• Topics: Environmental Horticulture  | Nursery Weeds  | Landscape Herbicides  |  Norcini, Jeffrey.  |  Stamps, Robert H  | Rock, Diane K 

Preemergence Herbicides for Use in Ornamentals¹

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. In field or landscape situations, the soil should be freshly tilled, with large clods of soil broken up. The growing medium, whether soil or soilless, should be settled and firm at the time of herbicide application.

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 is generally employed to reduce herbicide losses from volatility and photodecomposition, but it also serves to activate some herbicides. This is commonly accomplished by applying water using overhead irrigation, often about 0.5 inch [1.2 cm], after the herbicide has been applied. 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.

Many of the preemergence herbicides used in ornamentals are formulated as granular materials and should be applied evenly over the treated area. Avoid applications to moist foliage or to whorled or other foliage types that will accumulate and trap granules. Application equipment used to broadcast these formulations should not grind the granules or increase the amount of dust; this may generate potentially dangerous conditions to nearby plants and people. Use materials that have been screened of fine particles, if available.

For liquid applications, spray pressure should be constant and adequate to maintain proper spray pattern for uniformity and ensure droplet penetration through the plant canopy. Excessive pressure can lead to drift and damage to non-target 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 is a special type of liquid application in which herbicides are applied using the irrigation equipment. It is the least labor-intensive method to apply herbicides; however, few herbicides are labeled for application in this manner (see Table 1). 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 that safety equipment must be provided if chemicals are to be injected into irrigation systems. Some of the equipment necessary to prevent contamination of the water supply include check valves, vacuum breakers, low-pressure drains, shutoff valves, remote chemical storage tanks and interconnected power supplies to injector and irrigation pumps.

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 so those formulations should be weighed.

It is a good practice to keep records of all pesticide applications, even of nonrestricted use pesticides — including EPA registration and product lot numbers. 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 should 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. Treat a limited area or number of plants and wait 2 to 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 volatilization. Also, damage may not occur the first time a herbicide is applied, but may show up with repeated applications. Several formulations of a herbicide may be available with varying concentrations; consequently, recommendations on the manufacturer's label should be followed explicitly. These herbicides formulations are labeled for application in certain environments (outdoors, in shadehouses and/or in greenhouses) and specific locations (containers, 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) (see Table 1). 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 a 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 your supplier when you purchase the product; otherwise, you will be using it illegally.

Many of these herbicides have low solubilities in water and are quite strongly held to binding sites in growing media and are, therefore, not readily leachable. However, the relatively long persistence (long degradation half-lives, T_½) of some of these products creates the potential for leaching losses to still occur with potential effects on ground water. Relative leaching potential indexes (RLPI) for the listed products range from 3 (high potential to leach) to >2,000 (low potential for leaching, very immobile) and are given, along with relative run-off potential indexes (RRPI), in Table 1. RRPIs range from 1 (high potential to be lost in runoff) to 333 (lower run-off potential). For more information about pesticide leaching and run-off, read Managing Pesticides for Ornamentals Production and Water Quality Protection (University of Florida, Institute of Food and Agricultural Sciences, Cooperative Extension Service, Soil Science Department Circular 991, available at http://edis.ifas.ufl.edu/).

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 according to primary sites/modes of action (Weed Technology 11:384–393 [1997]) and can be used to select herbicides that have differing sites/modes of action. Repeated use of a herbicide or herbicides having the same site of action can, over time, cause development of weeds resistant to those herbicides.

Store herbicides in their original labeled containers, 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 your local, state and federal disposal laws).

In Table 2, weeds are listed alphabetically by scientific name and 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). The information in these tables is derived from product labels, and published and unpublished research results. A herbicide may provide variable weed control depending on soil, irrigation 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.

Table 4 lists the tolerances of ornamentals 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. This information is derived from product labels. It is only presented as a guide and does not imply that a herbicide can be used in or around that ornamental species in all sites (nursery, landscape, greenhouse). In Table 4, Y indicates that the crop is on the product label and 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.

To help in situations where the reader does not know the scientific name of the crop, Table 5 is included. It lists the common names beside their scientific names; however, readers should be aware that the same common name may be used for more than one plant so the true identity of the crop should be ascertained before herbicides are applied.

When selecting a herbicide, both of the factors listed below must be determined to legally use a 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 on which sites the herbicide can be used, that is – nursery (container or field); landscape (commercial, public, or residential); enclosed greenhouse.

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

Thank you.

Tables

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 extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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