SP707

Choosing theRight Herbicide

Greg Breeden
Weed Science Extension AssistantGreg Armel
Assistant Professor- Extension Weed Specialist2Introduction

Herbicides are chemicals used to control weeds in different crop andnon-crop environments. Choosing the right herbicide for a weed
control issue can be confusing, intimidating and a time-consuming

process. In this day and age, shopping for any consumer product can bedifficult, even if comparing prices is your major concern. However, the mostfrustrating part about shopping for herbicide products is trying todetermine if you are actually getting the right product to control yourppws.vt.edu/weedindex.htm) andthe University of Missouri Weed
ID guide (http://weedid.missouri.
edu/). After you have made apositive identification of yourproblem weed, you may start theprocess of selecting an herbicide
product that will give you maximum
control of the target weed.

problem weeds at an affordable price. Many retailers who sell herbicide
products might not have a staff member specializing in this area, so,
unfortunately, having a knowledgeable person to discuss the situation withis not always possible. In this situation, knowing how to interpret a productlabel can be extremely helpful. The label contains all the information you
need to properly apply the product. However, there are a few things youneed to consider before you purchase an herbicide.

What to know before
you goProper identification of the target
weed is the first and most important
step in finding the best control
measure. Weeds that you may
encounter will either be classified asa broadleaf (i.e., wild carrot,chickweed, dandelion, etc.); a grass(i.e., johnsongrass, crabgrass,goosegrass, etc.); or a sedge (i.e.,yellow nutsedge or nutgrass). In
addition, weeds may also beclassified by their life cycle.
• Annual weeds (i.e., crabgrass,chickweed) reproduce from seeds
and complete their life cycle
within one year.
• Biennial weeds (i.e., wild carrot,etc.) reproduce from seeds andcomplete their life cycle in twoyears.

• Perennial weeds (i.e., yellow
nutsedge, dandelion,
johnsongrass, etc.) continue toreproduce over an indeterminate
number of years by producingspecialized underground
reproductive structures (i.e.,rhizomes, tubers, etc.). Thesestructures make perennial weeds
very difficult to control.
If you are unable to identify theweed yourself, you can contact yourlocal county Extension office forhelp. There are also many useful
Internet sites to aid in identificationof weeds. Some of the most userfriendly of these Web sites are theUniversity of Tennessee Weed ID
guide (http://turfweeds.utk.edu/weeddb/default.asp), the Virginia
Tech Weed ID guide (http://www.

Timing the application
Now that you have identified yourtarget weed, figuring out how tomanage it will be easier. Always
remember, the more you know
about your weed issue, the easier itwill be to select the proper herbicideand decide the optimal application
timing for maximum control. There
are two main types of herbicides tochoose from: preemergence (pre)or postemergence (post).
Preemergence herbicides areapplied before the weed emerges
and prevent the germinating
seedlings from developing once thegermination process begins. For
preemergence herbicides to work
properly, they must be applied
before weed emergence and needapproximately one-half inch ofrainfall or overhead irrigation
within one week after application.
Preemergence herbicides are mosteffective against annual weeds likecrabgrass, and generally provide
poor control of most perennial
weeds like dandelion.

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Postemergence herbicides are used
to control weeds that have emerged
and are actively growing.
Postemergence herbicides can either
be classified as selective or nonselective. A selective herbicide
controls certain weeds without
injuring the desired plants (i.e.,fruits and vegetables, ornamentalplants or turf) it may contact.
Selective postemergence herbicides
are available for control of annual
and perennial broadleaf weeds,grass weeds and sedges. Nonselective postemergence herbicides
will severely injure or kill all weeds
as well as desirable plants. Nonselective herbicides (products
containing glyphosate, glufosinate,
diquat or pelargonic acid, just to
name a few) can be used for spot
applications around desired plants
as long as the user is extremely
careful to avoid contact of theherbicide with the desired plant.
You must use caution in applying
these herbicides around any plants
you want to keep.

Useful information
found on the herbicidelabel
The weed has been identified andnow, armed with additional
knowledge on the types ofherbicides available, you canpurchase a product for your specificweed control issues. When you go
to a store to make your purchase,
you likely will encounter a broadoffering of herbicides. The question
now is, where do I start to find the4right herbicide product? The place
to start is comparing the various
product labels. The herbicide
product label contains all theinformation you will need to know
for selecting the best product, andmore importantly, it contains all theinformation you need to know
when it comes to safely handlingand applying the herbicide productyou choose. The most useful
information needed for successful
weed control includes:

1. Ingredients. The ingredientsection lists which activeingredients (i.e., herbicides) andinactive ingredients (i.e., inerts)
are contained in the product andat which percentage
these active andinactive ingredientsare in relationship to
the total volume ofthe product. Many
similar products will
have only slightdifferences in theamount of one ormore activeingredients.
Therefore, knowing
the percentage of theactive ingredient in a
product will help todetermine how muchof a product toapply. For example,
if there are two
products that
contain the sameactive ingredient(s)and the first productcontains half the percentage ofthe active ingredient(s) comparedto the second product, then apply
twice as much of the first productto get similar levels of weed
control provided by the second
product. Checking the ingredients
section will also allow you to be
sure the product you arepurchasing has the appropriate
active ingredient or combinationof active ingredients
recommended for control of yourspecific weed issue.

DO NOT rely on THE
trade name forselectingappropriate
herbicide products.

Often trade names will change or
can be similar for products
containing vastly different activeingredients.

2. Precautionary Statements andRestricted Use. Herbicides aregenerally classified for safety as“Caution” − the least toxic,“Warning” − mildly tomoderately toxic, or “Danger” −very toxic/poison. The label gives
specific instructions for what an
applicator is to do if the productcomes in contact with a person’s
skin or clothing, is inhaled oringested, or if the product gets
into the eyes. Some herbicides areregistered as restricted use. This
classification can be due tospecific potential dangers toapplicators or to the environment
if the product is handled in an
inappropriate manner. When an
herbicide is classified as restricted
use, it must be purchased andapplied by a certified applicator
or a person under his or her
direct supervision. If you are nota certified applicator and would
like to become one, please contact
your local county Extension
office for additional details.

3. Specific Uses. The label gives
you specific information about
where the herbicide can beapplied. For many products, there
is a broad range of differences inallowable use areas. For example,
one product may be registered forweed control in specific situations
such as tall fescue home lawns,

azaleas or bermudagrass hay
fields. However, other products
may be less specific, allowing foruse on all turf types, in all typesof ornamental plant beds or all
types of forages. Always makesure the desired use for theproduct being purchased iscovered on the label. If the use is
not listed, then severe injury or
plant death may occur todesirable plants. In addition, it isalso illegal to use an herbicide ina manner not consistent with its
labeling.

4. Personal Protective
Equipment (PPE). This sectionof the label contains information
about the safety
equipment thatapplicators andhandlers must wear.
If you do not havethe type ofequipment listed,
then make sure topurchase it and use it
appropriately. Typesof safety equipment
may include the useof safety glasses orgoggles, chemicalresistant footwear,gloves, aprons orcoveralls.

application rate and timing ofapplication (postemergence orpreemergence). The application
rate can be represented in several
different ways. Application rates
will often be expressed in pounds,
ounces or grams per thousandsquare feet (1000 ft2) or acre fordry formulations; or quarts, pints
or fluid ounces per thousandsquare feet (1000 ft2) or acre forliquid formulations. Dependingon the product, there may also be
specific recommendations for“spot treatments” of certain weed
problems, whereby an applicatorwill mix a certain amount of theherbicide product in a specified
amount of water and then spray

5. Application
Directions.
This section of thelabel contains
information about
the product5the target weed with this mixture
through a pressurized boom orwand sprayer until the foliage iswet. Many liquid herbicide
products are pre-formulated forpostemergence spot spraying, socontainers bought in the store are
already equipped with a nozzle
and some type of pump for
immediate application without
pre-mixing. Other liquid-based
postemergence products areconcentrated with a higher
percentage of the active
ingredient and should be mixed
with water and broadcast-applied
at a specific rate of material per
area of land with a pressurized
sprayer. In contrast, many
granular types of preemergenceherbicide products arerecommended for use with aspreader-type applicator.
The most important thing toremember is that no matter which
herbicide product is found on theshelf, this section of the label will
specify exactly how best to apply
the product, even if it may not be
obvious by looking at thepackaging. In addition, specificinformation regarding calibrationof the appropriate sprayer/
spreader and whether the productneeds any special additives (suchas a surfactant) may also befound in this section of the label.
Basically, everything you willneed to know about safely andaccurately applying the productcan be found in this section of thelabel.

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6. Weeds Controlled. Thissection has a list of weedsthat the product controls
when applied at theappropriate rate andapplication timing. Someherbicides will need to beapplied before weeds emerge,
while others may have weed
size restrictions when appliedpostemergence to certain
weeds. Usually, specificinformation about the rates
and timings for control ofspecific weeds will be foundin this section.

7. Use Precaution
Statements.
This area of the label has
information about specificthings to do or not to do to limit
potential risks. Information
regarding irrigation, herbicide
movement, sensitive plant
precautions and other usefulhints are contained in this sectionof the label.

Final Thoughts
Always remember that the label isthe law. Pesticide use is governed bystate and federal regulations, so be
sure to read the entire label andfollow all label directions. If youhave questions or concerns, contact
your local Extension agent for help.
With proper handling and use,
herbicides are safe and effective
tools for rapidly controlling manyweeds.

7Visit the UT Extension Web site at
http://www.utextension.utk.edu/SP707-5M-6/08

E12-5115-00-013-08 08-0235

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