The first goal of attaining spring blooming iris is to plant bulbs that naturally mature and bloom in the spring. Dutch iris, dwarf iris and bearded iris bloom in early to mid spring while Japanese iris tend to bloom later in the early summer. Iris are hardy in zones 7 through 10 depending on the variety of bulb. Fall planting, adequate water, good fertilizer and sound cultivation practices all contribute to healthy flowers in the spring.
Plant iris bulbs in the fall into rich well drained soil that receives a full sun to a half day of sun exposure year round. Half days of sun are tolerated in warmer climates, but iris bloom most profusely when they receive day long sun. Bury the bulb 5 inches deep leaving the top of the rhizome exposed above the soil. Space bulbs 12 to 14 inches apart to prevent the need to divide in several years. Plant the rhizomes closer together for a more immediate massing effect.
Water the bulbs in well at planting and through the root establishment phase. Water deeply once in the early spring to support bloom and thereafter only in times of drought or in arid climates.
Mulch over the bulbs with several inches of shredded bark, compost, leaf mold or pine straw to insulate the bulbs from cold and moisture loss and ensure they overwinter in peak condition. Mulch will also contribute to the nutrient quality of the soil, thereby boosting bloom.
Feed your iris with a good quality granular bulb fertilizer in the spring to boost bloom. Follow dosing directions on the package. Water in the fertilizer well after application to drive the nutrients down to the bulb and roots. Make a second application in mid to late summer to recharge the bulb for next season's bloom.