Answer: Iris borers can ruin iris rhizomes. If you suspect your irises have borers, the best approach is to dig them all up, cut away the sections where you find borers, amend the soil with organic matter, and replant your rhizomes. Amending the soil will help them spread out (it will help your other plants, as well). When spreading plants don't, it's usually because growing conditions are less than optimal. Amending the soil with compost will loosen it and allow easy penetration of plant roots so they can creep and spread according to their nature. Management of iris borers is difficult. However, there are several steps that can be taken to reduce their damage in gardens.
Plant resistant varieties. Although all types of iris may be infested by iris borer, Siberian iris are more tolerant to iris borer attack.
Maintaining good garden sanitation is important. Check iris during spring for evidence of chewing damage and water-soaked streaks. This injury can be easily overlooked so look carefully. If you discover iris borer damage early in the season, you can crush the insect while it is inside the leaf or remove the infested leaf.
Sanitation in July is critical if you are having problems with iris borer. If an iris plant has above-ground symptoms (brown leaf tips, early senescence), dig it up and examine the rhizomes. Discard rhizomes containing iris borer caterpillars and those with tunnels.
During fall, remove and destroy (e.g. burning or burying) old iris leaves, stems, and any nearby plant debris. This removes and kills overwintering eggs, minimizing the risk of iris borers next year. Clean up iris beds anytime after we get a hard frost when female moths are no longer laying eggs.
Finally, a well-timed insecticide application can help reduce an iris borer infestation. Two options are acephate (Orthene) or spinosad (Bulls-Eye). It is important to time the application when eggs are just hatching. This is approximately when new growth is about four to six inches high. A repeat treatment 10-14 days after the first application may be necessary.
As for your junipers, trim them back in early spring, while they are actively growing and they will quickly fill in to cover your pruning cuts.
Best wishes with your garden!
Q&A Library Searching Tips