Chemical Weed Control and Types of Herbicides

Herbicides can substitute for long hours of tedious labor when properly used, but they must be used carefully to avoid injuring desirable ornamental plants. Herbicides should be used in combination with nonchemical control methods such as mulches whenever possible. The success of chemical weed control depends on the type of herbicide used, where it is placed, when it is applied, and what environmental conditions prevail at the time of application. Herbicides can be grouped into several categories depending on how they affect plants and when they are used.

• Preemergence herbicides - applied before weeds emerge
• Postemergence herbicides - applied to the foliage of emerged weeds
• Chemical characteristics of selected landscape and ornamental herbicides

Herbicide Selectivity

Selectivity is the ability of herbicides to affect some plants but not others and is due to biological differences between plants or due to the physical placement of the herbicide and the behavior of the herbicide in the soil. Preemergence herbicides commonly used in landscapes generally have little affect on emerged plants (i.e., no postemergence activity) because they inhibit seed germination and seedling emergence from the soil. Furthermore, preemergence herbicides recommended for use in desert landscapes have very limited mobility in soils so the herbicide molecules stay near the surface of the soil where seeds germinate and do not reach or affect the roots of ornamental plants that are deeper in the soil profile. Some postemergence herbicides are selective due to biological differences between plant types. Fluazifop and sethoxydim are herbicides that kill grasses but do not affect broadleaf plants because broadleaf plants don't contain the type of enzyme that binds these herbicides. Generally, most of the postemergence herbicides used in landscapes are non-selective meaning that they kill all green tissue. However, these postemergence herbicides can be used selectively in desert or rock-mulch landscapes by carefully directing the herbicide spray underneath or at the base of desirable ornamental plants so that only small weed seedlings are contacted by the spray. Many of the desirable ornamentals in desert landscapes have woody, non-green bark that protects the plants from the effects of postemergence herbicides.

Herbicide Names

The herbicides discussed in this publication are identified by their common name because they are often sold under a variety of trade names. Furthermore, herbicides are often mixed with other herbicides or with fertilizers and sold under a different trade name than when they are sold individually. The label on pesticide containers lists the common name, the chemical name or both names. Be sure to read the label each time a purchase is made because commercial companies occasionally change herbicide trade names, change the herbicides in a product or change the concentration of a herbicide in a product. Table 3 lists the common name, chemical name, trade names, and the oral LD₅₀ of herbicide formulations commonly available and used in landscape situations.

Herbicide Toxicity

Companies the manufacture herbicides must extensively test potential new herbicides to determine their physical properties, fate and impact on the environment, and their toxicity to animals. Herbicides are classified according to their acute toxicity or oral LD₅₀ for test animals, usually albino rats. The oral LD₅₀ is the single dose of herbicide expressed in mg of herbicide per kg of body weight taken orally that will kill 50% of the population of test animals. The lower the LD₅₀ value the more toxic the herbicide. If a herbicide has an LD₅₀ of 50 or less, the herbicide container will have the signal word: DANGER, indicating very high toxicity (approximate amount needed to kill an average adult is a taste to a teaspoon). Herbicides with an LD₅₀ from 51 through 500 have the signal word: WARNING on the container (approximate amount needed to kill an average adult is a teaspoon to an ounce). Herbicides with an LD₅₀ greater than 500 have the signal word: CAUTION on the container (approximate amount needed to kill an average adult is greater than an ounce). Most herbicides on the market carry the CAUTION signal word and are less toxic than caffeine (250 mg/kg) and aspirin (780 mg/kg), and many are less toxic than table salt (3320 mg/kg) or ethyl alcohol (3800 mg/kg). Refer to the worker protection safety information on the herbicide label and take appropriate measures to reduce exposure to any herbicide regardless of toxicity. Table 3 lists the common, chemical, and trade names, and the oral LD₅₀ of herbicides that are commonly found in hardware and retail nursery outlets and are commonly used in landscape situations.

It is useful to consult herbicide labels prior to making an application since they contain a large amount of useful information. Herbicide labels and MSDS (manufacture's safety data sheet) can be found on the web at the primary manufacture's web site or at:

• Bluebook online edition of Turf and Ornamental Reference by Chemical and Pharmaceutical Press, Inc.
• Crop Data Management Systems, Inc.

Herbicide labels also contain worker protection safety information (WPS) for applicators. Worker protection safety information can also be found on EPA's website:

• Pesticide Safety Programs from the EPA Office of Pesticide Programs

Protective equipment worn during a herbicide application may include rubber gloves and rubber boots (without liners) with long pants worn outside the boots. Applicators should also wear a long sleeve shirt and wear eye protection. Mix and use herbicides in well ventilated areas to avoid breathing herbicide vapors. Do not smoke or eat during or after a herbicide application unless you thoroughly wash.