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Publication #OH-94

Preemergence Herbicides for Use in Ornamentals1

Robert H. Stamps, Heidi M. Savage, Diane K. Rock, and Jeffrey G. Norcini2

Preemergence herbicides, by definition, are herbicides that are applied prior to weed seed germination. Control of weeds using preemergence herbicides is most successful when the correct herbicide is applied in the correct manner to a weed-free growing medium prior to weed seed germination.

Preemergence Herbicide Application

In field or landscape situations, the soil should be freshly tilled and large clods of soil broken apart. Additionally, the growing medium, whether soil or soilless, should be settled and firm at the time of herbicide application to prevent the applied herbicide from channeling downward into the root zone of the crop. A couple of irrigation cycles or a good rainfall is often enough to settle the growing medium.

Herbicides should be applied uniformly to the target area and then immediately incorporated into the growing medium. Uniform herbicide coverage is dependent on good application technique and, for mechanized application, on well-maintained and calibrated equipment. Incorporation using water is generally employed to reduce herbicide losses from volatility (Table 1, Vapor pressure) and photodecomposition, but it also serves to activate some herbicides. Soon after the herbicide has been applied, about 0.5 inch (1.2 cm) of water is applied using overhead irrigation for incorporation/activation. Herbicides can also be mechanically incorporated by mixing the herbicide into the top layer of the growing medium, generally to a depth of 1–3 inches (2.5–7.5 cm). Recommendations regarding the depth of mechanical incorporation and/or the amount of irrigation water to apply to activate/incorporate the herbicides should be followed carefully in order to minimize the possibility of crop injury. Read the product labels for specific incorporation instructions.

Preemergence Herbicide Formulations

Many of the preemergence herbicides used in ornamentals are formulated for application as dry granules. The active ingredient(s) is (are) carried on small, inert particles or pellets (clay, paper, sand, corn cob, etc.). These carrier particles typically have low concentrations (≤ 10%) of herbicides and should be applied evenly over the treated area. The granules should not be applied to moist foliage or to whorled or other foliage types that will accumulate and trap the granules. Application equipment used to broadcast these formulations should not grind the granules or increase the amount of dust as this may generate potentially dangerous conditions to nearby plants and people. Whenever possible, use granular formulations that do not have extremely fine particles.

Preemergence herbicides are also formulated as liquid solutions (aqueous solutions, AS; emulsifiable concentrates, E, EC; flowable solutions, F) and dry, water-soluble formulations (dry flowable, DF; disperable granules, DG; water-dispersable granules, WDG; water-soluble bags, WSB; wettable powders, WP) for spray applications. Spray pressure should be constant and adequate to maintain proper spray pattern for uniformity and to ensure droplet penetration through the plant canopy. Excessive pressure can lead to drift and damage to nontarget plants. Spray volume should be sufficient for thorough coverage. Herbicides should be thoroughly mixed (agitated) in spray tanks to obtain uniform results, and spray nozzle openings (orifices) should be checked regularly and replaced when wear becomes evident. Spray adjuvants that enhance coverage, penetration, and/or persistence of herbicides can be added to some spray mixtures, especially if the mixtures are applied so that the crop foliage is not treated. Test spray solutions that contain any new adjuvant for phytotoxicity on a small part of your crop, just as you would test any new pesticide.

Chemigation

Chemigation is the least labor-intensive method for applying herbicides; however, few herbicides are labeled for application in this manner (Table 1, Labeled for chemigation). In addition, some types of irrigation systems (e.g., drip) do not apply water to all areas that need to be treated and/or do not have adequate uniformity for this use. Herbicide formulations applied through irrigation systems must not clog nozzle orifices. Florida and other states have laws that require safety equipment to be provided if chemicals are to be injected into irrigation systems. Some of the equipment necessary to prevent contamination of the water supply includes check valves, vacuum breakers, low-pressure drains, shutoff valves, remote chemical storage tanks, and interconnected power supplies to injector and irrigation pumps. Check with the agency that deals with pesticide compliance in your region for specific details and requirements.

Good Herbicide Practices

Liquid herbicides should be measured volumetrically (i.e., using measuring cups, graduated cylinders, etc.). Since dry pesticides vary in density, it is not possible to give accurate volumetric conversions across brands and formulations. Therefore, if a volumetric conversion for an herbicide is not listed on its label, those dry formulations must be weighed.

It is a good practice to keep records—including EPA registration and product lot numbers—of all pesticide applications, even of nonrestricted-use pesticides. These records can be useful for planning future weed control measures. In addition, they can be invaluable if crop damage occurs.

Regardless of the herbicide or application method used, it is strongly recommended that the effects be evaluated against untreated controls under your particular conditions before treating large areas. This is especially important when there is a statement on the label permitting use of the product on an ornamental not specifically listed on the label (Table 1, General crop labeling). Treat a limited area or number of plants and wait 2–3 weeks, or longer if the label recommends, for any phytotoxic effects to appear. Be aware that the larger the area treated, the more likely that phytotoxicity may occur, especially due to herbicide volatilization. Also, damage may not occur the first time an herbicide is applied, but it may show up with repeated applications. Several formulations of an herbicide may be available with varying concentrations; consequently, recommendations on the manufacturer's label should be followed explicitly. Note also that there may be limits on the amount of herbicide that can be applied at each application and additional restrictions on the total amount that can be applied to a given area in a year's time. Further, herbicide formulations are labeled for application in certain environments (outdoors, in shadehouses, and/or in greenhouses) and specific locations (container, field, and/or landscape) and can only be applied in those specified environments and locations.

Although most preemergence herbicides labeled for use in ornamentals have relatively low acute mammalian toxicities, they are potentially dangerous if handled improperly and, therefore, the safety precautions on the label(s) must be followed. Read the entire label, including the small print, before buying or using the herbicide. Wear personal protective equipment (PPE) and comply with restricted-entry intervals (REI) (Table 1, REI). Keep the telephone number and address of the nearest County Poison Control Center listed in a convenient location in case of an accidental poisoning. Have clean copies of herbicide labels and material safety data sheets (MSDS) available to be taken to the Poison Control Center or hospital in the event of an emergency. If an herbicide is labeled for use specifically in a particular state (for example, 24(c) Special Local Need labeling in Florida), obtain a copy of the supplemental label from the supplier when you purchase the product; otherwise, you will be using it illegally.

Herbicide Persistence and Movement

There are many processes affecting preemergence herbicide losses and movement in the environment. Volatilization, the vaporization of the active ingredient into the atmosphere, has already been mentioned. Breakdown of the active ingredients can occur by means of biological, chemical, and photodegradation processes. Removal of preemergence herbicides from the growing medium surface can reduce weed control. Downward movement of herbicides can lead to crop damage and has the potential to contaminate water resources. The main avenues by which properly applied herbicides may reach surface and ground waters are runoff and leaching. Runoff is the physical transport of herbicides over the ground surface by water that does not percolate down through the growing medium. The source of the water can be rain or irrigation. The herbicide may be in solution in the water and/or attached to organic matter carried along by the water to a surface water body. As more nurseries install tailwater recovery systems, the potential for runoff of herbicide-laden water to retention ponds is increased. When the stored water is later used for irrigation, it has the potential to cause crop damage. Leaching occurs when the herbicide moves down through the soil with water that is percolating. In many areas of Florida, soils are sandy and permeable, and leaching is likely to be a more serious problem than runoff. Fortunately, many of the preemergence herbicides labeled for use on ornamental crops have low solubilities in water and are quite strongly held to binding sites in growing media (see Sorption below); therefore, they are not readily leachable.

As relates to runoff and leaching, the fate of the herbicides applied to growing media depends largely on two of the herbicide's properties—sorption and persistence. Sorption is the process whereby the herbicides are bound to the growing medium. Depending on the herbicide's solubility, more or less of the herbicide will dissolve in the water held in the growing medium and be likely to leach. The growing medium organic matter content, temperature, pH, and moisture content can all affect the sorption/desorption process.

A useful index for quantifying herbicide sorption is the organic carbon partition coefficient, Koc (Table 1). Herbicides with high Koc values are typically not very water soluble and preferentially adhere to the growing medium rather than dissolve in water. Herbicides that are strongly bound to the growing medium are unlikely to be a problem because of leaching, unless they are very persistent (see Persistence below). However, if there is erosion of the growing medium, the herbicide will move with medium particles. The California Department of Pesticide Regulation has determined that pesticides with Koc values less than 1,900 have the potential to contaminate groundwater.

Nonetheless, as alluded to above, the relatively long persistence (long degradation half-lives, T½) of some herbicides creates the potential for leaching losses to still occur with potential effects on groundwater. However, from a weed control standpoint, good preemergence herbicide persistence at the surface of the growing medium can be beneficial.

Herbicide Sites of Action and Weed Resistance Management

As an aid to help delay the selection of herbicide-resistant weeds, the herbicide group to which each product belongs is listed in Table 1. The groups are based on primary sites/modes of action (Mallory-Smith and Retzinger 2003) and can be used to select herbicides that have differing sites/modes of action. Repeated use of an herbicide or herbicides having the same sites of action can, over time, select for weeds resistant to those herbicides.

Herbicide Storage and Container Disposal

Store herbicides in their original labeled containers and out of reach of children. All pesticides should be locked in secure facilities. When containers are empty, rinse with water at least three times and pour the rinse water into the spray tank. Dispose of empty containers promptly and safely (according to local, state, and federal disposal laws).

Herbicide Efficacy Tables

In Table 2, weeds are listed alphabetically by scientific name along with the preemergence herbicides that control (C), suppress (S), or do not control (N) each weed. Table 3 is similar except that the weeds are listed alphabetically by common name(s). Unfortunately, common names can be misleading, and several weeds may have the same common name. It is important to correctly identify weeds since two very similar weeds may require completely different herbicides for effective control. The information in the efficacy tables is derived from product labels as well as published and unpublished research results. An herbicide may provide variable weed control depending on growing medium, irrigation, weed biotypes, and other variables. Where the weed entry in the tables lists only a genus, the labels must be checked to see which species in that genus are controlled by each herbicide.

Crop Herbicide Tolerance

Table 4 provides a guide to ornamentals' tolerances to preemergence herbicides, with the table subdivided into trees and shrubs, groundcovers, and vining plants, and annuals and perennials. The plants are listed alphabetically by scientific name. Information in Table 4 is derived from product labels and does not imply that an herbicide can be used in or around an ornamental species in all sites (nursery, landscape, greenhouse). The symbol "Y" indicates that the crop is listed for the common chemical name but may not be listed on all labels because of formulation differences. Always read the labels for species/cultivar tolerance, special use, and injury instructions. The symbol "N" indicates that the product should not be used on the crop. Please note that not all plant species and cultivars listed on the herbicide labels are included in the table, and some plants are listed in the table even though, at the current time, they are not listed on any of the herbicide labels. Hopefully, research and future label changes will help alter this situation for this latter group of plants.

Table 5 provides a reference of common names for the ornamentals presented by scientific name in Table 4. Readers should be aware that the same common name may be used for more than one plant (genus/species), so the true identity of the crop should be ascertained before herbicides are applied.

When selecting an herbicide, both factors listed below must be determined to legally use an herbicide in or around an ornamental species. The information below is on the product label:

1. Determine the tolerances of specific species, varieties, or cultivars, if applicable,

AND

2. Determine which sites the herbicide can be used on (i.e., nursery [container or field]; landscape [commercial, public, or residential]; open and enclosed shadehouses, and greenhouses).

Please report any errors, omissions, and/or updates to Bob Stamps (rstamps@ufl.edu).

Thank you.

References

Mallory-Smith, C. A., and E. J. Retzinger. 2003. "Revised classification of herbicides by site of action for weed resistance management strategies." Weed Technology 17 (3): 605–617.

Footnotes

1.

This document is OH-94, one of a series of the Environmental Horticulture Department, UF/IFAS Extension. Original publication date January 1999. Revised March 2012 and August 2013. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Robert H. Stamps, professor, Heidi M. Savage, biological scientist, and Diane R. Rock, former research assistant, Mid-Florida REC, Apopka, UF/UFAS Extension; and Jeffrey G. Norcini, horticulturist, OecoHort, LLC, Tallahassee, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label. Use pesticides safely. Read and follow directions on the manufacturer's label.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.