The Irises in my Garden Are Developing Poorly. How can I Help Them?

Do your irises have leaves with irregular tunnels and damaged or rotted rhizomes?

Do your irises have leaves with large, ragged holes?

Are the flower buds on your irises dying and are the petals distorted or is the plant's growth stunted?

Do your irises have leaves with yellow, brown or black spots?

Do your irises have leaves with water-soaked spots and rotted soft rhizomes?

Do your irises have mottled or streaked leaves?

Leaves with irregular tunnels and damaged or rotted rhizomes are caused by Iris borers. These pinkish, up to 2 inches long larvae enter the plant on the top and tunnel down to the rhizome. They often introduce soft rot bacteria into irises as they feed. In the fall, remove dry leaves as they might carry borer eggs and destroy badly infected plants in spring. In the spring, you can also dust the base of the plants with pyrethrin to kill emerging larvae.

Large, ragged holes on the leaves are caused by slugs and snails. Slugs and snails feed on the plant leaves usually at night and most species leave slimy trails of mucus. To control slugs and snails use diatomaceous earth around your plants. This will irritate their soft bodies and their numbers will drop. You can also handpick them during the night or use shallow dishes filled with beer as a trap for them.

Dying flower buds, distorted petals, and stunted growth are caused by thrips. Thrips are tiny, yellow to black flying insects. They hide under leaves and inside flowers. Severely infected plant parts should be pruned off and destroyed. Control thrips by introducing lady beetles or lacewings, their natural predators. Insecticidal soap helps in severe infestations.

Leaf spots are caused by several different species of fungi. Fungi love wet weather, so prevent leaf spots by ensuring good air circulation around irises. Infected plant parts should be removed and destroyed.

Water-soaked spots on leaves and rotted, soft rhizomes are caused by bacterial soft rot. It usually attacks the plant during wet seasons and in poorly drained soil. Remove and destroy rotting rhizomes and wash tools when cultivating to avoid transmitting the disease. Control borers as they can because of soft rot infection.

Mottled or streaked leaves are caused by the mosaic virus. There is no cure for infected plants. Remove and destroy all irises that have the symptoms of mosaic virus. Mosaic virus is spread by aphids and leafhoppers, so the best way of preventing the disease is to control these insects.

If your the stems of your irises rot at the base, the most likely cause is a fungal disease - Crown rot. Leaves and stems turn brown at the base, the foliage turns yellow and black spores may appear on the stems. Dig and divide iris rhizomes every few years to avoid overcrowding. Plant irises in well-drained soil and avoid damaging crowns when cultivating.