environmental factors, particularly warmth, moisture, and soil fertility. In their native tropical and subtropical habitats (USDA hardiness zones 8-11), where they are perennial and remain in the ground year-round, Canna lilies can spread aggressively. A single rhizome can establish a dense, multi-stemmed clump that expands outward significantly in a single growing season.
In ideal conditions—rich, moist soil and full sun—the plant’s growth is strong, allowing the rhizomes to quickly extend their reach. Gardeners in warmer zones may find their plants need thinning every two years to prevent overcrowding, which can negatively affect both foliage and flower production. Without intervention, a Canna clump can easily spread several feet in diameter over a few seasons, forming a thick stand of plants.
For gardeners in cooler climates (zones 2-6), where the rhizomes must be lifted and stored over winter, the spread is naturally contained to a single season’s growth. Even so, the plant is a fast grower, and the rhizome will increase in size and complexity each year before the fall frost. Closely spaced rhizomes, planted as little as 12 to 18 inches apart, will merge to form a continuous, dense border by the end of the summer.
Controlling and Managing Canna Spread
Managing the spread of Canna lilies involves physical containment and regular division to prevent overcrowding. For gardeners who wish to limit the plant’s lateral expansion, planting in a large container is an effective strategy. The container should be at least 18 inches in diameter for a single rhizome, providing enough space for the plant to grow and spread for one to two seasons before becoming root-bound.
If planting directly in the ground, you can utilize a physical root barrier to contain the rhizomes, which typically grow close to the soil surface. The most common and practical method for long-term management is periodic division of the clumps. Overcrowding reduces plant vigor and flowering, so dividing Canna lilies every two to five years is recommended to rejuvenate the stand.
Division is best performed in the spring just before new growth begins, or in the fall when lifting the rhizomes for winter storage. The entire clump is dug up, and the rhizome mass is separated into smaller sections using a sharp, clean knife. Each new section should contain at least one to three healthy eyes, or growth points, ensuring a viable plant when replanted.