Weed Killers

Eliminating pesky weeds from your lawn or garden is a never ending struggle, but there are products available that make it a lot easier to get rid of dandelions, crabgrass and other unwanted vegetation. Weed killers, also called herbicides, are chemicals that kill plants or inhibit their normal growth. The best product for you depends upon what types of weeds you are trying to eradicate. Herbicides vary according to their effects. Some affect germinating seeds, while others kill weeds after the plants are up and growing. Before you buy an herbicide, consider the following:

• What types of weeds are you trying to kill?
• Should you protect the plants around the weeds you are trying to eliminate?
• After you apply an herbicide, will you be planting or seeding in that area?
• Would a contact or translocated herbicide work best to kill weeds in your lawn?
• Do you prefer applying herbicides in liquid or granular form?

Types, Characteristics and Application

There are four basic characteristics that describe how all herbicides work -- emergence, selectivity, persistence and contact or translocation. Understanding these concepts will help you decide the type of herbicide that you need. Because these products are highly effective, it is very important that you understand how to apply them correctly so you do not damage other plants in the same area. Product labels are full of information covering all aspects of use, handling and storage, so take time to thoroughly read the label before using an herbicide.

Emergence: Preemergence herbicides target germinating seeds and seedlings before they grow. To be effective, apply this type of herbicide two to three weeks before the weed seeds germinate. For this reason, preemergence herbicides work well to combat annual weeds, or weeds that live for only one growing season, reproducing only from seed. Postemergence herbicides work on plants that have already started growing and are visible. An herbicide with both preemergence and postemergence materials controls existing vegetation and prevents the emergence of new seedlings.

• Common ingredients in preemergence herbicides are oryzalin or trifluralin
• Apply preemergence herbicides to soil cleared of visible weeds
• Water preemergence herbicides into the soil for the best effect
• Apply preemergence herbicides just before weed germination begins
• The best way to apply postemergence herbicides is to spray it on the leaves

Selectivity: Herbicides are either selective or nonselective. Selective herbicides kill weeds without damaging nearby plants and are best used to treat plants that are grown in crops, lawns and ornamental plantings. Nonselective herbicides generally kill all plants in the vicinity of application. They work best in areas where no plant growth is desired, such as fence rows, irrigation and drainage ditches and greenhouse floors. Selectivity can vary according to the rate of application, the time and method of application, environmental conditions and the stage of plant growth.

• Use a selective herbicide to kill dandelions without damaging the lawn
• Use a nonselective herbicide to kill unwanted vegetation growing in sidewalk cracks
• When using a nonselective herbicide, you must protect other desirable plants in the area
• Glyphosate is a common non-selective herbicide
• Increasing application rates of selective herbicides may kill more than the targeted weeds

Persistence: The term persistence describes how herbicides vary in the length of time they remain active after application. Nonselective herbicides that kill everything growing on a site and prevent regrowth are persistent. Those that provide no lasting prevention of weed regrowth are nonpersistent. Nonpersistent herbicides break down quickly after application. If using a persistent herbicide, consider how long the herbicide will prevent regrowth. For example, if you use a persistent herbicide in your lawn, make sure the product will not interfere with your reseeding or planting plans.

• Persistent herbicides provide long-term weed control
• By retaining their chemical structure, persistent types stay in the soil after application
• Sensitive plants placed where a persistent herbicide has been applied may not thrive
• Microorganisms and sunlight easily break down nonpersistent herbicides
• Nonpersistent herbicides perform their work quickly after application and then become inactive

Contact/Translocated: Some herbicides kill plants on contact when applied while others operate by translocation or by working their way throughout the plant's system. Contact herbicides only kill the part of a plant where the herbicide is applied. This type is best used to kill annual and biennial weeds and usually causes plants to die quickly. Translocated herbicides are absorbed through a plant's leaves or roots. This type works well against perennial weeds because the chemical attacks the whole plant, including the root system.

• Contact herbicides are ideal for clearing ground prior to sowing or planting
• Contact types don't kill perennials but repeated application may exhaust reserves
• Common contact herbicides include paraquat, diquat, glufosinate and sodium chlorate
• Translocated herbicides may take two to three weeks to work, or longer for woody perennials
• Common translocated herbicides include glyphosate and amitrole

Application: Each type of weed killer comes with its own unique guidelines for application, so it is important to read the manufacturer's instructions thoroughly, both for safety reasons and to ensure the product works as expected. Use compressed air sprayers or sprayers attached to a garden hose for liquid application. Note, however, that the higher the pressure and the finer the mist, the greater the chance of injuring surrounding vegetation. Use sprinkler cans on small areas, but make sure not to use the same applicators when spraying ornamental or garden plant herbicides. Always use a separate sprayer for killing weeds and clean completely after use.

• Apply herbicides to perennial weeds late in the growing season (early fall) for maximum effectiveness
• Don't mow for two days before and two days after applying an herbicide to your lawn
• Use only the recommended concentration for best results and safe use

The herbicide you select will more than likely embody a number of characteristics. For example, a selective, postemergent solution is ideal for removing broadleaf weeds from your lawn without damaging desirable grasses. Use the following chart as a simple reference when trying to determine which characteristics best meet your needs.

Characteristic	Target	Application and Considerations
Contact	External Annual weeds	Effects only the areas touched by the herbicide Causes plants to die quickly
Nonpersistent	Elimination of weeds without lingering effects	Works quickly and then gets broken down by microorganisms or sunlight Won't interfere with plans to replant or reseed areas
Nonselective	All weeds, grasses and actively growing plants in an area	Use to control weeds in areas where no vegetation is desired Ideal for sidewalks, driveways, patios Can be used to prepare your lawn for replanting
Persistent	Regrowth of weeds	Provides continuous protection against weeds Length of prevention time varies from product to product
Postemergent	Visible weeds Mature plants	Spray directly onto the weed's leaves Apply in early fall to control perennial and winter annuals
Preemergent	Germinating seeds Seedlings	Spray into the soil two to three weeks prior to germination Apply in spring to control growth of annual weeds
Selective	Specific weed type or types	Kills weeds without harming surrounding vegetation Ideal for treating lawns, crops or ornamental gardens
Translocated	Internal Perennial weeds	Absorbs through the roots or leaves Penetrates the system including the root structure

Features

Liquid: Liquid herbicides are applied in small amounts using a hand sprayer. Sometimes they are mixed with water before application. Contact with these chemicals can injure your skin, eyes or lungs so follow necessary safety precautions.

Granular: Granular weed killers are solid and can be applied using drop or centrifrugal spreaders. Contact with the grains can injure your skin, eyes or lungs so follow necessary safety precautions.

Soap Salts: Formulated from potassium and ammonium salts of fatty acids, soap salts help kill moss and algae in mulched landscape beds, concrete pavers and patio surfaces. They have also been packaged as a nonselective weed control product. Soap salts are contact herbicides, meaning they only affect the tissue that comes in contact with the herbicide.

Organic: Some nonselective herbicides have been formulated from organic oils and acids, such as citric acid, acetic acid, garlic oil, clove oil and sodium lauryl sulfate. These types are generally not translocated but instead are applied on contact.