The genus Iris, named after the Greek goddess of the rainbow, encompasses hundreds of species known for their distinct, showy flowers. These perennial plants are widely popular in gardens, but their presence often leads to the question of whether they spread aggressively. The answer is yes, irises do spread, and the rate and method of that spread depend entirely on the specific type of iris being grown. Understanding the plant’s natural propagation habits is the first step in successful cultivation and control.
The Primary Mechanism of Iris Spreading
The main method of spread for most cultivated irises is vegetative propagation through specialized underground stems called rhizomes. These fleshy, horizontal stems function as storage organs, accumulating nutrients and energy for future growth. The rhizome produces new lateral buds that develop into separate plants, which is the mechanism by which a single iris plant can expand into a large clump over time.
Bearded irises (Iris germanica) are the most common example of this, where the rhizome often lies partially exposed at or just below the soil surface. New “fans” of leaves arise from these rhizomes, extending the clump outward each season. This continuous process of producing new growth is what causes overcrowding and necessitates periodic intervention.
A secondary method of propagation is through seeds, which are formed in pods after the flower is pollinated. Seed spread is generally less predictable and slower than rhizome spread but can lead to new plants appearing in unexpected locations. Growing irises from seed often results in genetic variation, meaning the resulting flower may not resemble the parent plant. Preventing seed formation is an easy way to control unwanted seedlings and ensure the parent plant’s energy is redirected elsewhere.
Clumpers, Creepers, and Runners: Understanding Different Growth Habits
Iris species exhibit distinct growth patterns determined by the structure and behavior of their rhizomes, which can be broadly categorized into three types: clumping, creeping, and running. Clumping irises, such as the Siberian iris (Iris sibirica) and Japanese iris (Iris ensata), have short, tightly branched rhizomes that expand slowly and remain localized. These species form dense, compact mounds of foliage that maintain their shape for many years. Siberian irises, for instance, can often go a decade or more without needing division due to their non-aggressive clumping habit.
Creeping irises, a category that includes the popular Bearded irises, feature thick, fleshy rhizomes that grow horizontally just under the soil line. They spread at a moderate pace, with new growth developing adjacent to the old, spent rhizome, leading to a crowded cluster that expands incrementally. This creeping behavior is why bearded iris beds become congested and require division every few years to maintain vigor.
The running iris is the most aggressive type, characterized by long, thin rhizomes that travel widely beneath the soil before sending up new shoots. Certain species irises and some types of Louisiana irises exhibit this habit, allowing them to colonize large areas quickly. These runners can emerge several feet away from the original plant, making them the most challenging to contain within a defined garden space.
Management Techniques for Controlling Spread
Controlling the spread of irises is primarily achieved through division, which is necessary every three to five years for most creeping varieties like the Bearded iris. Overcrowding depletes soil nutrients, reduces air circulation, and leads to diminished flower production and increased disease susceptibility. Division involves lifting the entire clump to remove the old, spent rhizome sections, which no longer produce flowers.
The optimal time for dividing Bearded irises is late summer to early fall, approximately six to eight weeks after they finish flowering, allowing the divisions time to establish roots before winter. The process requires cutting the new, healthy rhizomes, each with a fan of leaves, away from the older, central parts. Discarding soft, withered, or damaged rhizomes is important, as they can harbor pests or disease.
For beardless types like the Siberian iris, which are sensitive to root disturbance, division is done less frequently. It is best performed shortly after flowering or in early spring. When replanting, position the healthy young rhizomes at the correct depth—partially exposed for Bearded irises or slightly deeper for Siberian and Japanese varieties. Rhizomes should be spaced 12 to 18 inches apart to allow for future growth.
Preventing seed spread is a simple management technique known as deadheading. When the flowers fade, removing the spent bloom and the green, swollen sheath at its base prevents the plant from forming a seed pod. This action conserves the plant’s energy, redirecting it toward strengthening the rhizome for the following season’s blooms.