How to Grow Bearded Irises
Site Selection and Preparation
A location with full sun is best. Too much shade will reduce or prevent flowering. That said, some of the delicate pink and blue Bearded Iris do hold their color better in partial shade.
Good soil drainage is essential to prevent the rhizomes from rotting. It may be necessary to plant the rhizomes on a slope or in raised beds (at least 6 inches high) to obtain proper drainage. Iris will grow in many soil types but a light, loamy soil that has been amended with organic matter is preferred.
The best time to plant Bearded Iris is late summer through early fall in colder growing zones. This will allow them to become well established before winter. Warmer growing zones may plant later. Container-grown iris can be planted in the spring.
Prepare garden bed by loosening soil to a depth of 12 to 15 inches. Mix in a 2 to 4 inch layer of compost. Dig a shallow hole large enough to accommodate the rhizome or clump of rhizomes. The hole should be about 10 inches in diameter and 4 inches deep. Form a mound or ridge of soil in the center of the hole for the planting base. The rhizome will be placed on the top of this mound, so make it high enough so the top of the rhizome is slightly above e soil level. Spread the roots down the sides of the mound. Fill the hole with soil, firm it gently and then water. Bearded Iris rhizomes grow best when planted at or slightly below the soil surface with feeder roots penetrating the moist, but not wet, soil below.
For a mass of color, plant at least three rhizomes (spaced 8 to 10 inches apart). Each grouping of Irises should be spaced about 2 feet apart.
Fertilization of Bearded Iris is important to obtain best results, but must be done in moderation. Nitrogen, potash, and phosphorus are essential for Iris, but excessive nitrogen promotes lush growth that is more susceptible to rot diseases. At planting, use a low-nitrogen fertilizer such as 5-10-10 or something similar. Half of a pound per 50 ft2 (1 ½ oz per 10 ft2) is recommended.
Care and Maintenance
Before flowering, water plants often enough to keep the soil just moist. After flowers fade, cut flower stalks back to an inch or two above the rhizome. This will prevent seed formation in single blooming Iris and encourage a second bloom on re-blooming iris.
Mulching of Bearded Irises is not recommended during the growing season. If you desire to mulch the bed for appearance, you should NOT cover the rhizomes. The sun must reach them to facilitate development of next year’s increase of the rhizome. Only apply a thin layer of compost around the base of plants each spring, leaving the fleshy root of the rhizome exposed.
Plants that are growing well (good green foliage) may not need fertilizing. If you fertilize, apply ½ cup of fertilizer per iris clump after flowering. Fertilizer can burn the rhizomes, so it should be applied around but not directly on them. Re-blooming iris should be fertilized in the spring as new growth begins and after spring flowering ends.
In early fall, trim away dead foliage and cut healthy leaves 6 to 8 inches from the ground. It is strongly recommended that newly planted Irises be mulched to keep from losing them. Freezing weather will not harm established rhizomes, other than causing their roots to heave out of the soil as a result of alternate freezes and thaws. Use pine needles or another material that does not hold water, after the ground has frozen, to reduce heaving. In the early spring, usually late March, the mulch must be removed. If heaving does occur, simply cover the rhizomes and the exposed roots with a thin layer of soil.
Divide Bearded Iris every 3 to 5 years, as they can become overcrowded. Iris can be divided any time, but 4 to 6 weeks after the flowering period is best.