Louisiana iris is the name used worldwide for a unique group of Louisiana-native iris species and, in particular, their hybrids. They are now grown in gardens from Europe to Australia.

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, says LSU AgCenter horticulturist Dan Gill.

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. Although they will tolerate shade for part of the day, irises need at least six hours of direct sunlight for good blooming.

Louisiana irises prefer a soil high in fertility and organic matter.

The farther apart they are planted, the longer they may be left without being divided. If the plants become too crowded, they will not bloom as well, so plant them about 12 inches apart.

Dormancy is the time to divide established iris plantings, making now through the end of September the ideal time to divide them.

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, Gill says.

To divide your irises, dig up a clump using a shovel or garden fork. The young rhizomes that have green foliage at their tips will bloom next year.

Cut those rhizomes from the old ones that do not show new growth.

Discard the old rhizomes and replant the divisions immediately back into the bed or into containers 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.

Got a gardening question? Write to GardenNews@agcenter.lsu.edu.