Water irises are flowering perennials grown from underground rhizomes that thrive in a range of very moist to wet soil conditions. They reside happily submerged under a few inches of fresh water, hence the moniker "water iris." Depending on the cultivar, they flower in hues of yellow, purple, blue and white for a few months over the summer. Good cultural practices and regular feeding with an acid-rich fertilizer will help support optimal blooming.
Feed your water iris several times per year with a complete balanced granular or spike fertilizer designed for acid-loving plants. Nestle the fertilizer in the planting soil and cover over with soil to secure. Apply the fertilizer according to package label dosing directions and do not over feed.
Refrain from pruning your water iris except to remove dead, diseased or damaged stalks. Undisturbed plant clumps with lots of foliage bloom more profusely. Deadhead flowers when they fade to keep a tidy looking plant. Deadheading spent flowers will also allow the plant to put more energy into making more flowers.
Place your water plants where they will receive a full sun exposure of at least five to six hours daily to encourage optimal bloom. Whether planted directly into the underwater soil or in a pot placed underwater, iris will not bloom properly without significant sunlight.
Plant the water iris so that the crown of the plant is submerged 3 inches or less under the surface of the water. Planting deeper will cause the plant to struggle for light and oxygen and that stress will reduce flowering.
Dig and divide very large mature stands of water iris when the flowering begins to diminish but the foliage remains healthy. Do this only rarely, every four to six years or so. Re-plant the divisions in wet ground soil or in pots at intervals of at least 8 inches to prevent fast over-crowding and the need to dig and divide more frequently. Water irises will not bloom as robustly if they are too crowded.