Irises are popular perennials known for their spectacular colors and forms. The genus Iris includes two main types: Bearded Irises, which grow from fleshy, horizontal rhizomes, and true bulbous irises, such as Dutch Irises. Achieving successful blooms in Oklahoma requires precise timing due to the state’s variable climate (USDA Hardiness Zones 6b to 7b). Optimal planting ensures the underground structures establish a robust root system before seasonal extremes inhibit growth.
Identifying the Right Planting Window in Oklahoma
Success depends on differentiating the planting schedule based on the iris type. Bearded Irises (rhizomes) should be planted in Oklahoma during late summer to early fall, specifically July through September. This timing allows the rhizomes six weeks to develop anchor roots before the first hard frost, which is necessary for winter survival and spring flowering.
Planting too early, when summer heat is intense, can cause the rhizomes to rot. Conversely, planting too late prevents root establishment, leading to “winter heave” from freezing and thawing cycles. True bulbous irises, like Dutch varieties, have a slightly later optimal window, extending from September into mid-to-late October. These bulbs need cooler fall soil temperatures to initiate their growth cycle but must be planted before the ground freezes solid.
Preparing the Soil and Site Selection
Selecting the proper location and soil environment is necessary before planting. Irises require a minimum of six hours of direct sunlight daily to produce abundant flowers, making a full sun site preferred. Good drainage is essential for both rhizomes and bulbs, as standing water quickly leads to rot and plant failure.
Oklahoma’s dense clay soils often require amendment to improve drainage. This involves incorporating organic matter, such as well-rotted compost, into the soil to a depth of at least 12 inches. Materials like coarse grit or gypsum can also be mixed in to break up the heavy clay structure. Creating a slightly raised bed is another effective technique to ensure excess water drains away from the iris roots.
Essential Planting Techniques for Long-Term Success
The physical planting process differs significantly between the two iris types, and incorrect depth often prevents flowering. Bearded iris rhizomes must be planted very shallowly. In heavier Oklahoma clay soils, the top of the rhizome should be barely exposed to the air and sunlight.
To plant rhizomes, create a small soil mound in the center of the hole. Set the rhizome on top, spread the roots down and out over the mound, then backfill and firm the soil gently. True iris bulbs, such as Dutch Irises, must be planted deeper, typically four to six inches below the surface, with the pointed end facing upward.
For both types, spacing the units 12 to 24 inches apart allows for adequate air circulation and future growth. A thorough initial watering helps the soil settle around the roots, but subsequent watering should be moderate. Avoid placing heavy mulch directly over Bearded Iris rhizomes, as this traps moisture and heat, increasing the risk of rot.