INFO: Frequently Asked Questions
This file will be posted to Iris-L at regular intervals.
It is also available on Jim Wilson's web page
IRISES: FREQUENTLY ASKED QUESTIONS
Prepared by Jim Wilson, copyright 1995
1. Why do my irises grow fine and not bloom?
The most common cause of decreased bloom is an
overcrowded clump in need of division. Bloom will have
decreased steadily for a few years, probably most
clearly in the middle of the clump. Another possibility is
insufficient sunlight. Finally, the nutrient balance may
be wrong--lawn fertilizers, for example, can cause
heavy foliage growth but do little to promote bloom.
If a rhizome has only been planted for a year or two and
not bloomed, especially if it had been stressed (maybe
by overcrowding) beforehand, it may take that much
time to be established before blooming again. Time is the
best remedy for that, but be sure the plant gets
2. Can irises be made to bloom longer?
Several approaches are possible to extending iris
bloom. First, most modern varieties have a high bud
count--more blooms per stalk--than many of their
ancestors. Consequently, each of their stalks will be in bloom
longer. Some have as many as three buds in each
socket. Second, some bearded irises are classed as
rebloomers--they will bloom a second time in the summer
or fall if grown well (sometimes with watering and
feeding after spring bloom). More irises fit this category
in areas with longer growing seasons, but two that are
reliable most places are SDB Baby Blessed and TB
Immortality. Some beardless irises exhibit extended or
repeat bloom as well, e.g., SIBs Lavender Bounty
and Reprise. Third, growing multiple species of irises can
stretch bloom from the late winter appearance of
I. reticulata through the early- or mid-summer bloom of the
3. My tall bearded irises sometimes fall over. What can I do?
This is one of the possible problems with TBs. The main
things that lead to it are the variety, the weather while
the stalks are growing, and the weather when the flowers
bloom to catch the rain and the wind. Some people
say that the nutrients in the soil, especially the potassium,
are a factor too. Clumps at their peak do tend to
produce taller stalks and more and larger flowers, which
together can outpace the increase in stalk strength.
Finally, shade on one side can cause a plant to lean, which
can contribute to the problem.
One thing that hybridizers strive for are stalks that will
stand up. This isn't always successful--there is one
recent top award winner, Victoria Falls, which is a fantastic
iris in many respects, but when you mention its
name to an experienced grower, you are likely to hear
"Yes, she does" in response. These irises, and maybe
yours, are best grown in protected areas or by people
willing to stake them. Or cut the fallen ones quickly
before anyone sees.
I like to grow some of the shorter TBs and other bearded
irises of intermediate height (16-27"): Intermediate
Bearded (IB), which bloom just before the TBs, and
Border Bearded (BB), which bloom with the TBs. And I
especially like the Miniature Tall Bearded (MTB), or
Table Irises, which have slender, flexuous stalks and
small flowers. These shorter ones are good for even
moderately windy locations.
There are lots of moderate height TBs that will generally
stand up well, though. Even a few of the big,
large-flowered ones are built with strong stalks--
my favorite in that category is Dusky Challenger. Many of the
beardless irises stand up real well too.
4. How often should irises be divided?
They should be divided when the clump begins to get
crowded. Often a clump in need of division will bloom
poorly, especially in the central portions. For most
modern bearded iris hybrids, three or four years of good
growth can be expected, although some varieties are
exceptions on both sides. Japanese Irises will last about the
same amount, while many Siberian Irises will go a year
or two more. Louisiana Irises tend to spread so
extensively that containment and tracking are more
an issue than crowding. Many Spurias can go a decade or
more without need for division.
5. What time of the year should irises be divided?
Bearded irises typically continue underground growth for
a month or more after spring bloom season, then
enter a dormant period, which is a good time to divide.
This is in July in the northern USA, extends to mid
August in much of the middle section, and as far as early
September for parts of the south. Replanting at this
time gives the plants time to grow in the fall and develop
good root systems.
Siberian Irises and Japanese Irises often work well replanted
a month or so later, since they are planted deeper.
Dividing these beardless varieties soon after bloom
season works well in areas where heat and drought is not
expected over the following month or two. Spring
division can also work well in areas where the soil is not too
wet to work during that period. They also require regular
watering for the remainder of their establishment
6. How do I divide them?
Dig up the clump, being careful to include at least a
few inches of roots with it. Shake off or remove excess
soil. Most clumps can be pulled apart by hand, but with
some well established and matted Siberian Iris clumps,
you may need a pair of pitchforks to pry portions apart.
You'll notice that the fan is connected to a bulge in the
rhizome, and then there are necks and bulges corresponding
to previous growth periods (one per year in many
cases). As a general rule, save only the fan and its
associated rhizome part, discarding previous growth. Wash
them off to minimize transfer of soil-borne diseases.
If they are beardless, keep them wet from the time you
dig them until replanting. Bearded ones can be dried,
and even do better with a drying out period in that it can
minimize fungus problems. Cut the leaves back to
6 inches (shorter on SDBs, longer on vigorous TBs). If
you're doing multiple clumps, it helps to write the
variety name on a leaf with a permanent marker until it is
replanted in its new mapped and/or labeled location.
7. How do I replant them?
Choose a sunny spot, one with good drainage if you are
planting dry-land varieties such as bearded ones. It's
best if the rhizomes can be replanted within a few days,
but two weeks will do little harm, and there have been
cases where they have survived a year in a cool dry place.
Soak the rhizomes and roots for a few hours before
replanting. If soft rot may be a problem, include some
agricultural streptomycin in the soaking solution. (You
can use a little household bleach [10 parts water : 1 part
bleach] instead, but be sure to rinse it off before
Dig a hole with a hill in the middle. The object is to plant
the rhizome on a firm base of earth with no air space
underneath and with the roots spread out and extending
down and out. Fill in the soil, pack it down well, and
water very well. If you are planting bearded irises, their
tops should be barely covered with good soil. (If your
soil tends toward clay, leave the tops of the rhizomes exposed.)
No extra watering should be necessary. If you
are planting beardless irises such as Siberian Irises or
Japanese Irises, they should be two or three inches
deeper. Keep beardless irises well watered for the
remainder of the growing season.
8. What do I do about iris borers?
Borers are a problem especially in the northeastern
quadrant of the USA and nearby points. The single most
important step to take in avoiding borer problems is to
do a very thorough cleanup well before the expected
last frost, removing all withered foliage and debris. These
were likely where eggs were laid last fall and
overwintered, and taking it away before they hatch
goes a long way toward eliminating problems.
Beneficial nematodes have been suggested as a biological
control mechanism. They need to be applied and
active when the eggs hatch and the larvae are making
their way to target leaves, triggered by the day that the
temperature first goes above 70-72 degrees, often soon
after the last expected frost date.
The most effective active measure that can be taken is
to spray with a systemic insecticide like Cygon-2E, a
powerful chemical. Two or three sprayings should be
done, the first at or right before hatching and repeated
after 10 days or two weeks or until the beginning of the
tall bearded bloom season. After that point, the borers
are so large that no reasonable biological or chemical
control will be effective. Sometimes they can be seen in
the bottoms of the leaves and killed by squeezing there.
9. What is the smelly yellow ooze in the rhizome and at
the leaf bases?
The irises have become the victim of bacterial soft rot.
Remove the affected leaves and survey the extent of the
problem. Small attacks can sometimes be treated in the
ground; more extensive ones require digging and heavy
surgery. Scoop out all of the softened tissue, then cut into
the good tissue with a clean knife. (Wipe off and then
disinfect the knife in a bleach solution between cuts.)
Leave the cut open to air and sun. Sprinkling with
agricultural streptomycin or even Comet might help too.
Prevention is much easier than dealing with problems
in progress. Plant bearded iris rhizomes so that their top
is at or just under the soil level, even showing above it
if your soil tends toward clay. Insure good drainage. Do
not mulch them or fertilize them heavily. Control borers.
10. What are the small rust/brown spots on the leaves?
This is iris leaf spot. It is a fungus that in small amounts
will not do significant harm to the plant. If it gets out
of hand, the leaves and hence the plant will be weakened.
Cut off the portions of the leaves that are starting to
form spots and/or spray with Daconil to control. Discard
these diseased portions of leaves rather than
11. Should irises be cut back in the fall?
If bearded irises are growing in good health, they need
not be cut back at all. Unfortunately, leaf spot and other
fungal diseases can sometimes attack the leaves, and many
growers contain these problems by cutting off the
diseased portions of leaves. This is probably the best for
the plants, preserving all the green leaves possible
while containing disease. Some growers regularly cut back
all leaves, either because of severe disease outbreaks
or because of habit--most of the time this is not a good idea,
although the irises usually come through the stress ok.
Beardless varieties like Siberian Irises and Japanese Irises
regularly turn brown in the fall, some time between
the first frost and six weeks later, and they should be cut
back to an inch or two at that point.
Another time irises are cut back is during the division
process. New roots can only support a reduced leaf
mass, and smaller fans are less likely to blow over in winds.