The bearded iris in our church memorial garden bloom and grow well but have an infestation of borers. Besides trying to keep them free of dead leaves and debris, are Merit granules, by Bayer, an acceptable preventive treatment? And because iris rhizomes need sun, there is a pesky weed that springs up thickly where there is no mulch in the summer. What could we treat them with to prevent that?
The Gardener’s Answer
Hello, Sharon: The most effective method of getting rid of borers in your iris is to dig them up and separate the infected rhizomes from the newer healthier ones. Dispose of the infected rhizomes and transplant the healthy ones. You may see a decrease in blooms this year only because there are fewer plants but they will spread quickly. Borers live in older plants so removing them will drastically reduce the population. Iris like to be divided every few years. They spread and become overcrowded, forcing them to compete for nutrients, and as a result we get fewer blooms. So if you have not divided them recently you have two reasons to start digging–this is the time of year to divide them. I am sure you could get help from some other members. It will be obvious which ones are infected. There will be entry and exit holes. Borers only feed in the larvae stage so removing them now before the temperatures rise and they hatch will be very beneficial. Imidacloprid is the active ingredient in Merit. This granular systemic is effective against borers if applied during the spring while the borers are in the larvae stage and feeding. This is not an organic product and is highly toxic to honey bees. This systemic pesticide should be used only as directed. As you know, keeping the iris garden free of plant debris is important for a healthy garden so continue to do so. As for the weeds, the best solution is hand pulling. The trick is to get as much of the root system as possible so it cannot continue to spread. If it is possible to mulch this area this would prove to be helpful as well.