Canna lilies (Canna indica) are renowned for their vibrant, tropical foliage and showy flowers. While highly valued for their ornamental appeal, these fast-growing plants are notorious for their aggressive spreading habit. Proliferation occurs through two distinct biological mechanisms: the production of hard, round seeds above ground and the rapid multiplication of fleshy, underground stems called rhizomes. Controlling a canna lily patch requires a two-pronged strategy that addresses both reproductive methods to halt their spread.
Managing Above-Ground Spread
Preventing the plant from setting seed is the first step in controlling its overall footprint. Once blooms fade, the flower stalk forms a spiky capsule that matures into a seed pod. Allowing these pods to ripen and burst permits the seeds to disperse, leading to new, unwanted plants in the surrounding area.
The practice of deadheading should be performed as soon as the bloom fades to redirect the plant’s energy away from seed production. Cut the entire flower stalk back to the point where it meets a main leaf stem or a new emerging bud. Dispose of any mature, hard seed pods in a sealed refuse bin rather than composting, as composting may not destroy the tough seeds.
Physical Containment and Removal of Rhizomes
The primary mechanism for local spread and dense clumping is the thick, horizontal growth of the rhizomes beneath the soil surface. To eliminate an established patch, mechanical removal is necessary, involving digging out the entire network of these underground structures. This task is best undertaken when the plant is dormant, typically in late fall after the foliage has died back or in early spring before new growth emerges.
When digging, work several inches away from the visible base of the plant to capture the full extent of the rhizome mass. The fleshy rhizomes can grow deep, so use a sharp spade to loosen the soil, ensuring the entire root structure is lifted out intact. Even small fragments of the rhizome left in the soil can sprout into new plants, meaning thorough removal of every piece is necessary to prevent immediate regrowth.
Using Physical Barriers
For gardeners who wish to keep the plants but control their spread, installing a physical barrier is highly effective. The barrier material (heavy-duty plastic, metal, or concrete) must be sunk into the ground at a depth of at least 18 to 24 inches. This depth is designed to block the horizontal movement of the rhizomes, confining the plant to a designated area. A physical barrier provides a long-term solution for maintaining the plant within a controlled border without requiring annual digging.
Chemical Suppression Methods
The use of systemic herbicides offers a non-physical method for suppressing or eradicating canna lilies, particularly in hard-to-reach or large infestations. Systemic chemicals, such as those containing glyphosate, are absorbed by the plant’s leaves and then transported internally down to the rhizomes. This translocation is designed to kill the entire plant, including the underground storage structures, which is an advantage over simply removing the foliage.
Herbicide application is most successful when the canna lily is actively growing, typically during the mid-summer months. At this time, the plant is vigorously moving nutrients from the leaves down to the rhizomes, which facilitates the transport of the chemical to the roots. Applying the herbicide as a spot treatment is recommended to protect surrounding desirable plants, as glyphosate is a non-selective chemical that will harm any vegetation it contacts. Always read the product label thoroughly, as it provides specific instructions on application rates, timing, and safety precautions.