For fuller displays, it's time to divide German irises
Iris germanica, commonly called German or bearded iris, is a flamboyant, late-spring bloomer with glamorous flowers made up of parts with names such as standards, falls and beards. Thousands of cultivars are available in every color, or combination of colors, of the rainbow. They can be as small as 6 to 8 inches or more than 3 feet tall.
Their bold, swordlike foliage is attractive in the garden long after the flowers have faded and their stems cut. German irises spread by rhizomes, forming large clumps.
If plants are packed tightly together in the garden, they aren't blooming like they once were or it has been several years since they were planted, it is time to divide them. Any time after they have finished flowering through the end of August is suitable for dividing.
It is quick work to dig up shallow-rooted irises. The root system is made up of rhizomes (thick, horizontal stems with roots and buds). Cut apart rhizomes with a sharp knife and gently brush off the soil or rinse them in a bucket of water. Divisions should have a generous section of rhizome with some roots, two or more buds for flowers next year and a fan of leaves. Larger divisions flower the first year after division; smaller ones may not bloom for a year or two.
Replant rhizomes that feel firm and are free of blemishes or other injuries. Discard old, unproductive and soft sections. Soft rhizomes may be diseased or under attack by iris borers. A sign of iris borers is yellow streaks in the leaves caused by bacterial rot in the iris borer's tunnel as it makes its way to the rhizome. Remove any brown leaves and cut back the fan of foliage by two-thirds or to 6 to 8 inches tall.
If there is any sign of disease, dip the knife in a 10 percent bleach solution between each cut to avoid spreading the fungus. Or better yet, be safe instead of sorry and use the bleach solution when dividing all irises.
German irises prefer a spot with well-drained soil in full sun. Prepare the soil before planting. Remove any weeds, work organic matter into the soil and spread a balanced fertilizer (10-10-10) according to the directions on the package, mixing it in the top 6 inches of soil.
The most-often mistake made when planting irises is they are planted too deep. Dig a hole, mounding the soil in the middle. Set the rhizome on top of the mound and spread the roots down the edges. Fill in the soil over the roots and gently firm. The soil should only lightly cover or hug the sides of the rhizome.
Divisions should be planted 12 to 24 inches apart for best flowering. Water thoroughly after planting. They will not need additional watering unless there is an extended dry period.
Named after the Greek goddess of the rainbow, the iris plant's spring display of flowers is worth a little extra attention throughout the season. Cut back flowering stems after the flowers have faded in late spring. If mulch is used in the garden, spread it very lightly around irises.
Periodically check for signs of disease or borers and remove and destroy any suspicious foliage as soon as possible. After frost in the fall, clean up brown leaves and cut back fans of foliage to 6 to 8 inches tall.
That is not a lot of work to enjoy the beauty and fragrance of irises — in the garden or in a vase. Today, there are even reblooming German irises available at local garden centers that bloom again in the fall.
• Diana Stoll is a horticulturist, garden writer and speaker. She blogs at gardenwithdiana.com.