How to Grow Bearded Iris

By Guest Author Dr. Leonard Perry, Extension Professor, University of Vermont.

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, Irises need at least a half-day of full sun (full sun is best), 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 Bearded Iris rise above the leaves.  The Siberian iris will tolerate most soils including wet soils, while the Bearded Iris requires well-drained soils.

Iris flowers: Standards and Falls

Iris flowers have three upright petals called “standards” and three downward petals 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 blooms appear in the Northeast in mid to late May and late season blooms in the middle to end of June.

Bearded Iris care tips

When to plant Bearded Iris

For best establishment, plant Iris from mid to late summer so they will establish new roots by winter. They prefer a weed-free, rich soil, 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 indicates a need for phosphorus—good for root growth—add rock phosphate (organic) or superphosphate mixed in prior to planting. In most cases, soils contain enough phosphorus.

How deep to plant Iris

The most common mistake is planting Iris too deeply. The elongated rhizome should be horizontal, with the top at or just above the soil surface.  To plant Iris correctly, dig a shallow hole, with a mound in the center. Place the rhizome on top of the mound, and then spread the roots down around the mound. Then cover the roots with soil.  If the planting bed gets intense summer heat, plant the rhizomes just below the surface so they won’t get sunburn. Space plants about 15 to 18 inches apart. Then water well, and do not let the soil dry out.

How often to water Iris

Once the plants are established, and in subsequent years, only water if you experience a prolonged drought or if you live in arid areas. Bearded Iris will tolerate dry conditions well, but too much water can lead to soft, mushy rhizomes—a sign of rotting. It’s better to under water rather than over water.

Fertilizing Iris

Avoid high nitrogen fertilizers, as these may result in lots of leaves and no blooms, and may lead to root rots. Also avoid high-analysis fertilizers—those with high numbers. Choose a fertilizer with higher middle (phosphorus) and third (potash) numbers than the first (nitrogen), such as an organic 2-3-3 fertilizer. 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 Iris cultivars have more flowers and may need more fertility than older ones.  Even if you don’t fertilize, the plants should grow and bloom, just less abundantly.

After Iris blooms

Cut off the stalks after the Iris blooms. Remove any dying leaves, especially in fall, and cut the 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 the plants but to mulch around them. Apply the organic material in late fall and remove it in early spring.

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 the leaves to about 4 to 6 inches and the roots to 2 to 4 inches long, and remove any rotted parts (these may contain borers, so dispose of them in the trash, not in compost). Discard the original old rhizome as it won’t bloom again unless from side shoots. If it’s a large clump, you can divide it in pieces containing one or more roots with leaf  “fans”, using a knife or just your hands. Then replant the rhizome as you would a new root piece.

Identifying Iris pests and diseases

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 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 leaf spots, but to keep the disease from spreading, cut off the parts of the most infected leaves or use an appropriate fungicide spray (read the label to determine if the product controls this disease, and follow its proper use). If the rhizomes are rotting, dig them up and remove the rotted parts, and replant in drier soil.

More on Irises can be found at the American Iris Society (www.irises.org).

A version of this article, Growing Beautiful Bearded Iris, appeared on the University Of Vermont Extension Service website.

We garden in Pennsylvania, United States, zone 6B. Much of the info we share is based on gardening in our temperate area. But many of the flowers, vegetables, and fruit we write about can be grown in soil other than ours (clay) and significantly different weather conditions and elevations. You might need to choose specific varieties for your region, modify your garden, add soil amendments, or adjust the soil pH to match our results. Please check your local university extension website for specifics for your area.