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Bearded iris grow best in well-drained soil. Sloped or raised beds help ensure good drainage. Coarse sand or humus may be added to improve drainage. The ideal soil pH is 6.8 (slightly acidic). Iris do best with at least 6 hours a day of full sun. Insufficient sunlight will result in iris producing foliage, but no blooms.
When to Plant
For best results, iris should be planted at least six weeks before the first hard frost in your area. Establishing a good root system before the growing season ends is imperative.
Plant iris so the tops of the rhizomes are exposed and the roots are spread out facing downward into the soil. In very light soil or extremely hot climates, covering the rhizome with a light covering of soil by be used to shade the rhizome and prevent scorching. A common mistake is to plant bearded iris too deep. Ideally, the tops of the rhizomes should be visible above the soil level. Bearded iris planted 12 to 24 inches apart will require less frequent thinning. Watering in newly planted rhizomes will help settle the soil and encourage root growth.
Moisture helps establish root systems of newly planted rhizomes. Deep watering at long intervals is preferred to frequent shallow waterings. Unless the growing area is very dry, iris don’t need to be regularly watered. Over-watering is a common error.
Use only a low-nitrogen fertilizer (6-10-10), bone meal or superphospate. A light application in the spring when springs bulbs such as tulips are blooming and a second light application about a month later is all the is required. Avoid the use of anything high in nitrogen. Over feeding iris with anything high in nitrogen will prevent the plant from producing blooms.
Growing Iris in Containers
Use a roomy pot with good drainage. Bearded iris need a one gallon pot or larger. Fill to one inch below the rim of the container with well-draining soil that is low in nitrogen. Plant rhizomes so that the tops of the rhizomes are exposed. Water only when the top two inches of soil are dry. Over-watering will encourage rot. Containers can be over-wintered outdoors, but protect pots from freezing by moving into an unheated garage or other unheated, protected location when sustained daytime temperatures are below 32F or nighttime temperatures are below 25F. In climates with harsh winter weather, sink the pot into the soil and cover with mulch. Divide and transplant iris every few years or as they begin overcrowding the pot.