For best results, iris should be planted in July, August or September, but if weather allows, also plant in October and November. It's imperative that the roots of newly planted Iris be well-established before the growing season ends. In areas with hot summers and mild winters, September or October planting may be preferred.
Iris need at least a half day of sun. In extremely hot climates some shade is beneficial, but in most climates Iris do best in full sun. Be sure to provide your Iris good drainage, planting either on a slope or in raised beds.
Iris will thrive in most well-drained garden soils. Planting on a slope or in raised beds helps ensure good drainage. If your soil is heavy, coarse sand or humus may be added to improve drainage. Gypsum is an excellent soil conditioner that can improve most clay soils. The ideal pH is 6.8 (slightly acidic), but Iris are tolerant in this regard. To adjust the pH of your soil, lime may be added to acidic soils or sulfur to alkaline soils. It is always best to have your soil analyzed before taking corrective measures.
Iris should be planted so the tops of the rhizomes are exposed and the roots are spread out facing downward in the soil. In very light soils or in extremely hot climates, covering the rhizome with 1 inch of soil may be desirable. Firm the soil around each rhizome and then water to help settle the soil. A common mistake is to plant Iris too deeply.
Iris are generally planted 12 to 24 inches apart. Close planting gives an immediate effect, but closely planted Iris will need to be thinned often. Plants spaced further apart will need less frequent thinning.
Newly set plants need moisture to help their root systems become established. Specific watering information depends on your climate and your soil, but keep in mind that deep watering at long intervals is better than more frequent shallow waterings. Once established, Iris normally don't need to be watered except in arid areas. Overwatering is a common error.
Specific fertilizer recommendations depend on your soil type, but bone meal, superphosphate and 6-10-10 are all effective. A light application in the early spring and a second light application about a month after bloom will reward you with good growth and bloom. Avoid using anything high in nitrogen, as nitrogen encourages rot problem
Iris need to be thinned or divided before they become overcrowded, generally every 3-4 years. If Iris are allowed to become too crowded the bloom will suffer, some varieties may crowd others out and disease problems may be aggravated. Old clumps may be thinned by removing the old divisions at the centers of the clumps and leaving new growth in the ground. Or, you may dig up the entire clump and remove and replant the large new rhizomes.
Keep your Iris beds clean and free of weeds and debris, allowing the tops of the rhizomes to bask in the sun. Bloom stems should be cut off close to the ground after blooming. Healthy green leaves should be left undisturbed, but diseased or brown leaves should be removed