Garden calendar: For the week of Aug. 5
Perennials: You can divide German bearded iris at this time through the end of September. Cut the leaves back about halfway so the plants are easier to handle. Yellowing foliage can be a result of infestation by iris borer larvae, and you may see whitish frass (larval feces) that are small roundish pellets. If you see these symptoms, dig up the rhizomes and check them for iris borers. The borers are mature at this time, and are pinkish and about 1½ inches long. Remove them from the rhizome and dispose of them. A fact sheet on iris borers is available at https://pddc.wisc.edu/. Look under the fact sheets tab for the title “Iris borer.” Also, adding insult to injury, a bacterial soft rot may set in following the borer damage and further deteriorate the rhizome, leaving it mushy and foul-smelling. If there is unaffected tissue remaining, cut off the infected material and wipe off the cut end of the rhizome with rubbing alcohol or hand sanitizer. Also disinfect the cutting tool with rubbing alcohol. Discard the infected material. After the rhizome is cut back to healthy tissue, let it dry and callus off in your garage for a couple of days. Then, re-plant making sure the rhizome is horizontal in the soil and about half of it is above and half below the soil grade. Firm it in well so it doesn’t fall over and watch to be sure that squirrels don’t dig it up, as they are attracted to areas with freshly disturbed soil, especially at this time of year. Some folks temporarily use landscape staples (also called sod staples) which are narrow U-shaped pieces of wire about six inches long to hold the rhizomes in place until they re-root. These are available in most home improvement stores and garden centers. You could make your own out of wire coat hangers if you have a wire-cutter. Water the rhizomes in after planting.
Boxwood blight now in Wisconsin: The fungal disease boxwood blight has recently been identified in Wisconsin for the first time. The groundcover perennial plant pachysandra is also susceptible to this disease. The fungus causes brown spots on the plant’s leaves that enlarge until they merge and the leaves drop off. Black lesions also form on the plant’s woody stems. Boxwood blight thrives in warm, humid conditions. It is most commonly spread via infected plants, but it may also be carried on garden tools, clothing, and vehicles. Even when infected plants are removed, reproductive spores may remain in the soil for up to 6 years. While fungicides labeled for the disease may help prevent it, they cannot cure it and need to be applied annually. Once infected, plants and leaf litter should be disposed of to avoid spreading the disease. You can dispose of them by burning if legal in your area, burying the affected material at least 2 feet deep, or double-bagging it and putting it out with your garbage to be landfilled.
If you suspect you have boxwood blight, contact the Plant Disease Diagnostics Clinic on the UW-Madison campus to submit a sample for a test; there is no charge for this. Visit the clinic website https://pddc.wisc.edu/ for information on sample submission.
To prevent spreading boxwood blight, only purchase plants from reputable suppliers, carefully inspect them and make sure to choose a resistant variety (note that resistance does not mean immunity; the plants may still get the disease if conditions are right). Some resistant varieties include “Green Beauty” (one of the most resistant), and “Winter Gem,” “Green Gem” (moderately resistant). Isolate new boxwoods from existing boxwood/pachysandra plantings for at least a month to make sure they are not infected. Some cultural methods to help discourage boxwood blight as well as other fungal diseases are to space plants out enough for air to circulate around them, avoid overhead watering that keeps the leaves wet, and avoid working with plants when they are wet.