Louisiana irises are found around the world. (Photo by Dan Gill, LSU AgCenter)

For Release On 08/28/15

By Dan Gill
LSU AgCenter Horticulturist

One of my favorite native plants is the Louisiana iris. When these plants bloom in the spring, their flowers are among the most colorful and beautiful to be found in Louisiana gardens. This time of the year, however, they may need some attention.

Louisiana iris is the name used worldwide for a unique group of Louisiana native irises species and, in particular, their hybrids. Their extraordinary beauty and reliability in the garden have made these irises increasingly popular, and they are now grown in gardens from Europe to Australia.

The interbreeding of these native species has resulted in the modern hybrid varieties we grow today. Their large, attractive flowers cover a broad range of colors, including many shades of blue, purple, red, yellow, pink, gold, brown, lavender, burgundy and white. Varieties with bicolor flowers of contrasting colors, bright yellow signal markings or ruffled petals add to their beauty.

The best time to plant Louisiana irises is in August and September when they are at their most dormant, or October just as they begin to grow. You may find a few available in nurseries now.

Louisiana irises are not at all attractive in late summer when they are dormant, so expect to see lots of yellow and brown foliage. Plants are more commonly sold in spring when they are in bloom, but now is the time to plant if you can get them. If you have a friend with Louisiana irises, now is a great time to share.

Although the original species often grow in swampy or wet conditions in their native habitats, the species and hybrids can also grow in ordinary garden beds with excellent results. Of course, they are also outstanding planted in boggy areas, grown in containers in aquatic gardens or planted in the ground at the edge of ponds. Their culture is really quite easy as long as you provide the right growing conditions and are familiar with their seasons of growth and dormancy.

Louisiana irises should be grown with as much direct sun as possible. Although they will tolerate shade for part of the day, at least about six hours of direct sun are needed for good blooming. Avoid locating beds near large evergreen trees that create shade and extensive root systems that would compete with the irises.

When preparing a spot to plant Louisiana irises, incorporate a generous 3-inch layer of compost, composted manure or composted soil conditioner and a general-purpose fertilizer following label directions. They prefer a soil high in fertility and organic matter.

The farther apart the irises are planted, the longer they may be left without being divided. If the plants become too crowded, they will not bloom as well. Crowding and insufficient sun are common reasons for poor bloom. If you are planting several plants in a bed, plant them in a group, spacing them about 12 inches apart.

Dividing Louisiana irises

Speaking of dividing irises – now is the time to divide established iris plantings. Louisiana irises are at their most dormant state in the late summer, making now through the end of September the ideal time to divide them.

Each year Louisiana irises grow and spread, creating more rhizomes and shoots. Eventually, the plants can become crowded, which leads to lower vigor and less flowering. This generally occurs a few years after the bed is planted, depending on how close they were planted to begin with. Dividing clumps of irises is a way to control the size of the clump, to invigorate clumps that have become overcrowded and to propagate irises to plant in other areas or share with friends.

To divide your irises, dig up a clump using a shovel or garden fork. Be careful not to damage the rhizomes. Locate the young rhizomes that have green foliage at their tips. These will bloom for you next year. Cut those rhizomes (generally about 6 inches long or less) from the old rhizomes that do not show new growth. Discard the old rhizomes. Replant the divisions immediately back into the bed or into containers.

Before replanting, take the opportunity to improve the bed. Remove any weeds remaining in the bed, making sure to remove their roots. Spread a 2-to-4-inch layer of compost or other organic matter and sprinkle a general-purpose fertilizer following package directions over the area, and work them into the upper 6 to 8 inches of soil. Do not let the exposed roots of the irises dry out while you do this.

After the bed has been reworked, plant the rhizomes horizontally with the fan of foliage facing the direction you want the plant to grow and carefully cover all of the roots. The top of the rhizome should be about one-half inch below the soil surface. Mulch the bed about 2 inches deep and water thoroughly.

Your overcrowded irises will thank you by blooming more prolifically in the spring. And, if you share some of the divisions with your friends, wonderful new plants are likely to come your way when your friends have plants to share.