Growers can add Marengo^® to their weed management toolbox for long-term control.

An effective weed management program includes a variety of tools and tasks, such as sanitation, cultivation and barriers, and the application of herbicides. Although weed management requires labor hours, the proper use of preemergence herbicides will help alleviate time spent weeding, allowing crews to perform other tasks.

Marengo^®, a preemergence herbicide available in granular and liquid form, has the potential to provide up to eight months of weed control. Marengo is labeled for use on 270 plants with applications timed in fall or spring for enhanced flexibility. It is also labeled for use inside greenhouses and can be applied for weed control on floors and beneath benches. It’s available in two formulations—as an 18-ounce and 64-ounce liquid soluble concentrate or as a 50-pound granular with Verge^® carrier technology.

The active ingredient in Marengo herbicide is indaziflam, which has a unique mode of action that inhibits root development during weed germination by blocking cellulose biosynthesis.

“Marengo is the only active ingredient in the Weed Science Society of America (WSSA) Group 29, making it an effective rotational product in IPM programs,” says Dr. Aaron Palmateer, senior technical support with the Envu Ornamental business segment.

Dr. Palmateer suggests that for best results, growers should apply in fall for continuous weed control into the following growing season. He also recommends irrigation within 21 days of application.

Growers can apply Marengo herbicide using directed spray around ornamentals or broadcast in non-crop settings to prevent broadleaf weeds, grasses and annual sedges from seed. For spot treatments, the equivalent rate is 0.17-0.36 fluid ounces per 1,000 square feet.

/// Trials

Hannah Mathers, owner of Mathers Environmental Science Services and former professor and state extension specialist at The Ohio State University, trialed Marengo herbicide with spray applications on a variety of dormant (prior to bud break) field-grown boxwood and Taxus at three different sites.

“We found it performs really well as a dormant application. When applied in December, growers could easily get 21 weeks of control from Marengo when applied at the 15-ounce rate,” she says. “That takes growers into May and hopefully they are almost done with shipping by that point and can go in with another application of a different product—something a little bit lighter—in June.”

In Mathers’ trials, Marengo worked well on winter annual weeds that come up in early spring such as mustard and bittercress. Some of the more difficult weeds such as Canada thistle will likely need spot sprays to knock them down.

Mathers says dormant applications provide the ability to use nursery staff in winter (which is traditionally a “down-time” labor-wise versus spring). Dormant applications also provide insurance that applications will be completed before weed germination, versus waiting for spring when other production tasks often take precedence.

“These dormant applications also seem to be key in providing the ‘power’ and ‘duration’ of efficacy necessary to clean up nursery fields,” she adds.

Mathers also trialed a fall granular application of Marengo herbicide on dormant daylilies, with efficacy that lasted well into the growing season.

“In the perennial trial, we noticed the efficacy doesn’t change that much with the 200-, 400- or 800-pound rate. We also did an application around June/July on the daylilies, but there was some phytotoxicity, most likely on plants that were already weak. But they did fine with the fall application,” she explains.

Marcelo Moretti, assistant professor at Oregon State University’s Department of Horticulture, trialed Marengo in container-grown crops. He found that it performed well on annual grasses (crabgrass, foxtails, annual bluegrass) annual broadleaves like oxalis, bittercress, spurge and willowherb.

He also found that it controls liverwort. “Not many chemicals do that,” he says.

Although Marengo is a preemergence herbicide, Moretti found that it has some post-emergence activity, as well.

“I observed excellent control of willowherb with Marengo applied in post-emergence and suppression of black nightshade. Just to be clear, Marengo really shines as a preemergence herbicide, which is how it should be positioned in a [weed management] program. [But] the little post-emergence comes as a bonus,” he explains.

In his trials, Moretti was also screening for crop tolerance and efficacy.

“For crop safety, I noticed that conifers, in general, are very tolerant to Marengo that is applied over the top. Same with boxwood, roses and other species,” he says. “The exception to that is hydrangea, and this year we noticed that Ilex x meserveae ‘Blue girl’ was sensitive to Marengo. The key is to do a small test whenever trying Marengo on a new crop, but that is true of all herbicides.”

The newly revised Marengo herbicide label allows for testing.

“If a desired plant is not listed, treat several plants at the maximum use rate and evaluate one to two months later for acceptable tolerance. The user assumes responsibility for application to plants not listed on this label,” Palmateer explains.

/// The cost-effective solution

Properly applying preemergence herbicides helps reduce labor costs related to weed management. When evaluating different preemergence solutions, it is important to look at the total program cost, which is best measured as cost per acre per month of control.

This includes:

• Cost of product and labor for the initial application
• Cost of product and labor for cleanup
• Cost of product and labor for any reapplication treatments.

Products that are less expensive upfront but have a short residual will require more frequent reapplication and/or more cleanup between applications. This increases both labor and product costs. Herbicide products offering longer residual control, such as Marengo, will reduce the frequency for reapplication and cleanup applications because they last longer, reducing labor and product cost. No product lasts longer than Marengo, making it one of the most economical program options for weed management based on cost per month per acre of control.

A product that lasts up to 4 months will have to be applied twice to achieve similar results to a product that lasts up to 8 months. That means the labor cost will be twice as expensive for the shorter-lived product. Every herbicide application includes labor for site preparation; application prep (equipment, handling, mixing, etc.); application; and cleanup (equipment, disposal, etc.).

Envu offers an easy way to track your herbicide costs with its cost-in-use calculator. Users can compare any herbicides and retrieve cost analysis to maximize your investment. Plug in information that fits your exact parameters and see side-by-side numbers for any herbicides you choose.

/// More information

For additional information about Marengo herbicide, contact your Envu Ornamental Specialist.

This article originally appeared in the August 2020 Nursery Management Supplement – State of the Market Weed Control Report, sponsored by Envu.