Iris (botanical name Iris spp) a perennial plant that grows from rhizomes. It produces showy flowers with three petals on talk stalks. Iris come in a variety of cultivars with hardiness levels that can vary from USDA plant hardiness zone 3 to 9. Different species of iris will also vary in how long before they bloom after planting. To determine the exact blooming time, research the cultivar carefully or talk with an expert at your local garden center.
Types of Iris
Iris come in thousands of varieties. The most common bulb iris is the Dutch iris. Rhizome iris come in bearded, beardless and crested varieties. Siberian iris and Japanese iris are beardless varieties. Miniature dwarf, standard dwarf, intermediate, miniature tall and tall are bearded types of iris. Cultivars are available in many colors and sizes. Most varieties of iris may take several years to become fully established. The first year’s growth may only produce small, fragile flowers. Poor flowering can also occur when plants are too crowded, according to North Carolina State University's Department of Horticultural Science.
Iris Bloom Times
The various types of iris will bloom at different times, and bloom time also depends on environmental conditions. A clump of iris generally bloom three to four weeks after planting. Some varieties will re-bloom four to eight weeks after the initial bloom as well. Iris need 15 to 20 weeks of cold to initiate blooming, according to the University of Rhode Island Landscape Horticulture Program website. This is provided over the winter in cold climates, or you can “force” blooms by chilling the plant in the refrigerator or in an unheated garage. Provide moisture for the plants during this chilling time. Remove after the scheduled amount of time and place them in a sunny area. Plants should bloom in three weeks.
Iris prefer the sun, but will also grow in shade. They will need at least one half-day of sunlight, according to Schreiner's Iris Gardens. The ideal pH for iris is 6.8, which is slightly acidic. Plant it so the rhizome is slightly exposed and no more than 2 inches below the soil, with the roots facing into the hole. Iris plants need plenty of room to grow, roughly 12 to 24 inches per rhizome. Do not crowd with other plants. Water well after planting to aid new root growth. Ensure that the soil drains well to prevent root rot. Water during dry periods fertilize with a 6-10-10 fertilizer or bone meal. Apply one application in the spring and a second one month after the plant blooms.
Iris are resistant to many pests but may be troubled by root borers. Avoid high nitrogen fertilizers, which contribute to root rot. A 10 percent solution of Murphy’s Oil in water, poured onto the soil around plants is an organic method of controlling root borers. Aphid, thrips and whiteflies are controlled with malathion sprays. Divide iris plants every three to five years to give them space for good blooming.
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