Beginner's Corner: Back to the Basics
Mona French
The ideal time of year to plant bearded iris is July, August and September. September seems the best in Oklahoma, considering our blasting heat during summer. Plants and people alike droop at temperatures like we have seen this year. The iris should be planted at least one month to six weeks before our first freeze, allowing the roots to get a good hold and become established.
Iris do best if planted in full sun, but can also be placed in a location where they will receive at least a half day of sunlight. Slight shade is acceptable, but you may have less bloom. It is imperative that the soil have good drainage, so planting on a slope or in raised beds is advised. Standing water will rot iris quickly, so give them a good place in the sun where the water will shed easily. In our own garden, we have hard-baked clay, very alkaline soil. Each time we rework a bed, we add alfalfa pellets (also used as rabbit food, and found inexpensively at feed stores), Humore and/or peat moss. And, of course, if you are industrious enough to have your own compost pile, add compost. These additives help fluff the soil and give it more tilth, and ease your weeding chores. I add a small handful of super phosphate and Humore into the soil in the bottom of the hole as I am planting each rhizome. When first working a bed, gypsum will condition and improve clay soils. Sulfur can be added to lower the pH (the ideal pH for iris is 6.8 - slightly acidic), although iris will tolerate higher pH's.
Plant iris between 12 and 24 inches apart. Planting closer gives you a fuller effect quickly, but the clumps will need to be thinned more often. Plants spaced further apart can stay in the same place an extra year or two, and you can fill in the spaces between with other plants that have similar watering requirements. Plant the iris so that the tops of the rhizomes are exposed and the roots spread out facing downward into the soil. Firm the soil around the rhizomes and water to help settle the soil. Do not plant the iris too deeply, as this will decrease blooms.
Freshly planted iris will need more frequent watering than established plants. In Oklahoma's hot summers, it is usually best not to water when the temperatures are searing. Overhead watering is discouraged, as this can help cause rot and spread leaf spot. Soaker hoses are ideal, but you can also dig a trench next to the rhizomes and fill the trenches with water. If you have a small planting, you can use a gallon plastic milk jug (discard the cap) with three or four holes poked into the bottom. Fill the jug from the top with water, and set the jug on the ground near your plants. Gravity will slowly pull the water into the soil. The only drawback to milk jugs is that when they become empty, the Oklahoma winds can send them blowing into your neighbors yard.
Iris should be fertilized in early spring using a high middle number fertilizer (for instance, 6-55-10). Do not use a fertilizer high in nitrogen (first number) as too much nitrogen can encourage rot problems. Apply a second application of fertilizer about one month later. Take care to keep the fertilizer from coming into actual contact with the rhizomes - spread it about six inches away from the plants. Apply as a top dressing using a handful for each square foot, then work the fertilizer into the soil. Water in well to get the nutrients down to the iris roots. Reblooming iris will need additional watering and fertilizer application to help assist in the late blooming. Iris other than bearded iris have different fertilizer and watering requirements.
Keep your iris beds as clean and weed-free as possible, allowing the rhizome tops to bask in the sun. Bloom stalks should be cut off close to the ground after blooming, or grab the old bloom stalk and bend it sharply towards the ground, snapping the stalk off close to the soil line. Snapping instead of cutting is suggested by some iris growers, as there is a natural breaking point in the molecules as the bloom stalk is dying back and it is believed that this will help the plant heal more quickly. If you use a knife to cut the bloom stalks, cut at a sharp angle so that water will run off of the wound, preventing water from collecting and perhaps causing some rot.
One of the very best weeding tips I have ever received comes from Nancy Kowalchyk, who weeds with a serrated knife. Cut the weeds underneath the soil line, being careful not to cut any neighboring plants that you wish to keep. This works well for weeds that need to reproduce from seed. It does not prevent weeds that reproduce by underground travelling from eventually coming back, but it surely does give you that pristine weed-free look just after weeding. You will be amazed at how quickly you can weed a bed with a serrated knife, and this is a boon to those of us with arthritic hands. Kitty and Perry Dyer's garden-helper bent his knife blade at an angle, so that the knife is comfortable to hold and easily enters into the soil. Weeds can also be prevented by using pre-emergence herbicide before weeds begin growing in spring, or Over the Top (available at garden centers) to kill grassy weeds during the growing season.
During the growing season, healthy iris foliage should be left in tact, but browning or diseased leaves should be removed. Healthy foliage should not be trimmed before fall, as the chlorophyll in the leaves helps feed the rhizome and its increases. In late fall or early winter, dying foliage can be trimmed back to about six inches. If you have newly planted iris that have not had a good chance to establish before cold winter temperatures begin, mulch the iris with straw, pine needles or leaves. Give your iris beds a good early spring cleaning, by clearing away all mulch, dead foliage and winter-blown debris. It is important for the good health of iris that, by early spring, the area around each plant is free from material that may hold moisture or you can find yourself watching the iris rot.
Bacterial soft rot can plague iris and is usually caused by excessive moisture. It is often associated with some type of iris injury (insect damage or careless cultivation), overwatering or over-fertilizing. It will appear first at the base of the plant, where the leaves join the rhizome and can be seen as yellowing leaves which drop parallel with the ground. Keep a watchful eye for rot during particularly wet weather. The rhizome will turn mushy and soft, and damaged parts must be removed quickly to save the health of the rest of the plant. You can dig out the rot with a spoon, scooping out the diseased area until you reach healthy rhizome tissue. To help assist in the wound healing over, disinfect the scooped out area with at 10% bleach solution (1 part bleach to 9 parts water), or sprinkle with chlorine cleanser such as Comet. After the wound has been disinfected, allow it to set for several days before covering it with soil, or leave it exposed to the sun. If you have a whole plant or entire area of plants that succumbs to soft rot, the soil may be infested, and any new rhizomes you plant will also get rot. It is best to completely remove the soil in that area and replace it with fresh soil taken from a place where iris have never grown. Another alternative is to plant daylilies or some other plant material where the rotted iris were and find a new home for your iris.
Aphids, thrips and whiteflies can cause iris damage and spread diseases. The best organic method of dealing with these sucking pests is using insecticidal soap (1/2 cup of dishwashing liquid mixed with a gallon of water and sprayed onto the plants). With heavy aphid infestation, you may need to spray once with insecticidal soap, wait a few days, and spray a second time after new aphid babies have hatched. Chewing insects, such as caterpillars, can be controlled by using bacillus thurgingiensis (B.T.). I have also sprayed garlic water (1 part crushed or pulverized garlic to 9 parts water, or use a product called Garlic Barrier found at TLC) to help prevent grasshoppers from eating up my precious newly sprouted iris seedlings. Any of these methods must be reapplied after any serious rain.
Leaf spot is a common fungal problem occurring with wet conditions such as rain, high humidity, fog or overhead irrigation. Small yellow and brown spots will appear near the top of the foliage and eventually spread downward, causing the leaves to die. If allowed to run rampant, leaf spot can also lead to rot. The spores that cause leaf spot overwinter in garden debris. A tidy iris bed will help control this - remove all dead foliage as quickly as possible. Do not water using overhead sprinklers and don't work in the garden just after rain or in early morning heavy dew. Most iris sellers suggest using a spraying program to prevent leaf spot from occurring or spreading. Spray six weeks after bloom, and again after rainstorms. Suggested fungicides to use are Captan, Fore, Bravo, Daconil, Kocide and Bayleton. For those of us who prefer organic gardening, tidy beds are the best option. Some iris varieties seem to be particularly susceptible to leaf spot, and some are particularly resistant. Consider growing iris that are less susceptible. Generally, iris with healthy-looking blue-green (darker) foliage are less susceptible and iris with lighter green foliage are more susceptible.
Iris should be divided every three or four years, before they become too crowded, causing the bloom to be sporadic. Some varieties are invasive and increase easily, crowding out their neighbors, with a potential to cause disease problems. This will also add difficulty when digging and dividing in deciding which plant is which. Lift the iris clump with a shovel or spading fork, and shake off the excess soil. Be sure to mark at least one fan of each clump with the variety. Cut between rhizomes with a sharp knife dipped in bleach water, and discard old rhizomes or diseased areas (any place where there are no fresh roots growing). For best hygiene, trim off roots and foliage to about six or eight inches, and soak rhizomes in a bucket of 10:1 water/bleach solution for about 10 or 15 minutes.
Allow plants and wounds to dry completely before replanting, placing them somewhere in the shade. Kitty Dyer hangs her freshly washed rhizomes upside down from the roots on a clothes line with clothes pins. This allows the foliage to shed water easily. Perry Parrish lays his rhizomes in his net hammock where the fresh air can easily dry the rhizome. Do not let the rhizomes lay out in full sun as the foliage will bleach out to a very light green/white color, depleting it of chlorophyll. If you are not ready to plant, iris can be stored for one or more months in a cool spot in your garage (if you are so lucky) or in your house. They can be placed in onion sacks & hung to store in a cool place inside. Each rhizome has a "heel and toe". The heel is the wider part from where the foliage grows. The toe is the narrower part where the fresh cut has been made. Plant iris together in groups of three, if you wish, with the toes facing each other. This will allow the plants to increase outward (increases grow from the heel). After planting, stand back and smile.