There are several reasons bearded iris have been popular with gardeners for many years: they’re hardy, they have gorgeous blooms in early summer in many colors and color combinations, and they are easy to grow. For the most beautiful blooms they need at least a half day of full sun (full sun is best, especially for newer cultivars), planting near the surface, and dividing every 3 to 4 years.
There are many types of irises, and thousands of cultivars (cultivated varieties), but the most common and hardy for the north are the very different Siberian and bearded irises. While the Siberian grow in a clump with tall (2 to 3 feet) narrow leaves, the bearded have much wider sword-shaped leaves only a foot or so high which arise from swollen storage organs called “rhizomes”. Flowers of the latter rise above the leaves. The Siberian will tolerate most soils including wet ones, while the bearded iris need well-drained soils. Iris flowers have three upright petals called “standards” and three downward ones called “falls”. What gives the bearded iris their name is the fuzzy attachment or row of hairs—the “beard”—near the base of each falls petal. While the colorful standards attract pollinators, falls give them a landing pad, markings guide them to the nectar, and the beard provides a means for them to hang on. Iris professionals recognize six classes of bearded iris, based on their height, from the miniature dwarfs with bloom stalks under 8 inches tall and blooming early, to the tall bearded with bloom stalks 28 inches or more tall and blooming late. Depending on location, early season in the Northeast may be mid to late May and late season the middle to end of June.
For best establishment, plant iris from mid to late summer so they will be settled with new roots by winter. They prefer a rich soil, weed-free, and amended with organic matter such as peat moss or compost. You may want to do a soil test first, particularly for soil acidity or pH, as they prefer a neutral to slightly acid (6.5 to 7.0) soil. If the soil test indicated a need for phosphorus—good for root growth—add rock phosphate (organic) or superphosphate mixed in prior to planting. Otherwise many soils already contain enough phosphorus.
The most common mistake is planting too deeply. The elongated rhizome should be horizontal, with the top at or just above the soil surface. You can make a shallow hole, with mound in the center. Place the rhizome on top of the mound, laying the roots downward around it. Then cover the roots with soil. If a planting bed gets intense summer heat, rhizomes can be planted just below the surface so they won’t get sunburn. Then water well, and keep new plants watered if the soil dries out. Space plants about 15 to 18 inches apart.
Once plants are established, and in subsequent years, only water if prolonged drought or in arid areas. Bearded iris will tolerate drought well. Too much water can lead to soft, mushy rhizomes—a sign of rotting. It’s better to under rather than over water. Avoid high nitrogen fertilizers, as these may result in leaves not blooms, and may lead to root rots. Also avoid those high-analysis fertilizers—ones with high numbers. Choose one with higher middle (phosphorus) and third (potash) numbers than the first (nitrogen), such as the organic 2-3-3. If you have rich soil and apply some compost around plants (not on top of them) in spring, this may be all that is needed. Fertilize in early spring, and perhaps again just before bloom. Newer cultivars have more flowers and may need more fertility than older ones. Even if you don’t fertilize, plants should grow and bloom, just less abundantly.
Cut off stalks after bloom. Remove any dying leaves, especially in fall, and cut leaves back in late fall to 4 to 6 inches high. If you mulch with an organic material such as straw or chopped leaves to prevent winter heaving, make sure not to cover plants but mulch around them. Apply it in late fall and remove in early spring.
If you see chewed leaf edges, long streaks on leaves, or mushy rhizomes, your plants may have iris borers. You may use insecticides on these, but keeping a clean garden, and squashing borers often as they appear may be all that is needed. If you see spots on leaves, these are likely from a leaf spot fungus. Plants can tolerate a fair number of these, but you can cut off the parts of the most infected leaves, or use an appropriate fungicide spray program (read labels to determine if a product controls this disease, and to follow its proper use). If rhizomes are rotting, dig up and remove rotted parts, and replant in drier soil.
To keep bearded iris blooming at their best, divide them when they are crowded—every 3 to 4 years. Simply lift the plants (a garden fork works well) in July to mid-August, cut back leaves to about 4 to 6 inches high and roots to 2 to 4 inches long, and remove any rotted parts (these may contain borers, so dispose of them in trash not compost). Discard the original old rhizome as it won’t bloom again unless from side shoots. If a large clump, you can divide in pieces containing one or more roots with leaf “fans”, using a knife or just your hands. Then replant as you would a new root piece.
More on irises can be found at the American Iris Society (www.irises.org).