of Rhode Island GreenShare Factsheets
Irises are among the earliest
blooming and easiest perennials to grow. The most popular
and widely grown iris is the tall
bearded iris. "Bearded" refers to the presence of a furry strip
on each of three drooping, petal-like sepals, called falls.
The true petals are called standards and are upright. Flowers
appear in late spring to early summer on stalks above the foliage.
Flower colors vary greatly and are even available in two-color
combinations. The leaves are stiff and strap-like and grow
in fan-shaped clumps.
Bearded irises are available in a wide range of heights;
miniature dwarf (up to 10 inches ), standard dwarf (10 to 15
inches), miniature tall (15 to 25 inches), intermediate (15-27
inches) and border (28 inches). Despite size differences, all
bearded irises thrive in full sun and well-drained soil.
Bearded irises grow from an underground
stem known as a rhizome. Healthy rhizomes increase in size
and develop branches
or "off-shoots" which can be broken off (do not remove attached
roots) and replanted. This method of propagation (division)
should be done one to two months after bloom (usually July
or August) every two to three years. When dividing, cut back
leaves to one-third their length to keep newly planted divisions
from being whipped by the wind and pulled from the soil. Otherwise,
cut back foliage in fall as it begins to brown.
The planting procedure is the same whether planting a
new plant or division. Cultivate the soil 12 to 18 inches deep
and mix in composted organic material. Dig a hole about six
inches deep and wide enough to accommodate the rhizome and
its roots. Build a small mound of soil in the bottom and place
the rhizome on it, spreading the roots carefully. Adjust the
height of the mound so that the rhizome is just beneath the
soil's surface. Fill in the hole and water. Bearded irises
should be spaced 12 to 18 inches apart.
Irises require a well-balanced fertilizer such as 5-10-10
or 6-10-6, applied once in early spring and again in early
summer following bloom. Ample moisture is required in spring
when flower buds are developing and during flowering.
The most serious iris pest is the iris borer, which overwinters
in the egg stage attached to leaves. The eggs begin to hatch
in late April, and the tiny caterpillars crawl up iris leaves
and begin chewing and mining their way down within the leaf
fold, eventually reaching the rhizome. Once in the rhizome,
they continue to feed, sometimes reducing the rhizome to a
hollow shell. Leaves which yellow and are easily detached indicate
damage to the rhizome. The fully-grown borer crawls out of
the rhizome and pupates in the soil in mid-August. The gray-brown
moth emerges in mid-September to October, lays eggs on dry
plant material and other garden debris and dies. Eggs overwinter
until the following spring, when they hatch and the cycle repeats.
Eliminate iris borer eggs by removing and destroying debris
in and around the iris planting in both spring and late fall,
or by digging rhizomes and physically removing the borer, then
replanting, in July or August.
Soft rot is a common problem of damaged iris rhizomes.
Soft rot enters the rhizome through any wound, including feeding
damage from iris borers. If soft rot is a problem, dig up the
rhizome, scrape out the affected tissue, allow it to dry in
the sun, then dip in a 10-12% solution of household bleach
for a few minutes. Rinse with water and allow to dry before
The beardless iris group includes those with smooth falls
and leaves which are long (2 to 4 feet) and relatively narrow.
Of this group, Japanese (Iris ensata), Siberian (Iris
siberica and Iris sanguinea), and Louisiana (hybrids
of Iris fulva, I. giganticaerulea and I. brevicaulis)
are the easiest to grow in the Northeast. Japanese and Louisiana
irises thrive in constantly moist soil or even standing water,
while Siberian irises can tolerate moist conditions as well
as those of a typical perennial border. They all prefer acidic
soil and require regular fertilizations through the growing
season. Unlike bearded irises, these should be transplanted
in fall or early spring (divide in early spring) and require
heavy watering during establishment. All but Louisiana irises
should be planted in a sunny location where they can remain
for several years. They do not thrive when disturbed too often.
Borers do not as readily attack these irises.
The crested or evansia irises have a cockscomb-like crest
along the lower half of the falls, instead of a beard. Iris
tectorum, I. cristata and I. tenuis range in height
from 6 to 12 inches, and bloom in May if provided with a moist,
shaded location and soil rich in organic matter.
The bulbous irises include the so-called English, Dutch
and Spanish irises, which have erect standards. Some have smooth
bulbs while others have a netted or fibrous covering on the
bulb. Bulbs are planted in fall. Iris reticulata and I.
danfordiae bloom in late February or March. Still other
lesser known irises--Iris pseudacorus, Iris laevigata, Iris
virginica and Iris versicolor--do quite well submerged
in a pond, at water's edge or in a moist part of the garden. Iris
versicolor also does well in ordinary garden conditions.
Adapted from Leslie Dybiec,
Ohio State University Extension, 2000
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
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The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program