University of Vermont Extension
Department of Plant and Soil Science
Summer News
Article
GROWING BEAUTIFUL BEARDED IRIS
Dr. Leonard Perry, Extension
Professor
University of Vermont
There are several reasons bearded
iris have been popular with gardeners for many years: they’re
hardy, they have gorgeous blooms in
early summer in many colors and color combinations, and they are
easy to
grow. For the most beautiful blooms they
need at least a half day of full sun (full sun is best, especially
for newer
cultivars), planting near the surface, and dividing every 3 to 4
years.
There are many types of irises, and
thousands of cultivars (cultivated varieties), but the most common
and hardy
for the north are the very different Siberian and bearded irises.
While the Siberian grow in a clump with tall
(2 to 3 feet) narrow leaves, the bearded have much wider
sword-shaped leaves
only a foot or so high which arise from swollen storage organs
called
“rhizomes”. Flowers of the latter rise
above the leaves. The Siberian will
tolerate most soils including wet ones, while the bearded iris need
well-drained
soils.
Iris flowers have three upright
petals called “standards” and three downward ones called “falls”.
What gives the bearded iris their name is the
fuzzy attachment or row of hairs—the “beard”—near the base of each
falls
petal. While the colorful standards
attract pollinators, falls give them a landing pad, markings guide
them to the
nectar, and the beard provides a means for them to hang on.
Iris professionals recognize six
classes of bearded iris, based on their height, from the miniature
dwarfs with
bloom stalks under 8 inches tall and blooming early, to the tall
bearded with
bloom stalks 28 inches or more tall and blooming late. Depending on
location, early season in the
Northeast may be mid to late May and late season the middle to end
of
June.
For best establishment, plant
iris from mid to late summer so they will be settled with new roots
by
winter. They prefer a rich soil, weed-free,
and amended with organic matter such as peat moss or compost. You
may want to do a soil test first,
particularly for soil acidity or pH, as they prefer a neutral to
slightly acid
(6.5 to 7.0) soil. If the soil test
indicated a need for phosphorus—good for root growth—add rock
phosphate
(organic) or superphosphate mixed in prior to planting. Otherwise
many soils already contain enough
phosphorus.
The most common mistake is
planting too deeply. The elongated
rhizome should be horizontal, with the top at or just above the soil
surface. You can make a shallow hole,
with mound in the center. Place the
rhizome on top of the mound, laying the roots downward around it.
Then cover the roots with soil. If a planting bed gets intense
summer heat,
rhizomes can be planted just below the surface so they won’t get
sunburn. Then water well, and keep new plants watered
if the soil dries out. Space plants
about 15 to 18 inches apart.
Once plants are established, and
in subsequent years, only water if prolonged drought or in arid
areas. Bearded iris will tolerate drought well. Too much water can
lead to soft, mushy
rhizomes—a sign of rotting. It’s better
to under rather than over water.
Avoid high nitrogen fertilizers, as
these may result in leaves not blooms, and may lead to root rots.
Also avoid those high-analysis
fertilizers—ones with high numbers.
Choose one with higher middle (phosphorus) and third (potash)
numbers
than the first (nitrogen), such as the organic 2-3-3. If you have
rich soil and apply some compost
around plants (not on top of them) in spring, this may be all that
is needed. Fertilize in early spring, and perhaps again
just before bloom. Newer cultivars have more
flowers and may need more fertility than older ones. Even if you
don’t fertilize, plants should grow and bloom, just less abundantly.
Cut off stalks after bloom. Remove any dying leaves, especially in
fall,
and cut leaves back in late fall to 4 to 6 inches high. If you
mulch with an organic material such as
straw or chopped leaves to prevent winter heaving, make sure not to
cover
plants but mulch around them. Apply it in late fall and remove in
early
spring.
If you see chewed leaf edges,
long streaks on leaves, or mushy rhizomes, your plants may have iris
borers. You may use insecticides on
these, but keeping a clean garden, and squashing borers often as
they appear
may be all that is needed. If you see
spots on leaves, these are likely from a leaf spot fungus. Plants
can tolerate a fair number of these, but
you can cut off the parts of the most infected leaves, or use an
appropriate
fungicide spray program (read labels to determine if a product
controls this
disease, and to follow its proper use).
If rhizomes are rotting, dig up and remove rotted parts, and replant
in
drier soil.
To keep bearded iris blooming at
their best, divide them when they are crowded—every 3 to 4 years.
Simply lift the plants (a garden fork works
well) in July to mid-August, cut back leaves to about 4 to 6 inches
high and
roots to 2 to 4 inches long, and remove any rotted parts (these may
contain
borers, so dispose of them in trash not compost). Discard the
original old rhizome as it won’t
bloom again unless from side shoots. If
a large clump, you can divide in pieces containing one or more roots
with leaf “fans”, using a knife or just your hands.
Then replant as you would a new root piece.
More on irises can be found at
the American Iris Society (www.irises.org).
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