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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.

Marengo’s active ingredient, indaziflam, 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 Bayer’s 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 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 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 growers 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 her trials, it 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 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 postemergence 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 postemergence 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 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 clean-up
• 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 clean-up between applications. This increases both labor and product costs. Products offering longer residual control, such as Marengo, will reduce the frequency for reapplication and clean-up 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 clean up (equipment, disposal, etc.).

Bayer 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.

To try the calculator, go to https://bit.ly/Bayer-Marengo- cost-in-use-calculator.

ALWAYS READ AND FOLLOW LABEL INSTRUCTIONS

Bayer Environmental Science, a Division of Bayer CropScience LP, 5000 CentreGreen Way, Suite 400, Cary, NC 27513. For additional product information, call toll-free 1-800-331-2867. environmentalscience.bayer.us. Not all products are registered in all states. Bayer, the Bayer Cross, Marengo, Ronstar and Roundup are registered trademarks of Bayer. © 2020 Bayer CropScience LP. ES-0720-T&O-0210-A-1

Photos by Kelli Rodda

Weeds are a continuous issue for nursery production crews since they can pop up in any area of the operation throughout the year. To win the war on weeds, nurseries should adopt best management practices (BMPs) to save labor, inputs and improve overall plant health. The University of Massachusetts Extension has provided these strategies for an improved weed management system.

Scouting

All areas of the nursery (nursery fields, container yards, hoop houses, greenhouses, potting and propagation areas, holding areas and areas adjacent to these locations) should be scouted for the presence of weeds on a regular basis. Give special attention to those weeds that might be new to a nursery. Correctly identify and record all weeds. Determine and record the life cycle of each weed. Regular scouting enables a nursery manager to plan and implement appropriate management strategies and evaluate the long-term effectiveness of those strategies.

Planning

Develop a weed management program before planting. This should be one of the first steps in the production process. A complete weed management program outlines all strategies to be implemented.

Preventive strategies

Weeds should be prevented from going to seed in all areas of the nursery. Control measures include mowing, hand-pulling and herbicides. Weeds should be controlled before they produce viable seed. Special attention should be given to weeds with unique seed dispersal mechanisms. Seeds can be moved by wind (dandelion, horseweed, groundsel, hawksbeard), splashing irrigation water (chickweed, pealwort) and force seedpod dehiscence (woodsorrel, bittercress). Optimize the production cycle and minimize the duration in which container and field nursery stock remains in the nursery. Weed problems increase over time and nursery stock that is held for long periods of time can be problematic. In nursery fields, a short production cycle allows for effective perennial weed control. Control weeds in nursery fields before planting. This is especially true for difficult-to-control perennial weeds such as mugwort, quackgrass, yellow toadflax, bindweed and Canada thistle. Once the nursery is planted, it becomes increasingly difficult to control these weeds.

Pre-plant applications of a non-selective, translocated herbicide will effectively control many perennial weed species. Cultivation, unless it is done multiple times over a period of time, may spread perennial weeds. Cultivation also can result in the loss of soil structure and organic matter. If weed populations are considerably different among nursery fields, cultivation equipment should be washed to remove soil and weed seeds. Maintain a weed-free ground cover in field nurseries.

Depending on the type of nursery stock being grown, fields can be maintained either vegetation-free or in a system in which the weed-free areas are maintained in the rows and a persistent sod grassway is established and maintained in the alleys. Weed control in the rows can be achieved with programs that include residual preemergence herbicides and both selective and non-selective postemergence herbicides. These areas can also be mulched. If cultivation is used to control weeds in the row, pay special attention to the movement of soil. Over time, a substantial ridge of soil may develop at the base of nursery stock and be detrimental. Sod grassways will support equipment and allow field access for digging, spraying, mowing and other practices to continue when soils are muddy and snow covered. Additionally, sod grassways can effectively compete with weeds and prevent their spread. These areas are traditionally managed by mowing; however, plant growth regulators can also be used. Maintain a weed-free area around the base of nursery stock. Nursery stock that is surrounded by weeds is prone to rodent damage. Weeds around the base of nursery stock can compete for water and nutrients. Maintain cover crops in fields that are not being used. Cover crops will not only improve soil but also reduce weed growth. Winter rye and oats can be used as cool-season cover crops. Buckwheat and sudangrass can be used as cover crops for the summer months. Maintain weed-free areas around and between greenhouses and hoop houses. These areas can be a source of weed seed that can infest container nursery stock. An early season preemergence herbicide or a postemergence non-selective and preemergence herbicide tank-mix can be used.

All weeds should be controlled in container areas. Nursery fabrics can be used in container areas to prevent weed growth. Container media that has spilled or has fallen from nursery container drain holes and all plant debris should be cleaned from container areas. While initially weed-free, growing media that is stored improperly or in an open area can quickly become contaminated with weed seeds. Covering the pile or storage indoors should be considered as strategies to keep weed seeds from entering the media. If soil or compost is a component of a container media the weed levels should be determined. Nursery liners and transplants should be weed-free. Inspect purchased liners and transplants for weed growth. If weeds are present, remove as many as possible, especially those near the surface, before planting. Ask the supplier questions about their production process weed management programs and weed levels before purchase. Nursery containers that are going to be reused should be washed thoroughly. Washing should be done in a manner that removes all leftover growing media and weed seeds. Many weeds are very small and can easily be missed when containers are inspected visually so washing is necessary. Routinely scout and monitor newly potted plants. Recently planted nursery containers can be very prone to weed growth. If weeds do appear, examine the nursery container closely and determine if the weeds are germinating from the liner or from the growing media. Take the appropriate action based on the location of the weeds. Implement management strategies that are targeted toward preventing weed growth in container nursery stock. Several container surface covers can be used to reduce weed growth in nursery containers.

Herbicides and applications

All characteristics of a particular herbicide should be considered when selecting an herbicide, including weed species controlled and longevity of effective control, weed growth stage at time of application, crop tolerance to herbicide and potential for injury, herbicide rate and activation requirement including rainfall/irrigation free period, application timing, herbicide formulation, herbicide mode of action, herbicide persistence and speed of degradation, potential for leaching and runoff, potential for injury from spray drift and volatility, selective vs. non-selective, contract vs. systemic/translocated, and cost. The product labels of all herbicide products should be read and understood before application. Make applications according to label directions.

Combine herbicides to increase the spectrum of weeds controlled. Most herbicides used singly do not control all the weeds at a specific site. A particular preemergence herbicide is generally stronger on “grassy weeds” or “broadleaf weeds” and therefore a tank-mix of “grass” and “broadleaf” herbicides will increase the spectrum of weeds controlled. Apply preemergence herbicides at frequent enough intervals to maintain effective weed control. Rotate herbicides based on mode of action and weed spectrum controlled. Using the same herbicide or herbicides with the same mode of action at a specific site can result in a shift in weed populations as well as increase the potential for herbicide resistance. If weeds that had previously been controlled with a specific herbicide are not currently being controlled, herbicide resistant weed populations may be developing. Steps should be taken to prevent suspected weeds from producing seed and spreading.

All herbicide application equipment should be calibrated on a regular basis. Check delivery rate and application pattern on all sprayers and spreaders and adjust accordingly. Replace worn or damaged spray tips. Apply herbicides based on the germination period and growth stage of the specific weeds. Postemergence herbicide applications should be made only to weeds that are actively growing and not under moisture stress. Weeds should not be mowed for two weeks prior to and one to two weeks after application.

An accurate record of all herbicide applications should be kept on file. Information recorded should include application date, herbicide used and formulation, herbicide rate and spray volume output, weeds present and their growth stage, crops and their growth stage, location and amount of area treated, weather (air temperature, dewpoint temperature, wind speed and direction, post application rainfall), soil moisture, application problems, and other information that might be helpful. Avoid over-watering container and field nursery stock. Over-watering can increase weed germination and establishment. The effectiveness of preemergence herbicides can be significantly decreased as a result of herbicide degradation as a result of over-watering. Thoroughly clean herbicide application equipment after application. Evaluate herbicide applications for effectiveness. Use the information collected to make appropriate adjustments to the herbicide program.

Best Management Practices information compiled by Tina Smith and Paul Lopes, University of Massachusetts Extension; weed management information edited by Randall Prostak, University of Massachusetts. Read the entire BMP publication at https://bit.ly/nursery-BMPs

Bayer is proud to sponsor the first-of-its-kind weed management special issue of Nursery Management and we are excited to examine the important trends for the industry that come out of this report. This year has brought many changes and challenges, especially related to the ongoing scarcity of available nursery laborers and the increasing costs associated with that labor. With these realities claiming center stage, it may be tempting to skimp on overall pest control. However, in an industry that is all about quality, this is not an effective or economical long-term solution as pest damage and reduction in plant quality can have long-term negative impacts.

Tackle complex weed challenges

When allowed to flower, some weed species are capable of producing thousands of seeds per square foot. Weeds are expensive to control, especially after they emerge. In fact, on many nursery operations, herbicides account for nearly 50 percent of pest management costs. However, the greatest cost of weeds is the cost of neglect. Weeds that are left unmanaged or treated ineffectively cause significant upticks in costs for labor required to scout and hand-weed. Growers must also consider the ever-changing inventory of plants and the potential for introducing new and potentially invasive weed species. Fortunately, the use of a preemergence herbicide to preventively control weeds before they germinate can result in dramatic cost savings over the course of the growing season.

Controlling weeds at the source is the key to success

Starting clean and staying weed-free is the best approach any nursery grower can take to be successful. Following strict sanitation guidelines to maintain a clean environment minimizes opportunities for introducing pests, diseases and weeds. Taking the time to install physical barriers (such as ground cloth) to reduce exposure to the soil beneath containerized plants minimizes the potential for weed contamination. Preemergence herbicides can be applied around adjacent buildings, on gravel roadways and in plant holding areas to effectively control weeds.

This approach minimizes the potential for weed outbreaks reducing labor costs for hand weeding and post emergence herbicide applications. In fact, a single application of Marengo® herbicide at the high labeled rate can provide weed control for up to eight months. Every time a grower has to make a pesticide application there is site preparation, application prep (i.e. preparing equipment and product mixing), application of the treatment and then any necessary clean up (equipment, disposal, etc.). Avoiding more frequent applications saves time and money and highlights the value of using Marengo® for long-lasting weed control.

Innovative weed management solutions

Ultimately, when it comes to managing weeds, growers are first and foremost looking for solutions with good performance and crop safety. At Bayer, we have several solutions that help growers effectively control weeds in the nursery. Marengo® preemergence herbicide demonstrates excellent crop safety and provides up to eight months of residual control of more than 90 weeds with a single application. Marengo® G preemergence herbicide is labeled for use on 266 plant species and provides up to five months of residual control of over 90 weeds. Ronstar® G preemergence herbicide offers exceptional root safety and is ideal for use on newly transplanted ornamentals and liners. It is labeled for use on more than 250 ornamental species with strong performance on several difficult-to-control weeds. And for postemergence weed control, Roundup PROMAX® and Roundup QuikPRO® are great options for putting a stop to more than 200 types of grassy and broadleaf weeds. Growers can learn more about these and other innovative solutions for pest control in nurseries by visiting es.bayer.us/ornamentals.

Last, but not least, thank you Nursery Management for sharing this important weed management special issue. We look forward to being part of this report for years to come.

Sincerely,

Steve Larson Nursery & Greenhouse Crop Specialist – Southwest US, Ornamentals, Bayer

Aaron Palmateer, Ph.D. Senior Technical Service Representative, Ornamentals, Bayer

ALWAYS READ AND FOLLOW LABEL INSTRUCTIONS Bayer Environmental Science, a Division of Bayer CropScience LP, 5000 CentreGreen Way, Suite 400, Cary, NC 27513. For additional product information, call toll-free 1-800-331-2867. environmentalscience.bayer.us. Not all products are registered in all states. Bayer, the Bayer Cross, Altus, Broadform and Marengo are registered trademarks of Bayer. © 2020 Bayer CropScience LP.