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08-07-03
Question and Answer
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.
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