Iris borers raise a big stink by midsummer

Garden Q&A

August 04, 2007|By Ellen Nibali and David Clement | Ellen Nibali and David Clement,Special to the Sun

Some rhizomes of my bearded iris smell rotten. When I dig them up, they're like mush. Leaves are dying, or gooey or fine. What do I do?

Iris borers are pale, hairless caterpillars of a moth. In early spring, the moth's eggs hatch into tiny caterpillars that chew inside leaves, causing brown streaks and brown tips in the plant, though the entire iris seldom dies. By midsummer, borers tunnel into the rhizome. Their feeding allows entry of a bacterial soft rot that makes rhizomes slimy, soft and foul-smelling.

Check iris leaves in spring for evidence of chewing. Remove the leaf or crush borers while they're inside. In July, cut off rotted rhizome parts. Spear borers or discard borer-infested rhizomes. Practice good sanitation in fall by destroying old iris leaves, stems and adjacent plant debris to remove overwintering eggs.

My bean plants have finished producing. Should I pull them out and mulch that empty spot until next spring?

Why not plant another vegetable for a fall crop? It's counterintuitive, but mid- and late-summer are prime times for planting dozens of fall vegetables. Check out our fact sheet Planting Dates for Vegetable Crops in Maryland. Look online or call us to get one. Your other options are mulching or planting a cover crop. The bottom line is to prevent weeds, weed seeds and erosion.

Checklist

It's too late to apply post-emergent crabgrass herbicides. Crabgrass problems should be dealt with next spring by using a pre-emergent herbicide. Over-seeding your lawn in fall to thicken the turf also helps combat crabgrass.

Poison ivy leaves turn red soon. Leaves retain irritating toxins until they decompose. Do not burn vines.

Ellen Nibali, horticulture consultant, works at Maryland Cooperative Extension's Home and Garden Information Center, and David Clement is the regional specialist. Call the center's "hotline" at 800-342-2507 or e-mail plant and pest questions through the Send a Question feature at hgic.umd.edu.

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