The frequency of flowering in irises (Iris spp.) depends entirely on the specific genetic makeup of the variety and the environmental conditions where it is grown. Irises are broadly categorized into two groups: those programmed to bloom only once per season and those that possess the ability to rebloom. Understanding this distinction is the first step in knowing what to expect from these popular perennials. The difference between a single spring display and an extended season of color often comes down to selecting the correct cultivar.
The Standard Bloom: Once-a-Year Irises
The majority of iris varieties follow a singular, annual bloom cycle, typically occurring in the late spring or early summer. These non-remontant types are genetically dictated to have only one floral flush per growing season. The classic Tall Bearded Iris is a prime example; the flower stalk emerges, produces its blooms, and the reproductive cycle concludes. This single event usually takes place between May and June, depending on the local climate.
The plant dedicates the remainder of the warm season to vegetative growth and energy storage within its thick, underground stem, the rhizome. This stored energy is crucial for the formation of the following year’s flower bud. Once-blooming irises require vernalization—a period of cold temperatures during winter—to complete the development of the terminal bud into a flower stalk. Without this chilling requirement, the plant cannot initiate the floral process necessary for a spring display.
Other once-blooming species, such as Siberian Irises (Iris sibirica) and Japanese Irises (Iris ensata), also adhere to this single annual cycle. Siberian Irises generally bloom slightly later than Bearded Irises, often extending the color into June. Japanese Irises are late-season bloomers, showcasing their large, flat flowers from mid-June through early August. After blooming, these plants focus on building robust root systems and dividing their rhizomes to ensure vigorous growth for the next spring.
Understanding Reblooming Irises
A subset of irises possesses the genetic trait known as “remontancy,” allowing them to flower multiple times in a single year. Reblooming irises set new bloom stalks after the initial spring flowering without needing a full vernalization period. While many types, including certain beardless varieties, can rebloom, the trait is most commonly found in the Bearded Iris class. This ability results from specific genes that bypass the typical cold requirement for floral initiation.
Reblooming irises generally display one of two distinct flowering patterns. The first is the “Cycle Rebloomer,” which completes two separate, full growth cycles within a season. This pattern typically involves a strong spring bloom followed by a period of vegetative growth, culminating in a second flush of flowers in the late summer or autumn. The second pattern is the “All-Season Rebloomer,” which produces flowers sporadically throughout the warmer months.
All-Season rebloomers are less dependent on day length and more reliant on sustained warm soil temperatures to trigger subsequent blooms. Specific cultivars within the Miniature Dwarf Bearded, Standard Dwarf Bearded, and Intermediate Bearded categories are often reliable rebloomers. Tall Bearded rebloomers, such as ‘Immortality’ or ‘Harvest of Memories,’ offer the dramatic size and color of their once-blooming relatives with the benefit of a second appearance. The actual number of reblooms is heavily influenced by the environment and the cultural care provided.
Cultural Factors That Influence Bloom Frequency
Even with the correct genetics, a reblooming iris must have optimal growing conditions to produce flowers more than once. Reblooming requires significantly more energy than a single annual flowering, demanding specific cultural interventions. Deadheading—removing the spent flower stalks completely after the spring bloom—is a simple action that prevents the plant from diverting energy into unproductive seed production. This allows resources to be redirected toward forming new bloom stalks.
Reblooming varieties require more consistent water than their once-blooming counterparts to support this accelerated life cycle. Allowing the soil to dry out for extended periods during the summer can cause the plant to enter dormancy and halt its second bloom cycle. While standard irises thrive in drier conditions after their spring show, rebloomers need regular, deep watering throughout the summer months.
The additional energy expenditure necessitates a specific fertilization regimen. After the initial spring bloom, a second application of fertilizer one month later is beneficial. This fertilizer should be low in nitrogen but higher in phosphorus and potassium, often represented by a ratio like 5-10-10 or 6-10-10. Avoiding high-nitrogen formulas is important because excessive nitrogen encourages leafy growth at the expense of flower production.
Climate plays a major role in the reliability of rebloom, with success diminishing in the coldest hardiness zones. Excessively hot summer nights can also trigger dormancy and stop the reblooming process. Mild summer temperatures and a long, cool autumn provide the best environment for remontant irises to set and mature their second set of flower buds.