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Weed Control in Iris (C 867-8)

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Figure 1. Japanese Iris (Iris ensata). Figure 1. Japanese Iris (Iris ensata).

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. Popular iris species used in the landscape include Crested iris (Iris cristata), Japanese iris (Iris ensata) and Siberian iris (Iris sibirica). 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. Fortunately, many 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 important for establishing healthy plants. Mulch is helpful in preventing weed growth, but it should be used sparingly (no greater than a 2-inch layer) to avoid disease problems. There are many herbicides labeled for use on irises. The preemergent herbicides in the following table control a large spectrum of broadleaf and grass weeds.

Table 1. Preemergence herbicides for controlling broadleaf and grass weeds in irises.
TRADE NAMES ACTIVE INGREDIENT
Barricade and RegalKade (Granular) prodiamine
Dimension dithiopyr
Gallery isoxaben
Freehand dimethenamid and pendimethalin
Pendulum, Corral (Granular) pendimethalin
Pennant metolachlor
Snapshot (Granular) isoxaben and trifluralin
Surflan oryzalin
Treflan and Preen trifluralin
XL (Amaze) benefin and oryzalin

Most preemergence herbicides listed 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 to 95 percent of the annual weeds normally found in irises. Many weeds not controlled with preemergent herbicides can be easily removed by hand. The herbicides listed are designed to control weeds germinating from seed. During iris establishment, and under heavy weed infestation, at least two herbicide applications should be made in most Southern states — usually in January / February and again in April /May — to control most spring and summer weeds. Additional preemergence herbicide applications may be necessary to control annual winter weeds. Preemergence herbicides tend to be more useful on large acreages.

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

Table 2. Postemergence herbicides for controlling annual and perennial grasses in irises.
TRADE NAMES ACTIVE INGREDIENT
Acclaim Extra fenoxaprop
Envoy Plus clethodim
Fusilade II, Ornamec, and Grass-B-Gon fluazifop
Segment sethoxydim

Postemergence grass herbicides 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 always read and understand the label before using any pesticide. As herbicides go off patent, some manufacturers market herbicides under different trade names, so the buyer must beware. For instance, glyphosate, the active ingredient in Roundup®, is now available from many suppliers under a wide range of trade names.

Herbicides containing the active ingredient glyphosate can be used to control problem perennial weeds that are unsuccessfully controlled by hand removal or mulches. Weeds growing among irises should first be carefully separated from iris leaves and then placed horizontally on bare ground or a piece of plastic for treatment. Next, paint or sponge a 5 percent solution of glyphosate (6 ounces of at least a 41 percent glyphosate concentrate per 128 ounces of water). A cover, such as a plastic bag, placed over the iris plant while treating the weeds will help shield the iris from the herbicide. Remove protective coverings once the herbicide has dried. The treated weeds will begin to die in 10 to 14 days. If weeds re-sprout, repeat the treatment procedure.

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 labeled selective herbicides available to control them in iris.

The University of Georgia has conducted experiments with both 2,4-D (various trade names) for controlling select broadleaf weeds and halosulfuron (Sedgehammer®) for controlling nutsedge. Neither product is labeled for weed control in Iris spp., but data has indicated labeled rates of these postemergence herbicides can be used on select Iris cultivars with little to no damage. It is suggested that users wishing to try this method test it on small areas of iris / weeds to be treated, wait two weeks and then evaluate the iris plants for unacceptable damage before treating an entire area.

Always read the product label and contact your county Extension office with any pesticide or plant culture questions.

Published with minor revisions on Sep 25, 2014