Yes, irises multiply, and this ability is a fundamental characteristic of the popular perennial flower. The iris plant, with its distinctive sword-like leaves and complex blooms, employs a robust reproductive strategy that ensures its spread and persistence in the garden. This natural tendency to increase in number is what allows a small initial planting to eventually form a dense, colorful display over time.
How Irises Increase in Number
The public often refers to all irises as growing from “bulbs,” but the majority of garden irises, specifically the Tall Bearded Irises, actually grow from a specialized structure called a rhizome. The rhizome is a thick, fleshy, horizontal stem that grows at or just below the soil surface, acting as a storage organ for starches and proteins. This structure facilitates the plant’s asexual propagation, creating genetically identical copies of the parent plant.
The main rhizome continuously produces lateral shoots, known as offsets, which develop their own fan of leaves and roots. These offsets mature and become new flowering units, which then produce their own offsets, creating a branching network that expands the clump outward. A single rhizome can branch multiple times, allowing the iris to spread rapidly across a garden bed.
Some smaller, early-blooming varieties, such as Dutch Irises or Iris reticulata, grow from true bulbs or corms. These structures also multiply by producing tiny offsets called bulbils, which form around the base of the main bulb. While the mechanism differs, both rhizomes and true bulbs are highly effective at generating new plants.
Recognizing When Division is Necessary
While multiplication is a sign of a healthy plant, the resulting overcrowding can eventually become detrimental to the iris’s performance. The clearest sign that a clump needs attention is a noticeable decrease in the number of flowers produced. As the rhizomes become densely packed, they compete intensely for limited resources like sunlight, water, and soil nutrients.
This competition often results in the center of the clump becoming barren or “woody,” with the foliage and blooms appearing only around the perimeter. A visual cue is when the rhizomes begin pushing themselves completely above the soil line, becoming exposed to the elements. This crowding necessitates intervention, typically every three to five years, to rejuvenate the plants and maintain vigorous blooming.
Step-by-Step Guide to Division
The best time to manage an overcrowded iris clump is in the mid-to-late summer, six to eight weeks after the flowers have faded. This timing allows the newly divided sections to establish a strong root system before the onset of winter dormancy. Begin by carefully digging up the entire clump with a garden fork, taking care to avoid piercing the fleshy rhizomes.
Once the clump is lifted, follow these steps for division and replanting:
- Shake or wash off the excess soil to expose the network of rhizomes.
- Select the healthy, younger sections of the rhizome that are firm and have a robust fan of leaves and new roots.
- Discard the old, spent center sections, which are woody and leafless, since they will not bloom again.
- Use a clean, sharp knife to separate the healthy rhizomes, ensuring each new division has at least one strong leaf fan and a section of the root.
- Trim the leaves back to about one-third of their height to reduce moisture loss and prevent the division from toppling over.
- Replant the prepared rhizomes in a sunny location, spacing them 12 to 18 inches apart.
- Place the rhizome right at the soil surface or slightly exposed, with the roots spread out below, to ensure proper light exposure and air circulation.