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Planting Your Bearded Iris in a Southern Garden
By Evelyn
Blalock
SITE SELECTION
Select a location that receives
at least 6 hours of sunlight a day. In extreme environments like ours, a little
afternoon shade is preferred, but avoid building your raised bed completely
under a tree. The iris need sunlight to bloom… and completely filtered
sunlight will not produce the results you desire. While you do not want to
burn your rhizomes, sunlight is your best asset in reducing moisture
accumulation and resulting soft rot. Your rhizomes also would prefer not
competing with a mature tree for nutrients. In addition to considering
sunlight, you should choose a site that can accommodate a raised bed.
Many soil
preparation guides will tell you to dig down four inches to cultivate and
then raise the bed four to eight more inches, for a total depth of twelve
inches of prepared soil. However, in heavy clay and gumbo soils (such as you
find in the more southerly regions), it is best NOT to dig down the four
inches. Doing so would create a lovely swimming pool for your rhizome roots
every time it rains, as the water runoff will congregate at the lowest point
of the least dense soil. Instead of digging, scrape the area free of grass
and weeds, apply gypsum to help break up the underlying soil for healthy
rhizome root growth, and build up eight to twelve inches in raised beds with
new soil.
SOIL PREPARATION
Other than providing a raised
site, soil preparation is the single most important factor in having a
successful bearded iris garden in our region. Why? Because we've got soil
like our food… hot, thick gumbo! Traditionally,
when you dig, you will find a heavy clay soil that retains lots of moisture….
which proves great for Louisiana iris, but is the kiss of death for bearded
iris. This gumbo clay proves way too difficult to amend for proper rhizome
growth, so mixing fresh soil with ingredients from your local garden center
is the best way to go, using a formula that includes all-purpose garden soil
that has not been amended with nitrogen or manure, coarse construction sand,
garden humus that has not been amended with nitrogen or manure, bone meal,
rock phosphate, granular dolomoite, wood ashes, and horticultural vermiculite. The "secret
recipe" of how to mix these ingredients in a combination that will yield
the most success for your garden will be dependent on the humidity in your
environment, how often it rains, how long the winters might be, how low your
winter temperatures will reach, if you have breezes to help dry out the soil,
and any number of other uncontrollable variables. The specific mix I use when
preparing my beds in this humid Zone 8B location is highlighted at the bottom
of the page.
In addition to the above
nutrients, use of a specially designed iris fertilizer is beneficial.
I have found this available from both Cooley's Gardens and Comanche Acres. Of
course, other vendors may carry such a fertilizer as well. Look for one that
includes a good balance of slow release phosphorus, potassium, boron, zinc,
iron, copper, molybdenum and manganese.
Your goal
in soil preparation is to create an environment that is rich in the nutrients
needed for healthy rhizomes without causing rot in our humid and hot
environment. Manure and high nitrogen will cause your rhizomes to get too hot
and basically cook them! High humus content in the soil helps provide
necessary nutrients without making the soil too heavy. High sand content in
the soil allows frequent rains and high humidity to run off faster, thus
reducing the potential for rot.
PLANTING THE RHIZOMES
Basically, planting a bearded
iris rhizome is the direct opposite of planting a Louisiana iris rhizome.
Whereas exposing a Louisiana iris rhizome to the sun will cause it to rot,
failing to expose part of a bearded iris rhizome to the sun will cause it to
rot! Louisiana iris thrive with high water, even growing submerged, whereas
high water is the enemy for bearded iris. Keep this in mind when planting and
do not go too deep! For best results, put some PCNB
in the planting hole along with generous amounts of bone meal and a
little PCNB on top as you place each rhizome. Do NOT use alfalfa pellets because they cause the worst
leafspot ever!
Though there seems to be no universal rule on spacing (opinions range from 8 to 24 inches!), placement should keep in mind your own personal objective. If you want a look of immediate abundance, plant closer together. If you want to divide clumps less often, plant further apart. Planting further apart provides more air circulation around each plant, but also can look sparse. Regardless of your spacing, planting so all rhizomes run in the same general direction looks better and helps to keep clumps spaced out as they increase. If you have several rhizomes of the same cultivar, give them a natural look by planting as a clump, angling each rhizome away from one another as if they sprouted from an imaginary mother rhizome. Cover only about 3/4 of the rhizome with
soil, leaving the top exposed above. In times of extremely high sun exposure
and heat (during the hottest part of the summer), a lightweight wood shingle
or similar item can be placed over the rhizome for protection, if needed.
Avoid the temptation to heavily mulch your rhizomes for heat protection or to
cut down on weeds. This will only accumulate moisture around the rhizome and
lead to soft rot. However, a light mulch, such as decomposing leaves or
crushed pine straw can be used between rows. Likewise, a very thin layer of
soil can be placed on top of each rhizome during the hottest months for
protection.
 
View from front, fans curving outward View from back, rhizome
exposed above soil line
Bearded
iris may also be planted in well-drained pots, using the same planting medium
that is described elsewhere on this website. Check for adequate drainage
holes and add gravel to the bottom of the pot to control water retention. For
permanent planting in pots, plant your rhizomes about 6-8 inches apart. A
two-gallon container can support one full-grown iris; a ten-gallon container
can support three. If you are planting multiple rhizomes of the same variety,
you can place them all at the center of the container, pointing their toes
together and circling the leaves toward the outside of the pot to create a
fan. This same method of planting multiple rhizomes of the same variety
together can also be used to create a more abundant look in your raised beds.
On a side note, here's a handy formula to roughly determine the gallon size of any round container. Measure the radius at the top of the container, and then measure the radius at the bottom. Use the average of those two as your overall container radius and plug it into the following formula: [(Radius * Radius) * 3.14 * Height] / 1728 (which is the number of cubic inches in a cubic foot). Take the result of that equation and multiply it by 6.428 (which is the number of dry gallons in a cubic foot). This result is the number of dry gallons of soil needed to fill your container to the rim. So... if you have a 12-inch tall container that has a 10-inch radius at top and an 8-inch radius at bottom, the calculation would be: 10+8=18 and then 18/2=9 to give you the average radius of the container. Then [(9 * 9) * 3.14 * 12] / 1728 = 1.76625, which is then multiplied by 6.428 to give you a capacity of approximately 11.35 dry gallons for your container. As potting soil is sold in a variety of sizes, you may also find the following conversions helpful when purchasing products:
1 dry gallon = 4 dry quarts
1 dry gallon = 4.65 liquid quarts or 1.163 liquid gallons
1 dry gallon = .155 cu. ft.
1 dry gallon = 0.125 of a bushel
1 dry gallon = 18.61 cups (versus 1 liquid gallon, which equals 16 cups)
WEED CONTROL
Weeds will in all likelihood find their way into
your bearded iris beds with no mulch. While many chemical weed killers will also kill your bearded iris, there are a few pre-emergent herbicides that are labeled for use in iris beds. Trifluralin (Treflan, Preen and others) is an example that has been successfully used in iris gardens for many years. Other gardeners report success with Snapshot, which has trifluralin mixed with isoxaben for a little more punch. However, trifluralin is highly toxic to fish, so it should never be used anywhere close to a waterway or a backyard pond with goldfish. Trifluralin also has a strong warning label for pet and human contact. Gallery is the product name for isoxaben by itself (the second active ingredient in Snapshot), and it is the herbicide recommended by the LSU Agricultural Extension Service for broadleaf weed control on irises. Gallery has not been found to be highly toxic to fish and appears to be a safer choice for the enviornment. LSU-Ag suggests combining Gallery with Pennant during the winter to successfully prevent not only broadleaf weeds, but also grasses, without harming the irises. Oryzalin (Surflan) is also labeled for use on irises, and has a broader range of approved ornamentals than trifluralin and isoxaben, but I have not found anyone with firsthand experience using this product on irises. Recently, there has also been a lot of attention on using corn gluten as a natural pre-emergent herbicide in iris and vegetable gardens. While this is fine for your vegetables, it's not an appropriate choice for your southern iris garden. Corn gluten incorporates a slow release of nitrogen to suppress the germination process... the same extra nitrogen you are trying to avoid in a southern iris garden! While it will suppress the weeds and give you lush foliage on your irises for a while, it may also lead to rotted rhizomes. For spot treatment of particularly troublesome weeds, Roundup may be used carefully, but do NOT use it before the bloom season. Mr. Walter Moores has reported that using Roundup after around January 1 will result in distorted blooms. Of course, hand pulling is also a choice if you are fearful of using herbicides near your children, pets or wildlife.
DIVIDING THE CLUMPS AS THEY GROW
An iris
rhizome will bloom only once in its life. It will then turn its energy into
producing baby rhizomes that spread out from the center. Over time, these
clumps can become overcrowded in your garden, putting a strain on the
plants as they compete to gather nutrients. Overcrowded conditions also
decrease air circulation in the beds, leaving the plants more vulnerable to
disease. Removing the center, non-blooming rhizomes and replanting their
offspring, while at the same time rejuvenating your soil, will improve the
bloom and overall performance of your iris garden.
Rebloomers
increase faster than once-bloomers, so clump division may be necessary
every two to three years. As some varieties need to be well established
before they bloom or rebloom, caution should be used in division.
Basically, you will want to divide out only half of the clump, leaving the
remainder to rebloom. However, as rebloomers are particularly heavy
feeders, you do not want to simply spade out half of the clump and leave
the rest completely undisturbed (though this is certainly tempting on hot
summer days!!!). Instead, dig each clump deeply to preserve as many roots
as possible, break the clump in half, discard the non-blooming center, and
remove all loose leaves. Set one half aside undisturbed as you rework the
soil. Use the other half either as a clump in another part of your garden,
or break it up into lots of lovely little babies to share with friends and
neighbors! Loosen and amend the garden soil as you go so that your
replanted iris clumps may thrive, and then replant. Do not cut back the
fans on these clumps, but simply reset them as undisturbed as possible to
encourage bloom and rebloom.
Discard the non-blooming mother rhizome and replant the daughters
TROUBLE SHOOTING
There are many potential
problems with planting bearded iris in the Deep South. Other than the
obvious fact that some varieties simply will not perform here because there
is not a long enough cold period, problems with root rot are much higher in
our region. Additionally, pests and diseases that may not be prevalent at
the northern farm where the rhizome was dug may very well be present in
your garden. A full discussion of pests and diseases that may affect irises,
along with suggested treatment plans, may be found on the Diseases
and Pests page of this website. Following is a list of preventative
steps you can take BEFORE problems arise:
1.
As a
preventative for soft rot, top dress beds in the spring with PCNB. Also in the spring, spray
your iris foliage with hand sanitizer with Tricoslan in it, mixing about 3
tablespoons to a gallon of water. Soft rot is a common problem of
waterlogged or damaged iris rhizomes. Soft rot can enter the rhizome
through any wound. If you detect soft rot, consult the Diseases and Pests page of this website for suggested treatment options.
2.
To minimize
rot potential, cut off all spent flower stalks as they occur. Before the
first true freeze (28 degrees or lower), cut off any remaining flower
stalks (blooming and spent). Removing the stalks minimizes the risk of
moisture accumulation as the stalks decay. Keeping your garden clean of
debris is the single best way to minimize onset of disease or heavy infestation
of pests.
3.
To minimize
leaf spot, do not use alfalfa in your garden. Though many guides will
suggest the use of alfalfa powder or pellets as a nutrient, in Southern
gardens it has been shown to increase the incidence of leaf spot.
4.
To minimize iris
borers, apply a systemic with Imidacloprid (Merit is an excellent example) in
early spring to kill the bad without endangering the good insect
population. Granular or spray forms may be used. BE PROACTIVE. A
once-a-year Imidacloprid application can almost completely eliminate your
borer population if timed correctly.
5.
To control
our most common pests, slugs and squirrels, just remember "salt and
pepper"! Sprinkle Epsom salt to reduce slugs and cayenne pepper to
reduce squirrel damage. Remember that they need to be reapplied after each
rain.
RECORD KEEPING
Be sure to keep complete
records of which varieties are performing well, and include a schedule of
when they bloom. Keeping these notes will help you decide which varieties
you want to recommend to friends or plant elsewhere in your garden.
Accurate records will allow you to update the Reblooming Iris Society
regarding those varieties that are successful.
You will also want to keep a
record of problems you encounter, treatments of chemicals, bed preparation
and rejuvenation, and other pertinent details so that you can eventually
formulate your own garden schedule.
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