Cooperative Extension Service
The University of Georgia College of Agricultural and Environmental Sciences
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Weed Control in Iris

Mark Czarnota, Ph.D., Ornamental Weed Control Specialist
Department of Horticulture, 1109 Experiment Street
Griffin, GA 30223

With their beautiful flowers, irises are coveted as one of the finest herbaceous perennials. Unfortunately, like most garden plants, weed control in irises can be difficult. Established perennial broadleaf weeds can be extremely difficult to control, requiring special removal techniques. The good news is that annual broadleaf and grassy weeds can be easily controlled with mulches and the judicious use of herbicides.

As with any garden plant, planting irises in a proper location is extremely important in establishing healthy plants. The use of mulch helps prevent weed growth, but use it sparingly (no greater than a 2-inch layer) to avoid disease problems with iris. Many herbicides are labeled for use on irises. The preemergent herbicides in the following table control a large spectrum of broadleaf and grass weeds.

Most of the herbicides listed above are available in both a granular and sprayable form. Granular herbicides are popular with homeowners because they require no mixing and are more forgiving when an application error is made. However, the herbicides listed do not control all possible weeds. There are no "Silver Bullets" when it comes to herbicides. Most herbicides or herbicide combinations will control 80 percent to 95 percent of the weeds normally found in irises. Many weeds not controlled with preemergent herbicides can be easily removed by hand. The herbicides listed are designed to work only if applied before the weeds germinate, and all of the herbicides will need to be applied at least two times (spring and fall) to maintain adequate weed control. Preemergent herbicides tend to be more useful on large acreages. Home gardeners might find hand removal of weeds adequate as well as invigorating.

Several postemergent grass herbicides are labeled for use in irises (Table 2).

Grass herbicides are concentrates that are mixed with water and sprayed over the top of irises to control grasses that are actively growing. These grass herbicides have no preemergent activity and will not prevent the germination of weed seeds. Herbicide labeling can change, so make certain you read and understand the label before using any pesticide. As herbicides go off patent, some manufactures may market herbicides under different trade name, so buyer beware. For instance, glyphosate, the active ingredient in Roundup®, is now available from many suppliers under a wide range of trade names.

Broadleaf and other perennial weeds can be difficult to control in iris. Nutsedge (Cyperus spp.) and Florida betony (Stachys floridana), for instance, are two problem weeds with no selective herbicides available to control them in iris.

Herbicides containing the active ingredient glyphosate can be used to control problem perennial weeds unsuccessfully controlled by hand removal or mulches. Weeds growing among irises should be carefully separated from iris leaves. To avoid contact of the herbicide solution with desirable iris foliage, lay the weed on bare ground or a piece of plastic for treatment. Then paint or sponge a 5 percent solution of glyphosate (6 oz of herbicide/128 oz of water). You could also cover the iris plant with a plastic bag or plastic containers, and treat surrounding weeds. Remove the protective coverings from the iris plants after the weeds treated with herbicides have dried. Make certain the glyphosate herbicide used to make the 5 percent spray solution contains 41 percent or greater active ingredient (glyphosate). If you get herbicide solution on iris foliage, immediately wash it off with water. In 10 to 14 days, the treated plants will begin to die. If weeds re-sprout, repeat the above treatment procedure.

Trade and brand names are used only for information. The Cooperative Extension Service, University of Georgia College of Agricultural and Environmental Sciences does not guarantee nor warrant published standards on any product mentioned; neither does the use of a trade or brand name imply approval of any product to the exclusion of others which may also be suitable.

Attention!

Pesticide Precautions

• It is the responsibility of the pesticide user to observe all directions, restrictions and precautions on pesticide labels. It is dangerous, wasteful and illegal to do otherwise.
• Store all pesticides in original containers with labels intact and behind locked doors. KEEP PESTICIDES OUT OF THE REACH OF CHILDREN.
• Use pesticides at correct label dosage and intervals to avoid illegal residues or injury to plants and animals.
• Use pesticides carefully to avoid drift or contamination of non-target areas.
• Surplus pesticides and containers should be disposed of in accordance with label instructions so contamination of water and other hazards will not result.
• Follow directions on the pesticide label regarding restrictions as required by State or Federal Laws and Regulations.
• Avoid any action that may threaten an endangered species or its habitat. Your county extension agent can inform you of endangered species in your area, help you identify them and, through the Fish and Wildlife Service Field Office, identify actions that may threaten endangered species or that habitat.

Circular 867-8/May, 2004

The University of Georgia and Ft. Valley State University, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service, the University of Georgia College of Agricultural and Environmental Sciences offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, sex or disability.

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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and the U.S. Department of Agriculture cooperating.

Gale A. Buchanan, Dean and Director