Ferguson: You can divide, plant, and fertilize Louisiana irises in the fall
Published 12:28 pm Friday, September 16, 2022
Many of our perennial flowering plants are putting on a lovely show now or did so earlier in the year. A lot of these plants will become dormant within the next few months, but the leaves of Louisiana irises — which have been named 2022 Louisiana Super Plants — provide green color in landscape beds through the winter and then flower in the spring.
Iris brevicaulis (zigzag iris), I. giganticaerulea (giant blue iris), I. hexagona (dixie iris), I. fulva (copper iris), Iris x nelsonii (Abbeville iris), and hybrids among these make up the group known as Louisiana irises. They grow well in wet areas and need approximately six hours or more of sun per day for abundant flower production.
Louisiana irises bloom around April in southern Louisiana and are largely dormant in summer and early fall. August through early October is a good time to divide them. While they aren’t always readily available in nurseries during these months, this is also a good time to plant containerized plants.
Louisiana irises spread by rhizomes, and if plants become too crowded, they won’t bloom well. When dividing iris rhizomes, keep the young ones that have leaves emerging from their tips and get rid of old ones.
Prepare soil well prior to planting. These irises are native to sites with relatively high organic matter levels. Plantings will likely benefit from addition of compost or another organic material if the soil is low in organic matter.
Iris rhizomes grow at or just under the soil surface. Cover them with no more than one inch of soil. If planted in water, they can be laid on the soil surface and pinned in place.
Rhizomes can be placed as close as eight to 10 inches apart. If you’d prefer to keep irises of two varieties separate, you can space them out more. Plants will grow in the direction of the end of the rhizome with leaves.
Unless the irises are planted in water, spread two to three inches of mulch on top of the soil. This helps prevent sunscald of rhizomes and suppresses weed growth.
Louisiana irises in landscape beds can be fertilized in October, when new growth begins, and again in February. Fertilizer can be applied at a rate of approximately 0.1 pound of actual nitrogen per 100 square feet. Examples of fertilizers and amounts that would provide 0.1 pound nitrogen include 0.75 pound (1.5 cups) 13-13-13, one pound (two cups) 10-10-10, and 1.25 pound (2.5 cups) 8-8-8.
Let me know if you have questions.
Dr. Mary Helen Ferguson is an extension agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa parishes. Contact Ferguson at firstname.lastname@example.org or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).