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Herbicides are chemicals which are used to kill weeds. A weed is defined as any plant growing where it is unwanted. This might include a petunia in the squash bed or a squash vine in the petunias. To plot a strategy for dealing with weeds, you should first know your enemy. There are basically three types of weeds with which we must cope—grasses, broadleaved weeds and sedges.

These classifications are important in that they not only help with selection of herbicides but help determine the best time of application. Below are important examples of these weeds.

Types and Examples of Weeds

Type of weed

Pattern of growth

Season of growth

Examples

Grasses

Annual

Summer

Crabgrass, yellow foxtail, goosegrass, sandbur, barnyard grass, witchgrass

 

 

Winter

Annual bluegrass(Poa annua), smooth brome

 

Perennial

Summer

Dallisgrass, orchardgrass, quackgrass, timothy grass

Broadleaf weeds

Annual

Summer

Black medic, purslane, carpetweed, spotted sprurge, prostate knotweed

 

 

Winter

Henbit, cranesbill, shepherd’s purse, mouseear chickweed, common chickweed

 

Perennial

Winter

Dandelion, wild carrot, white clover

 

 

Summer

Curly dock, broadleaf plantain, common yarrow

Sedges

Perennial

 

Purple nutsedge, yellow nutsedge, kyllinga

 

There are three methods of minimizing problems with weeds. An effective strategy may include one or all of these—cultural, mechanical and herbicides.

Turfgrass specialist conclude that at least 70% of lawn weeds may be prevented by effective cultural techniques. Translated, this means a healthy thick lawn will prevent weeds from invasion. Any bare spot in a lawn usually has a cause and if not solved will be an ideal area for a weed seed blown its way.        

Depending on your abilities and interests, small numbers of weeds in turfgrass and garden beds may be pulled by hand. Why buy a chemical, mix a sprayer full, don the proper cloths and spray a small yard with a few dandelions and some clumps of crabgrass? Pull or dig them out. It doesn’t take long, and it lets you release a little anger at them for invading your turf.

In flower or vegetable gardens, mulch is king for controlling weeds. It is effective, conserves water and modulates soil temperature and is much safer for the surrounding plants than any of the herbicides. Mulching is probably cheaper in the long run than chemicals.

Preemergent Herbicides

When used regularly and at the proper time, this group of herbicides is the most effective tool to minimize weed problems, other than a healthy and well tended lawn.  These are used by homeowners mid February through mid March to prevent crabgrass and other summer weeds. Used mid September through mid November, they prevent winter weeds such as henbit and annual bluegrass (Poa annua). Examples of preemergents available are listed below. It should be emphasized that there are many other preemergent chemicals not listed here which are available both to homeowners and commercial applicators and may have different coverage and duration of effects.

Preemergent Herbicides and Commonly Available Brands

Chemical name

Example Brand name

Comments

Dithiopyr*

Green light crabgrass preventer
Preen Stepsaver Weed Control
Ace Green Turf Crabgrass Preventer
Green Light Crabgrass Preventer
Sta-Green Crab-Ex with Dimension

Excellent crabgrass and other weedy grass control. Good broad-leaf weed prevention. Safe for all local turf grasses. No reseeding for 2-6 months according to individual label.

Pendimethilin*

Halts Crabgrass preventer
Scotts Crabgrass Preventer
Scotts Super Turf Builder with Halts

Same as above

Prodiamine*

Barricade
Pennington Crabgrass Preventer
Ferti-lome Lawn Food and Weed Preventer

Same as above

* Best choice for control of both crabgrass and broadleaved weeds. Some may require a second application which will be outlined on the product label. These products must be activated by irrigating its granular carrier with 1/2 inch of water. 

Postemergent Herbicide

The following table lists the main postemergent chemicals in herbicides available to the homeowner. These chemicals are sold in many different combinations and under many different brands. The table lists the generic chemical in the herbicide.

They are most often supplied as liquids for spray applications, although some are attached to fertilizer or inert granules.

Generally, they all are most effective when applied when the weed is young, small and actively growing, this tends to be spring and fall. Some postemergents, such as 2,4-D,  are variants of plant growth hormones and have much less effect once the weed has matured.  

Postemergent Herbicides for the homeowner

Chemical Herbicide

Weed target

Comments

2,4-D

Many broadleafed weeds

Some forms volatile, may drift long distances and damage desirable plants. Avoid windy days, best if used in combination, immobile once in soil

Mecoprop (MCPP)

Broad-leaf weeds

Mostly immobile in soil

Dicamba

Broad-leaf weeds

Mobile in soil, can enter trees and ornamental shrub roots in lawn causing damage. Taxus and junipers especially sensitive

Triclopyr

Woody and broadleaf weeds, labeled for poison ivy

Do not use on warm season grasses, some forms are volatile and may drift

Fluazifop

Annual and perennial grasses

May be used safely in labeled ornamentals, only effects grasses. Especially effective to remove Bermuda from gardens.

 Quinclorac

Some broadleaves, moderate effect on crabgrass

Used in some herbicide combinations for broadleaves and crabgrass, see below

Imazaquin
(Image)

Some broadleaves, nutsedge, Kyllinga, wild garlic

Cannot use on cool season grassed, will temporarily yellow bermuda

Halosurfuron
(Sedgehammer)

Most nutsedges, Kyllinga, wild garlic

Safe for all turf grasses

Glyphosate

Most all actively growing plants

Systemic activity, immediately inactivated with any soil contact.

 

The most commonly used postemergent broad-leaf herbicide is a combination of 2,4-D, Mecoprop and Dicamba,  found in sprays such as Weed-B-Gone, Triamec and many others.
The chemical 2,4-D is volatile, easily vaporizing and causing undesirable damage to other plants.

Triclopyr is found in most of the products labeled for brush. It is useful for elimination of  poison ivy. It may be sold as a lone herbicide or mixed with another, such as glyphosate, in one of the Roundup products.

Quinclorac, currently the most used crabgrass killerm but not as effective as the herbicide MSMA which was removed from the market. Quinclorac is very effective, however, for some of the tough broadleaves such as white clover, dandelion and dichondra.

 Glyphosate, found in Roundup and several others is legendary for its effectiveness in killing most all things green. One of its more unique applications is for killing green weeds in dormant (brown) Bermuda grass in January and February. Zoysia grass, which also goes dormant in the winter cannot be sprayed with glyphosate at any time.

It should be obvious that weed control and especially the use of herbicides is very complex. To negotiate this complexity, always try first to maintain a healthy lawn and if you use herbicides always read and follow the label. Never use a chemical on a turf grass or for a particular weed unless the labeling clearly permits it.