Growing Irises

How to Grow Irises

• Fertilize in early spring with an all-purpose fertilizer scratched in around the plants. Avoid applying high-nitrogen fertilizers to the surface or carelessly mulching with organic matter, which may encourage rhizome rot.  Reblooming Irises perform best if fertilized again after the first wave of flowering is finished.
• Do not overwater irises; too much moisture in the soil can cause the rhizomes (roots) to rot. Water consistently and deeply, especially during summer drought.
• Keep rhizomes exposed. Unlike bulbs, which thrive deep underground, iris rhizomes need a bit of sun and air to dry them out. If they’re covered with soil or crowded by other plants, they’ll rot. Irises may benefit from shallow mulching in the spring.
• Taller irises may need staking or they will fall over. 
• Watch for iris borers in the foliage (dark vertical lines that may appear watery show up in the leaves). See pest tips below.
• Deadhead (remove spent blooms) consistently; Bearded Irises will flower sequentially on buds spaced along the stems.
• After blooming is finished, cut flower stems down at their base. 
• But do NOT trim iris leaves after they have finished blooming. Leaves carry on photosynthesis for next year’s growth. Cut off brown tips—and cut the flowering stalk down to the rhizome to discourage rot.
• After hard frost in the fall, cut foliage back hard, remove any foliage that appears spotted or yellowed and dispose of all debris in the trash.
• If iris foliage is hit with heavy frost, remove and destroy it to eliminate borer eggs. See your local frost dates.
• For winter protection, cover the rhizomes with an inch or two of sand topped with a light layer of evergreen boughs, applied after the ground freezes and removed when the Forsythias bloom the following spring.
• In the early spring, remove winter mulch and any old foliage to allow for fresh, new growth and prevent Iris borers.

Dividing Irises

Over time, it’s not unusual for plantings of iris to become overcrowded, which causes the rhizomes to lose vitality and stop blooming. When this happens (usually every 2 to 5 years), it’s time to divide and replant healthy rhizomes in fresh soil. 

When to divide? Do this task after flowering finishes and then trim the foliage back to six inches. 

1. Shortly after blooming (mid-summer), carefully dig up the clump of irises. You’ll find that the original rhizome that you planted (the “mother”) has produced several offshoot rhizomes.
2. Separate these rhizomes from the mother with a sharp knife and discard of the mother, as it will no longer produce blooms. 
3. Inspect the rhizomes for any rotting tissue or other signs of disease, removing and discarding infected parts or entire unhealthy rhizomes.
4. Trim the iris foliage down to 3 to 5 inches in height so that the plants can focus on establishing new roots.
5. Plant these fresh rhizomes in a new bed, replant them where they were before (after adding new soil), or share them with friends and spread the joy of irises!