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HOW TO GROW IRIS |
| Tall bearded iris are one of the easiest and hardiest
perennials to grow. They are available in a rainbow of colors and will
reward you with beautiful blooms year after year. You will find these
suggestions on growing iris to be simple, and proper care is very easy.
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| WHEN TO PLANT |
| It is best to plant your iris
so the roots get well established before the end of your
growing season. Plant your new rhizomes as soon as possible after
receiving them, and at least six (6) weeks before your first frost.
The planting zone map and chart below is a general reference to determine
when your first frost date is, please click on the thumbnail view of the
map to determine your zone, then refer to the zone chart to determine your
approximate first frost dates and deduct 6 weeks from the date in your
zone. We normally start shipping starting to zones 1-4 during the first weeks of July,
zones 5-7 during the first weeks of August, and zones 8-10 until the
middle of September, so if you are in Zones 1-4 please take this into
consideration when placing your order. Rhizomes can stay out of the ground for a week or two if necessary as long
as they are stored in a cool dry location. |
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Zone 1
- Average dates first frost - July 1 to July 31
Zone 2 -
Average dates first frost - August 1 to August 31
Zone 3 -
Average dates first frost - September 1 to September 30
Zone 4 -
Average dates first frost - September 1 to September 30
Zone 5 -
Average dates first frost - September 30 to October 30
Zone 6 -
Average dates first frost - September 30 to October 30
Zone 7 -
Average dates first frost - September 30 to October 30
Zone 8 -
Average dates first frost - October 30 to November 30
Zone 9 -
Average dates first frost - November 30 to December 30
Zone 10 -
Average dates first frost - November 30 to December 30
Zone 11 - Free
of frost throughout the year. |
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| WHERE TO PLANT |
| Select a sunny, well drained location with
at least 1/2 day sun. Iris do best in full sun, but in very hot climates
some shade is good. Make sure you provide your iris bed with good
drainage, a raised sloping bed is ideal. |
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| SOIL PREPARATION |
iris will grow in any good
neutral garden soil.
To prepare an iris bed, till or turn and loosen soil to a depth of 6" to
8" and turn in a good application of compost. If you have heavy soils, the
addition of some coarse sand will improve drainage. |
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| HOW TO PLANT |
Plant iris rhizomes 18" to 24"
apart.
Dig a 6" - 8" hole with a hand spade, around 6" - 8" in diameter. Spade in
one (1) small handful of bone meal, and one (1) small handful of low
nitrogen fertilizer (approx. 5-10-10, or 6-to-6 is recommended). Do not
use high nitrogen fertilizer, as this will promote rotting of the rhizome.
Alfalfa pellets are also beneficial when incorporated into the soil around
newly planted iris.
Iris should be planted so the tops of the rhizomes are slightly exposed
and the roots are spread out and facing down. In extremely hot climates it
may be advisable to cover the rhizome with maybe 1" of soil. A common
mistake, however, is planting the iris too deep. Make sure you do not have
any soil covering the fan of leaves.
Firm the soil around the rhizome and water it in. |
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| WATERING |
Newly planted iris need
consistent moisture to encourage root growth, however over-watering can
encourage rot, so they should never be kept soggy. Plants should get
established in 2 - 3 weeks. If in doubt, simply tug lightly on the fan, if
you feel resistance, the roots are digging in, if the plant seems wobbly
like you could easily pull it out, then it needs more time to get
established.
Deep watering at long intervals is better for established plants than more
frequent shallow watering. Over-watering is a common error and can lead to
rot. Once established iris normally can tolerate some drought in
mid-summer. Re-blooming iris require a touch more water than spring only
bloomers. If re-blooming iris do not receive moisture during a three (3)
week period during the summer, they may not bloom in the fall.
I prefer the use of a weeping soaker type garden hose to conserve water,
as well as minimize the chance of disease growth. I add the hose at the
time a row of rhizomes are planted, the hose is buried to a depth of 1/2"
and placed near the newly planted rhizome. Many people have had success
with drip systems and other types of watering systems, some even rig up a
fertilizer injector at the water valve, and these are all great, but the
system you select is not as important as this one point: don't over-water
your iris. |
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| FERTILIZER |
Specific fertilizer recommendations depend
on your soil type.
Generally, a light application of balanced fertilizer (approx.
5-10-10, or 6-to-6, or super phosphate) as a top dressing around the
plants in about four (4) to six (6) weeks before spring bloom, then again
about our (4) to six (6) weeks after bloom, then again in very late fall
is desirable. Light application means just a handful, maybe 1/3 to 1/2
cup, sprinkled around each plant. Do this just before rain, or hose the
plants clean, so that all fertilizer that may have fallen on new growth is
washed off.
Never use high nitrogen fertilizer, or over fertilize, as this promotes
soft growth, thus a rhizome that will rot and die. |
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| GROOMING |
| It is not recommended to cut the green iris
leaves since plant food is stored in them. Keep your iris beds free of
weeds and debris, so that your iris can bask in the sun unobstructed. Old
and diseased leaves should be removed. Old flower stalks should be cut off
within about 6" of the rhizome. |
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| TRANSPLANTING |
| Iris need to be thinned or divided before
they become overcrowded, usually every three (3) to four (4) years. If
iris are allowed to become too overcrowded the bloom will suffer, and
disease problems may increase. It is best to transplant during the iris
dormant season in July-August-September, at least 6 weeks before your
first frost, this allows them to root sufficiently before winter. Dig the entire clump out of the ground,
wash soil off the roots, separate the rhizomes by cutting at
intersections, and trim leaves into a fan shape about six inches (6")
long. The fan will allow the plant to maintain it's food supply, which is
in the leaves, yet it will prevent dehydration during this 4-6 week
period. Discard all rhizomes without leaves. Replant rhizomes with fans. |
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IRIS PROBLEMS |
| Although most irises are tough and rarely succumb to
disease, you should be aware of the following types of infestations. |
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| Fungal Leaf Spot (rust) |
Leaf Spot is rarely seen in arid dry climates both warm and cold, like
Arizona and Colorado, but in the Coastal regions, Northwest, and Eastern seaboard,
it can be a continual problem.
After blooms appear, leaves
may become dotted with small brown spots with a watery streaked
appearance. This
fungus will not do significant harm to plants if it occurs in small
amounts, but if it gets out of hand and spreads to other plants, it can
weaken all of the leaves, thus eventually kill the plants. If fungus strikes,
remove all dead and diseased leaves, and spray with a fungicide containing CHLOROTHALONIL
(formerly Daconil), such as found in "Ortho Garden Disease Control".
Spray the infected parts of the plant. Usually you will need to spray four
(4) to six (6) times, within seven (7) to ten (10) days of each other, to
control this disease.
Fungal Leaf Spot runs a two year cycle, so if you had it last season,
you’ll have it this season. Start your spray program early, until the days
are warm and dry. The sun is nature’s cure, but if you live in an area
with rainy summers, the leaf spot can persist. Clean your garden
regularly, removing dried leaves and old bloom stalks, where spores can
over-winter. Keep aphids, thrips and whitefly in check, because they can
spread the spores. Spray in early morning so the morning sun dries the
foliage. Divide your plants every three or four years. The denser the
foliage, the worse the leaf spot. Allow good air circulation and always
morning sun.
If you want to use a non-chemical spray, the American Iris Society
recommends the following: To a gallon of water, add three (3) tablespoons
baking soda, two (2) tablespoons of shavings from a bar of Ivory soap, and
one (1) tablespoon vegetable oil. Apply the spray to infected parts of the
plant. This does prevent mildew, and may work on leaf spot as well.
Prevention is the best cure, so if you are in a moisture prone area, to
avoid fungal leaf spot it is best to plant irises in full sun, in a rich
well drained soil, also try to select disease resistant varieties, and do
not water with overhead watering systems (I use a weeping soaker type
garden hose system).
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| Leaf Spot Damage -Click
photo |
Leaf Spot Damage -Click
photo |
Leaf Spot Damage -Click
photo |
Leaf Spot Damage -Click
photo |
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| Iris Borers |
The most threatening to iris
are iris borers. They will attack all kinds of irises. In the northeast especially, the eggs are laid in the
fall on leaves. The first sign of their presence is ragged notches on the
leaf edge dripping with sap, or small accumulations of sawdust-like frass
(borers excrement), and streaks and wandering tunnels
on your iris leaves that later turn yellow and brown. Iris that are
severely infested may also fail to bloom. Each spring, the borers larvae
begin to hatch after the first two days of temperatures greater than 70
degrees. New England gardeners have observed that hatch begins about the
same time their tulips bloom, and it continues into June.
The ¼ inch long larvae crawl up the
iris leaves. Near the top they chew into the leaves. Then they eat their
way down inside the leaves to the rhizomes, where they gorge themselves
until they reach a length of about 1 to 1 ½ inches and are pink-gray in
color with dots on their sides and a brown head. Borers often will hollow
out whole rhizomes causing fans to collapse and the remaining tissue to
rot. Some time in the summer the borer larvae change into pupae with a
chestnut brown chrysalis or cocoon. These pupae reside in the soil for
about a month and then a moth emerges and lays eggs on
leaves, and the cycle starts over.
Borers in the pupae, moth and egg stages do not feed. Only
as a larvae do they eat and do damage. At this stage they are most
vulnerable to our efforts to control them. They
also can spin a tiny web and use it to hang-glide off to irises some
distance from where they hatched.
If iris borers strike, first remove all withered foliage and debris next
to and near your iris plants. This should be done in the spring as well as
late fall, as this is where the eggs will be. Do not compost the debris,
but dispose of it in the garbage. In the spring, when new growth starts,
spray the leaves with systemic insecticide containing ACEPHATE (formerly
Isotox) such as what is found in "Ortho Systemic Insect Killer".
To be on the safe side, spray a second time within seven (7) to ten (10)
days. You can do this up to bloom time. You should also feel the leaves,
if you sense a lump that might be a borer, squeeze it and kill it.
If the borer has chewed its way
further down the leaf, it may be easier to remove a portion of the leaf
than search for and destroy the borer.
If the problem is advanced, dig all the rhizomes after
bloom, usually July or August, and physically remove the borers by
squeezing them to death with gloved fingers.
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| Rhizome Damage -Click
photo |
Larvae - Click photo |
Pupae |
Moth |
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| Bacterial Soft Rot |
Although not as common as leaf
spot or iris borer infestation, bacterial soft rot can be a very serious
problem unless treated. What happens is bacteria enters the rhizome
through surface injuries or cuts, such as those that iris borers inflict,
or you accidentally may inflict on the rhizome when planting or weeding,
etc.. A yellow ooze soft rot sets in and causes the rhizome to become
mushy and smelly. To treat this you must dig up the rhizome, remove
all mushy leaves, and cut off all mushy tissue with a sharp knife.
Disinfect the knife after each cut in a solution of 10-20% bleach. After
the rhizome is free of all mushy tissue, soak it in the bleach solution
for several minutes. Rinse the rhizome, then place it in the sun or open
air to dry for 24 hours, then replant. If the rot is too extensive, the
rhizome must be discarded.
Bacterial soft rot is also caused by over-watering, poor drainage,
mulching, over fertilizing, using fresh manure, and the use of high
nitrogen fertilizer, prevention is the best cure. |
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| Crown Rot or Mustard Seed Fungus |
| The
disease earned its nickname Southern Blight because of its prevalence in a
geographical belt that runs from Tennessee to Texas, but it can be a
problem in any climate, from Maine to Southern California.
This disease is soil borne, and over 300 plants can be host carriers, from
beets to wheat, the list is extensive. To control this fungus, trim all
leaves to about six (6) to eight (8) inches to allow more sunlight and air
circulation. Remove and destroy all leaves that show evidence of disease.
You can recognize Mustard
Seed Mold usually only after it has progressed to a reproductive stage,
releasing “fruiting bodies” or spore sacks that resemble Mustard Seeds
with a reddish color. These will be found around the “crown” or top of the
rhizome, and around the soil particles near the plant. Eventually, the fan
falls over and bacterial rot sets in, making it worse. Without the
bacterial rot, the rhizome develops a chalky white look, flaking and
cracking like a bad skin disease, and root systems will have a white or
greyish webbing like spider-web or cotton strung through them. Treatment
is to remove the plant and burn. But if the plant isn’t too far gone, a
soak in TERRACHLOR (PCNB) will kill the fungus, and save the plant. Since
Mustard Seed Mold is soil-borne, Terrachlor must be drenched into the
ground where the fungus was found before replanting. The fungus can
persist in the soil for four or five years, maybe longer, and when you
replant iris there, it will infect them. Sterilization by composting or
burning will, of course, eradicate the fungus, but composting may not
reach adequate temperatures for total eradication, and burning may not go
deep enough into the soil. For this disease, Terrachlor chemical control seems
best. |
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| Snails and Slugs |
Snails and slugs are
members of the mollusk group and are similar except slugs lack the
external shell. They move by gliding along on a mucous-secreting muscle.
The dried mucous becomes a silvery slime trail that tells you they have
been dining in your garden. The common brown garden snail (Helix aspera)
was brought to California as a culinary source from France in the 1850s.
Moisture is critical to their survival and is why they are active only at
night or during cloudy days. On sunny days they are hiding in moist, shady
places. During hot, dry weather they seal themselves off with a membrane
while attached to tree trunks, fences or walls. During cold weather, they
hibernate in the topsoil. They are among the first pests to begin feeding
in spring and the last to stop eating your iris plants in fall. The adult
snail and slug each lay a mass of about 80-100 eggs and they may do this
up to 6 times a year. The eggs resemble small pearls (about 1/8 inch);
they can be seen about 1 inch below the soil level or under rocks, boards
or plant debris and they hatch when they come in contact with moisture. It
takes about 2 years for snails to mature but slugs mature in 1 year. The
garden snail may live as long as 12 years but slugs only live about 2
years. Given all this, it is easy to see why there are huge numbers of
these pests in the garden. Eliminating as many as possible requires
persistence. Snails and slugs will damage your iris plants by chewing
irregular holes with smooth edges in the leaves, as well as the bloom.
The best control is bait
containing METALDEHYDE, such as what is found in "Corry's Slug and Snail Death".
This bait is sprinkled around the plant in a circle, after physically
removing all active insects. |
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