By Mark Czarnota
University of Georgia
The good news is that annual broadleaf and grassy weeds, unlike perennials, can be easily controlled with mulches and the judicious use of herbicides.
As with any garden plant, planting irises in a proper place is vital to getting healthy plants established. Using mulch helps keep weeds from growing. But use it sparingly. Anything more than a 2-inch layer can promote diseases in irises.
Preemergent herbicidesMany preemergent herbicides are labeled to control a spectrum of broadleaf and grass weeds in irises. Most come in both granular and sprayable forms. Granular products are popular because they require no mixing and are more forgiving if you make a mistake in applying them.
Preemergent herbicides tend to be more useful on large acreages. Home gardeners may find that removing weeds by hand is good for the irises and invigorating, too.
For those who don't, here are some preemergent herbicides labeled for use in irises, with the active ingredient listed in parentheses: Barricade and RegalKade granular (prodiamine), Dimension (dithiopyr), Gallery (isoxaben), Pendulum or Corral granular (pendimethalin), Pennant (metolachlor), Snapshot granular (isoxaben and trifluralin), Surflan (oryzalin), Treflan and Preen (trifluralin) and XL (benefin and oryzalin).
No 'silver bullet'In that long list, no product controls all weeds. There is no "silver bullet" when it comes to herbicides. Most products or combinations will control 80 percent to 95 percent of the weeds normally found in irises. And many that they don't control can easily be removed by hand.
Remember, though, that these products work only if applied before the weeds germinate. All of them will need to be applied at least twice (spring and fall) to keep the weeds under control.
Several postemergent grass herbicides are labeled, too, for use in irises: Acclaim Extra (fenoxaprop); Envoy (clethodim); Fusilade II, Ornamec and Grass-B-Gon (fluazifop); and Vantage (sethoxydim).
Grass herbicides are concentrates that you mix with water and spray over the top of irises to control actively growing grasses. They won't keep weed seeds from germinating.
Check it outHerbicide labeling can change, so make sure you read and understand the label before using any product.
As products go off patent, some companies may market herbicides under a different trade name, so be careful. For example, glyphosate, the active ingredient in Roundup, is now available under a range of trade names.
Broadleaf and other perennial weeds can be hard to control in irises. Nutsedge (Cyperus spp.) and Florida betony (Stachys floridana), for instance, are two problem weeds with no selective herbicides available to control them in irises.
When hand removal and mulches aren't working, you can use products containing glyphosate to control problem perennial weeds. To use them, carefully separate weeds growing among irises from the iris leaves.
Take careTo avoid getting any of the herbicide solution on the iris foliage, lay the weed on bare ground or a piece of plastic for treatment. Then paint or sponge on a 5-percent solution of glyphosate (6 ounces of herbicide in a gallon of water). If you get the herbicide solution on iris foliage, immediately wash it off with water.
You could cover the iris plant with a plastic bag, too, and treat the surrounding weeds. Then remove the protective coverings after the weeds treated with herbicides have dried.
Make sure the product you use to make the 5-percent spray solution contains 41 percent or more active ingredient (glyphosate). In 10 to 14 days, the treated weeds will begin to die. If they resprout, repeat the treatment.
(Mark Czarnota is a Cooperative Extension horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.)
(Mark Czarnota is an extension horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.)