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Enjoy the glorious blooms while the color lasts

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Know It and Grow It
Lead Summary
By
George Bonnema, Luverne Horticulturalist

Today my flower gardens are absolutely glorious! This is the color I envision and plan for all through the cold, dark days of winter ... so enjoy every moment while it lasts.
If you are feeding your planters, they should be at their peak of color now. If you have one or two plants that seem to be determined to take over the space, prune them to keep them in bounds. Sweet potato vine is one that will often demand more than its share of room. Cutting some stems out will not hurt it but will give the other plants a chance to fill in … don’t worry about the sweet potato … it will be right back demanding as much room as you will allow.
I noticed some dead basal leaves and small yellowish spots on the leaves of my German bearded iris and confirmed that the spots are caused by iris borers. This is not untreatable but means more work than I was planning for today. My iris clumps are not large so I will dig the clump, wash the soil off the rhizomes and remove the damaged tubers. Then I’ll cut the leaves back to a 4-inch fan and soak the rhizomes in a 10-percent bleach solution for an hour. After that, they get to dry in the sun for a few hours and then I’ll replant them.
Iris borers are not new to our area, but I have not had to deal with them before. If left untreated, they will weaken the rhizome and you will end up with very few if any flowers. Total cleanup of all foliage in the fall is essential for control.
Iris borers spend the winter as eggs on old iris leaves and plant debris at the base of iris stalks. In early spring they hatch into tiny caterpillars and climb up new foliage. They chew pinprick-sized holes and tunnel inside leaves as they continue down toward the rhizome. Their feeding causes streaks that appear tan or water-soaked. Later the tips of infested iris turn brown and appear to senesce (age prematurely), although the entire plant seldom dies.
The caterpillar tunnels through the leaves, reaching the rhizome by midsummer (July in Minnesota). At that point, caterpillars have grown to about 1½ to 2 inches in length. Iris borers do severe damage to iris by consuming the rhizome. Iris borer feeding can allow the entry of a bacterial soft rot. Rhizomes infected by soft rot are slimy, soft and foul-smelling.
There are several iris diseases with similar symptoms that can occur when iris borer is absent, including bacterial soft rot, iris scorch (probably caused by a phytoplasma), and fungal leaf spot (Didymellina macrospora). All of these disease organisms will cause iris leaf tips to brown and iris plants to senesce, mimicking symptoms caused by iris borer.
In late July or early August, iris borer caterpillars move into the soil where they pupate. They emerge as moths in late summer or fall. Female moths complete the life cycle by laying eggs on old iris plants in August and September where eggs remain until next spring.

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