By Mark Czarnota
University of Georgia
The regal flowers of irises make these herbaceous perennials
among the most coveted plants in the garden. Unfortunately, weeds
can be hard to control in beds of irises.
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Established perennial broadleaf weeds can be extremely tough.
Removing them requires special techniques.
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 herbicides
Many 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 out
Herbicide 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 care
To 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.)
