The blooming period for irises, typically in late spring or early summer, is only the first phase of their annual cycle. Once the colorful display ends, the plants enter a post-bloom phase where proper care is paramount for long-term vigor. Decisions made during this time directly impact the health of the underground rhizomes, which store energy for the next season’s growth. Specific post-bloom maintenance ensures irises successfully prepare for a robust return of flowers the following year.
Removing Spent Flower Stalks
The first task after the final flower has faded is to remove the entire spent flower stalk, a process often referred to as deadheading. Leaving the stalk allows the plant to divert energy into producing a seed pod, which detracts from the rhizome’s energy reserves. Resources should be directed back into the rhizome to fuel next year’s bloom, and this practice also helps maintain a tidy appearance.
To remove the stalk, use sharp, clean pruning shears to make a single cut near the base of the plant. Make the cut at an angle to prevent water from collecting on the stub, which can lead to rot or fungal infections. Be careful to cut only the old flower stalk, avoiding damage to the fan of green leaves. Timely removal also prevents decaying material from becoming a breeding ground for pests like the iris borer.
Maintaining Foliage for Next Year’s Bloom
After removing the spent flower stalks, the fan-like green foliage must be left intact throughout the summer months. These leaves are the plant’s food factories, performing photosynthesis to convert sunlight into sugars. This energy is then transported and stored within the fleshy rhizome. The size and health of the rhizome directly correlate with the number and size of flowers produced in the subsequent spring.
A common misstep is to cut the leaves back or “braid” them immediately after blooming to tidy the garden. Doing this prematurely removes the photosynthetic surface area, severely limiting the energy the rhizome can store. This energy deficit results in weak plants and a poor bloom the following season. Only remove foliage if it shows signs of disease or has completely yellowed and withered, indicating the plant has reabsorbed its nutrients.
Post-Bloom Irrigation and Feeding
The watering needs of irises change significantly after the flowering period, especially as summer temperatures rise. While newly planted irises require consistent moisture for root establishment, mature irises with established rhizomes are quite drought-tolerant. Overwatering is a frequent mistake that creates soggy conditions, which can quickly lead to rhizome soft rot, a bacterial infection that causes the rhizome to become mushy and foul-smelling.
It is best to water deeply but infrequently, allowing the top layer of soil to dry out between waterings. Irises benefit from a post-bloom application to replenish nutrients used during flowering. Select a fertilizer formula that is low in nitrogen, such as a 5-10-10 or 6-10-10 ratio. High nitrogen levels encourage soft, leafy growth, which is more susceptible to rot and insect pests like the iris borer.
Apply the fertilizer in a ring around the plant, taking care to avoid placing it directly onto the rhizome itself. This is also the time to monitor for pest and disease issues, as the plant can be under summer stress. Watch the leaves for dark streaking or soft, discolored spots on the rhizomes, as these may signal the presence of iris borers or soft rot that require immediate removal and disposal of infected material.
Timing and Technique for Rhizome Division
Irises, particularly the bearded varieties, grow outward from their rhizomes, leading to crowded clumps that eventually reduce flower production. Division becomes necessary every three to five years to rejuvenate the planting and ensure continued robust blooming. The optimal window for this task is in the late summer or early fall, typically six weeks after the blooming period ends, allowing the plant to enter a semi-dormant state.
This timing gives the newly replanted divisions sufficient time to establish new feeder roots before the onset of winter frosts. Begin by carefully digging up the entire clump with a garden fork, gently shaking off excess soil to expose the network of rhizomes. Use a clean, sharp knife to separate the younger, healthier rhizomes, each with a fan of leaves and active white roots, from the older, woody, central rhizomes, which should be discarded.
Next, cut the leaf fans back to a height of about six inches, which reduces moisture loss and helps the plant focus its energy on root growth. When replanting, prepare the soil and create a small mound in the center of a shallow hole. Set the rhizome on the mound with the roots spread out, ensuring the top surface of the rhizome is just barely visible or thinly covered with soil; planting too deep is a common error that prevents flowering.