Bearded irises are a popular old-fashioned landscape plant with gorgeous blooms in a variety of colors. But irises do require a little extra care to maintain their beauty.
Irises multiply fairly quickly and when the plants become overcrowded they produce fewer of their lovely blooms. It is very easy to divide iris plants to rejuvenate them, and for the best display, bearded irises should be divided every three to four years.
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The best time to divide and transplant an iris is two to three weeks after it is done blooming for the season, through the end of August.
After carefully digging the thick roots known as rhizomes, wash the soil from them and inspect the rhizomes. Keep the youngest and healthiest rhizomes for replanting and discard old, shriveled rhizomes, along with any that have holes or soft, mushy spots. Holes in iris rhizomes could be caused by iris borers. Infected plants may also have dark streaks on their leaves. Discard any damaged rhizomes; they should not be added to your compost pile.
As you divide iris rhizomes, you may find some that have become very large, with several sections growing in different directions. Do not break apart these large rhizomes. Sections of rhizomes that are already separate can be loosened, but if a large rhizome needs to be divided, this should be done with a sharp knife. Make a clean cut in a natural place to split the rhizome into smaller sections. If a rhizome has forked, the fork would be a good place to make a cut. Each section of rhizome must be at least three inches long and have healthy roots. Make sure to disinfect the knife between cuts to help avoid spreading disease amongst the irises. The iris leaves should be cut back to be about six inches tall before replanting.
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Replant the divided rhizomes right away. If for some reason they cannot be replanted immediately, soak them in water for about an hour, then cover them with moist peat or sand and keep them in a shady spot until you are able to plant them. It is helpful to have their new bed ready for them before you begin to divide iris plants.
Bearded irises like to grow in full sun, so choose a spot for them that will receive at least six hours of sunlight each day. Irises will not perform well if they are planted in a shady spot. If the soil is heavy with clay, consider growing irises in a raised bed to get them up above the clay where they can flourish.
To replant the healthy rhizomes, dig a shallow hole with your trowel and set each rhizome in the hole about eight inches apart, carefully spreading out the roots on each rhizome. Add a granular bulb fertilizer to the hole at this time to keep your irises happy. If several rhizomes are to be planted in the same bed, arrange them so their leaves all point away from the center. This will encourage them to spread outwards, rather than toward each other.
Irises will bloom poorly or not at all if the rhizomes are planted too deeply. They should be barely covered with soil and the upper surface of the rhizomes should be at about the soil level. This may not seem right, but that's what irises like. Some seasoned gardeners simply toss the rhizomes onto the prepared bed, then gently press them into the soil a bit.
Your rejuvenated bearded irises will reward your efforts with an abundance of extravagant flowers next spring.