Iris, no matter the size, propagate through rhizomes. The iris’ rhizomes will begin to clump three to four years after the initial planting. It is recommended to divide and transplant the new iris rhizomes at least every four years to keep blooms strong and prevent rhizomes from developing on top of one another. The best time to transplant a tall iris, which blooms later than the smaller sizes, is in July and August, after the flowers have died back. Plant the divided iris rhizomes in well-drained soil with access to plenty of sunshine.
Break up the earth around the iris with a spade fork. Insert the fork in the ground under the iris and pry up the clump of rhizomes and roots, gently lifting them out of the ground.
Shake the soil from the plant. Cut rhizomes with healthy roots and one or two strong leaf fans from the older plant with the knife. Discard the old center rhizome and any rhizomes that are soft or damaged.
Cut the fan leaves back to a height of six inches.
Prepare the new planting site by digging a shallow hole twice the diameter of a rhizome. Make a slight ridge in the center of the hole. Set the cut rhizome on top of the ridge and spread the roots on either side of it. Adjust the rhizome so it sits one inch below the soil surface.
Cover the rhizome with soil. Firm the soil with your hands. Water thoroughly to remove air from around the roots and improve soil contact. Space each divided rhizome 18 to 24 inches apart.
Keep weeds from the new transplants by shallow cultivation with the garden hoe.
Tall bearded iris receives all of its nutrients from the soil. No additional fertilizer is required when transplanting.
Mulch the iris plants in areas where severe cold weather is a threat. Remove the mulch in early spring once all threat of frost has passed.