Herbicides and Application Methods

Herbicides are chemicals used to kill plants or to inhibit their growth.  All herbicides must be registered by the EPA for use in the United States.  Due to the uniqueness of the aquatic environment, very few herbicides are approved for use in water.  Compounds that effectively control aquatic plants must also meet the rigid environmental and toxicology criteria necessary for registration.

Herbicides can generally be divided into one or more of the following classifications:  1) contact, 2) systemic, 3) broad spectrum, or 4) selective.  Some herbicides may fall into more than one classification. For example, a contact herbicide may also be a broad spectrum herbicide.

1)      Contact herbicides act quickly and are generally lethal to the plant cells that they contact.

These herbicides are most effective on annual plants (plants that complete their life cycle in one year) and are usually applied to foliar parts of plants.

2)      Systemic herbicides are absorbed into plant tissues and are translocated through the plant.  Some herbicides are absorbed into the roots from soil applications, other herbicides are absorbed into the leaf or stem tissue.  These herbicides are effective on perennial plants (plants that persist from year to year).

3)      Broad spectrum or nonselective herbicides control a large variety of vegetation.  These are most often used when total vegetation control is desirable.

4)      Selective herbicides control certain plants, but not others.  Some herbicides control broad-leafed plants, but have minimal effect on grasses.  A plant’s susceptibility to a herbicide depends on many physical and biological factors.

When choosing a herbicide or combination of herbicides for a particular vegetation problem, district personnel consider a number of factors, such as non-target plants, invertebrates, fish, and mammals.  Also, wind speed and direction, dissolved oxygen of the water, salinity, time of day, rain possibility, water flow is considered.  Domestic uses of treated water must be examined—is the water body used for drinking, swimming, fishing, or irrigation?  All these factors must be weighed when choosing a herbicide.

Currently, there are fourteen different products that are registered by EPA for aquatic plant control, each with a specific mode of action.  All fourteen herbicides have met the most stringent safety standards under federal and state regulations.  None are restricted use herbicides, which means they are less toxic than many used in agriculture and even some used in the home.   For best management practices, these products are rotated when possible to reduce the potential for herbicide resistance.   The District utilizes aquatic herbicides in a safe, efficient, and economical manner to effectively manage nuisance weeds.