Herbicides (and other chemicals) are classified by their potential impacts on humans and animals. - Caution - Warning - Danger - are the three levels assigned by the U.S. EPA as advisory language.

The label affixed to the container must boldly display the herbicide's classification, and must present much additional information.

Specifications on the label
Although the content and legal obligation of the labels on herbicides varies, there is no questioning that following these instructions is the first priority in making decisions about herbicide applications.

In Florida as in the U.S., it is a violation of federal law to use a herbicide in a manner that is inconsistent with its label.

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LOCATIONS OR SITES WHERE HERBICIDES CAN BE USED
If certain types of sites are specified on the label of a herbicide, (such as, "in slow moving or quiescent bodies of water"), that herbicide cannot be used in other types of water bodies. (Fast-flowing water is the most likely type of aquatic site to be excluded for herbicides suitable for use in water). Occasionally, herbicide use in certain sites is limited to specified applicators; for example, diquat may only be applied to public waters of the United States by federal or state public agencies or by certified applicators authorized by such agencies.

PLANTS CONTROLLED
Most herbicide labels will list plant species for which the compound is most effective, frequently indicating the most appropriate rates of application. A herbicide may be used to control species that are not listed (provided they occur in approved sites) but in such cases the manufacturer will not guarantee any particular level of efficacy.

There is a frequent need to test herbicides on recently-arrived, non-native weeds so that they can be listed on the labels of effective herbicides. Some labels will also include a list of species which are not controlled by the herbicide, useful information if selectivity of weed control is desired. Some herbicides are effective only on certain types of plants and thus can be used selectively to remove only part of the vegetation; for example, 2,4-D and triclopyr typically affect only dicotyledonous (broad-leaf) plants and not monocots, such as grasses.

The DEP, herbicide manufacturing companies, universities and other research institutes like the U.S. Army Corps of Engineers frequently conduct research to find the most appropriate times and plants to control with aquatic registered herbicides. It is just as important to know which plants are not affected by herbicides as to know which are controlled in order to selectively remove weeds or invasive plants from beneficial native vegetation.

APPLICATION METHODS
Herbicide labels usually give directions as to how the herbicide may be applied. General language does not typically preclude any types of application that achieve the desired objective. For example, the label may state generally, "...use an application pattern that will provide a uniform distribution and avoid concentration of the herbicide", but alternative techniques are not permitted if specific methods are listed for particular sites.

Part of the preceeding was excerpted from, Role of Herbicides in Aquatic Plant Management Programs, by W.T. Haller and A.M. Fox, 2002.