|
back
| home
University
of Rhode Island GreenShare Factsheets
How
Herbicides Work
Herbicides are chemicals that kill plants or inhibit their
normal growth. Their means of doing this are varied and theoretically
as numerous as the processes essential to plant life. Herbicides
are most often and most effectively used together with good
cultural practices in a turf weed management program. The choice
of the best specific combination varies with agronomic, ecological
and economic factors.
In terms of their effects on plants, herbicides may be
described as selective or non-selective. Selective herbicides
kill or stunt weeds in turfgrasses with little or no harm to
the turfgrass. Non-selective herbicides kill or damage all
plants when applied at adequate rates. No herbicides belong
rigidly to either group. Non-selective herbicides may, under
certain situations, act selectively; if the dosage is excessive,
even a selective herbicide may become phytotoxic. Selectivity
is, therefore, a property of the type of treatment as well
as the chemical, regulated by such factors as time and method
of application, chemical formulation, environmental conditions,
the stage of turfgrass and weed growth, and particularly the
dosage.
Herbicidal Action:
Herbicides kill plants through either contact or systemic
action. Contact herbicides are most effective against annual
weeds and kill only the plant parts on which the chemical is
desposited. Systemic herbicides are absorbed either by roots
or foliar parts of a plant and are then translocated within
the plant system to tissues that may be remote from the point
of application. Although systemic herbicides may be effective
against both annual and perennial weeds, they are particularly
advantageous against established perennial weeds.
All herbicides can be grouped into classes. Each class
has a typical chemical structure and activity. The following
classes include herbicides commonly used in lawns.
Phenoxy Acid Type: Examples of this class are:
2,4-D (WEEDONE), mecoprop (MCPP) and 2,4-D plus 2,4-DP (WEEDONE
DPC). These herbicides are generally applied as foliar sprays.
They are systemic and are selective in that they generally
injure or control broadleaf plants without injuring grassy
plants. They act as a synthetic growth hormone, mimicking the
natural auxins that occur within plants. These herbicides are
readily absorbed by the foliage and usually translocate relatively
well and, in some cases, all the way down into the root system.
Selectivity is usually physiological in nature. General symptoms
include growth regulator effects such as twisting and downward
curvature (epinasty) of the stems and leaves.
Benzoic Acid Type: Examples are: dicamba (BANVEL,
TBA). This class of herbicides is very similar to phenoxy class
herbicides. They are applied as foliar sprays. They are systemic,
selective and primarily effective in controlling broadleaf
weeds. Dicamba is quite mobile in soils and can be taken up
by roots of sensitive trees and ornamentals. Selectivity is
physiological in nature. Symptoms are very similar to phenoxys.
Dinitroaniline Type: Examples of this class are:
pendimethalin (WEEDGRASS), trifluralin (TREFLAN), benefin (BALAN),
a combination of trifluralin and benefin (TEAM) and prodiamine
(BARRICADE). The herbicides in this class are used for preemergence
control of annual grasses in established turf. They are mitotic
(cell division) inhibitors and are primarily effective in inhibiting
root growth of germinating seeds. Selectivity is physiological
or chemical in nature. Some of these herbicides may be lost
by volatilization. All of these herbicides need to be watered
into the soil for proper activation. Some may persist for several
months.
Bipyridylium Type: Diquat (REWARD) is strictly
used as a foliar spray. It is a non-selective herbicide. It
is a contact herbicide and requires the addition of surfactant
for foliar activity. This type of herbicide is adsorbed quickly
and tightly to soil colloids. Therefore, diquat has no biological
activity in soil. These herbicides are membrane disrupters.
They intercept electrons from photosynthesis and transfer the
energy to various free radicals which damage cell membranes.
Symptoms resulting from these herbicides are quick wilting
and desiccation of plants. Activity of these herbicides depends
on light.
Substituted Urea Type: Siduron (TUPERSAN), linuron
and many others. These herbicides are selective. They are soil-applied
and are absorbed through roots and translocate in the xylem.
Unlike other substituted urea herbicides, siduron does not
affect photosynthesis but is a root growth inhibitor. Symptoms
include slow death of young seedling weeds. Siduron is the
only herbicide that can be used for selective control of crabgrass
and other annual grass weeds in newly seeded turf.
Arsenical Type: MSMA (WEED-E-RAD),WEED-HOE and
DSMA are examples of this class. Arsenicals are primarily contact
herbicides. They are strictly foliar-applied with a surfactant.
Symptoms include yellowing of foliage within 7 days of treatment.
They are tightly bound to soil. They are recommended for post-emergence
crabgrass and nutsedge control in established turf.
Miscellaneous Types: Glyphosate (ROUNDUP) is a
widely used, non-selective herbicide. It is strictly used as
a foliar spray and is systemic in nature. It is very effective
for perennial weeds because of its translocation characteristics.
It inhibits new plant growth by interfering with aromatic amino
acid synthesis. Visible symptoms generally occur on annual
plants in 2 to 4 days and on perennial plants in 7 to 10 days.
Symptoms on the shoots include yellowing and wilting, progressing
from the new to the older tissues. Glyphosate rapidly reacts
with and is inactivated by most soils. It is commonly used
for turf renovation.
Dithiopyr (DIMENSION) belongs to a new class of herbicide
known as pyridines. It is a selective herbicide primarily used
for preemergence annual grass control in established turfgrass.
It can, however, be used for post-emergence control of young
(1-3 leaf) grass seedlings. It inhibits cell division and cell
growth of meristematic regions (growing points of roots and
shoots). Dithiopyr is lost from soil by chemical and microbial
degradation.
Glufosinate (FINALE) is a non-selective herbicide which
severely limits glutamine synthesis in treated plants. It is
strictly a foliar-applied herbicide and can be used for turfgrass
renovation. It is absorbed through leaves and stems and degrades
into natural compounds such as carbon dioxide, nitrogen and
water. Glufosinate has no soil residual activity and therefore
sodding or seeding can take place soon after application.
Adapted from the University
of Massachusetts Extension, 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
|