Horticulture Principles and Practices

application, a particular herbicide may kill a certain species but fail to do so at a lower
concentration. However, even at lower concentrations, these chemicals remain toxic and
should be handled with care. As such, by spraying older cultivated plants with low application
rates, younger weeds may be controlled without harming the desired crop plants.
Nonselective Herbicides
Nonselective herbicides literally kill all plants exposed to them—weeds and crops alike.
These nondiscriminating herbicides are used to control weeds in areas where no plant
growth is desired, such as driveways, parking lots, and along railroad tracks. Examples of
nonselective herbicides are Roundup and atrazine. Nonselective herbicides may be made
selective through manipulation of the concentration or rate of application. For example,
using atrazine at low concentrations decreases its killing action to certain plant types.
8.12.2 CONTACT VERSUS TRANSLOCATED
Some chemicals use two modes of action. Contact herbicides kill by direct contact with
plants and are very effective against annual weeds. To be most effective, the application
must completely cover the plant parts. Systemic (translocated) pesticides are absorbed
through either roots or leaves. Those applied to the soil have residual action and thus are
most suitable for controlling perennial weeds. Complete coverage is not necessary when
using these chemicals.
8.12.3 TIMING OF APPLICATION
Regarding crop (or weed) growth cycle, three stages are important for herbicide application.
Systemic Pesticide
(translocated)
One that is absorbed and
moved from the site of
uptake to other parts of
the plant.
Preplant
Preplant herbicides are applied to the soil before planting the crop. Depending on the kind,
it may or may not require incorporation into the soil to be effective. Preplant applications
are made at low rates or concentrations and have the advantage of damaging weeds when
they are in the most vulnerable seedling stage.
Preemergence
Like preplant herbicides, preemergence herbicides are applied after planting the crop,
either before crops or weeds emerge or after crop emergence but before weed emergence.
These herbicides kill only germinating seedlings and not established plants. Whenever
soil is disturbed, weeds arise. A newly planted ground cover may be sprayed with a preemergence
herbicide to suppress weeds that may have been stirred up.
Postemergence
Herbicide application after cultivated plants have emerged is described as postemergence
treatment. In several situations, such as occurs in orchards, the grower has no choice but
to adopt postemergence application. However, the grower may choose to apply a herbicide
before weeds emerge (preemergence). It is important, therefore, that emergence
always be in reference to either the weed or the crop plant.
8.12.4 CHEMISTRY
Herbicides may be classified according to their chemical nature as either organic or inorganic.
Organic Herbicides
The various classes of organic herbicides include organic arsenicals and phenoxy herbicides.
Organic Arsenicals Organic arsenicals are translocated herbicides and thus are
effective against plant species with underground structures (e.g., rhizomes and tubers), as
8.12 Classification of Herbicides 271