Irises, with their elaborate blooms and foliage, are a beautiful addition to any garden and can be grown successfully in containers. They thrive on patios and balconies, provided specific cultural requirements are met. Success depends heavily on selecting the appropriate variety and managing the container environment, particularly drainage and seasonal care.
Selecting the Right Irises and Containers
Choosing a smaller variety of iris is the first step toward long-term container success. Miniature Dwarf Bearded Irises and Siberian Irises are excellent choices due to their compact root systems. While Tall Bearded Irises can be grown in pots, their large, spreading rhizomes require yearly division to prevent overcrowding.
The container must offer exceptional drainage for iris health. A minimum size of 6 to 8 inches in diameter is suitable for smaller dwarfs, while larger bearded varieties need at least a 12-inch pot. Terracotta pots are preferred over plastic because their porous nature allows excess moisture to evaporate, reducing the risk of waterlogging and subsequent rhizome rot. Wider containers are also better, as they allow the rhizomes to spread naturally beneath the soil surface.
Planting Techniques for Container Irises
Use a sterile, very well-draining potting mix rather than heavy garden soil, which compacts easily. A standard commercial potting mix should be amended with a coarse material like perlite, pumice, or grit (three parts soil to one part amendment) to ensure rapid drainage. This porous structure mimics the dry, airy conditions irises prefer around their rhizomes.
Plant bearded rhizomes horizontally with the roots spread out, ensuring the fleshy top is exposed or only barely covered by soil. Planting too deep prevents the sun from warming the crown, which inhibits flower bud formation and encourages rot. In a 12-inch container, you can fit three rhizomes in a triangular arrangement for a dense display. Place the container in a permanent, full-sun location, as irises need a minimum of six hours of direct sunlight daily to produce blooms.
Essential Year-Round Maintenance
Container-grown irises dry out faster than those in the ground. Water thoroughly until liquid runs from the drainage holes, but only when the top two to three inches of the potting mix feel dry. Allowing the soil to dry slightly between waterings is essential to maintain rhizome health, as overwatering is the primary cause of bacterial soft rot.
Fertilization should be approached cautiously, as irises thrive on minimal nutrients. Use a low-nitrogen fertilizer high in phosphorus and potassium (like a 6-20-20), applied lightly in early spring and again after blooming. Avoid high-nitrogen feeds because they promote soft leaf growth, making the plant more susceptible to pests like iris borers.
Overwintering is the greatest challenge in cold temperatures, as freezing soil can damage the rhizomes. In colder climates, move containers to a sheltered location, such as an unheated garage or cold frame, once the plants enter dormancy. Alternatively, bury the entire pot in the ground or surround it with insulating material like straw or burlap. Even in dormancy, the soil should not dry out completely, requiring occasional light watering on warmer winter days (above 45 degrees Fahrenheit).