Q. My Siberian iris plants grow and bloom well, but the plants have a dead spot in the center of each clump. What do I do? — Caryn Hildreth, Lafayette, Ind.
A. Many perennials, including iris, grow in an expanding circular pattern. This is their way of spreading to new locations. This growth pattern usually leaves behind a dead center, which works to the benefit of the plant since the new growth is more vigorous and healthy. As gardeners, we consider that empty center area to be an eyesore. In order to keep the plant vigorous and attractive, the plant should be lifted and divided. Siberian iris can be divided every three to five years.
I prefer to divide plants in early spring when the foliage is about 1 inch high. Siberian iris are tough enough that you can divide them at any time during the growing season, but the spring rains reduce the work required from you afterward.
Lift the clump and chop it into several pieces with a spade. Each piece should contain healthy roots and shoots. Discard the dead center portion of the plant. Plant the divisions and water regularly until they are established.
Q. The leaves on our black-eyed Susans are covered with small, dark brown spots. This happens every summer. It happens to plants in full sun as well as those in partial shade. What causes it? — Lynn Hegewald, Battleground, Ind.
A. Almost every Rudbeckia ‘Goldsturm’ (a cultivar of black-eyed Susan) I see this time of year is infected with Septoria leaf spot, caused by the fungus Septoria rudbeckiae. Symptoms begin as small, dark brown lesions, which enlarge to one-eighth to one-fourth inch in diameter.
The fungus overwinters in infected plant residue. Spores are produced in late spring and early summer, causing leaf spots on the lower leaves. As the season progresses, lesions develop on upper leaves as well. The spores of the fungus are dispersed by splashing water (either irrigation or rainfall), and can cause lesions throughout the growing season. Like most fungal leaf spot diseases, the spores require moisture to germinate and cause infection.
It is important to remove the infected leaves at the end of the growing season to reduce the amount of spores available the following year. Proper plant spacing will increase air circulation around the foliage and allow leaves to dry off quickly after dew or rainfall events. Since Rudbeckia plants spread quickly, this will involve pulling volunteer plants. Avoid overhead irrigation, which will promote leaf wetness and also splash spores from plant to plant.
While Septoria leaf spot is unsightly, the damage is primarily cosmetic, and infected plants will bloom. Infected leaves may die a little earlier in the fall than uninfected leaves. A general-purpose garden fungicide may help reduce the spread of the disease, but these chemicals are protectants and do not cure infected leaves. Application in early to mid June may help reduce initial infection, and result in a slower onset of disease symptoms. For maximum control, application of a protectant fungicide should be made periodically throughout the growing season (check label for instructions on spray interval and rate). This is impractical for most gardeners, however. I try to plant something in front of them to hide the unsightly leaves, instead.