Weed Control With Chemical Herbicides

How Do Herbicides Work?

Herbicides can act upon a weed in two different ways.

• Pre-emergent herbicides act upon weed seeds, seedlings or form a barrier in the soil to prevent weed seed germination or establishment. Pre-emergents are usually used in the spring to prevent crabgrass seeds from establishing when the soil temperatures begin to warm up. A properly timed application can provide control for several months. Products like Dimension (ai-dithiopyr) and Pre-M (ai-pendimethalin) work at suppressing all seeds in the soil, so any re-seeding projects are out of the question for a few weeks. Tupersan (ai-siduron) is notable as the only pre-emergent herbicide with the ability to suppress crabgrass and be used in conjunction with grass seed.
• Post-emergent herbicides work on actively growing weeds and can be further broken down into selective or non-selective categories.
• Selective herbicides can be applied to a lawn and target weeds while having little effect on grass. Products containing the active ingredient 2,4-D, dicamba, and MCPP are commonly used to control a wide variety of broadleaf weeds. Grassy weeds and grass-like weeds can be harder to selectively target with herbicides. Products containing the fenoxaprop ethyl, dithiopyr, or quinclorac can be used for post-emergent crabgrass control. Some products may require repeated applications for effective control. Nutsedge is a particularly stubborn weed to eradicate in the lawn, the relatively new product Sedge Hammer (ai- halosulfuron), formerly known as Manage, is the only herbicide to successfully target both yellow and purple nutsedge.

• Non-selective herbicides kill everything they come in contact with, including the lawn. Extra care must always be taken when handling non-selective herbicides to prevent spillage or accidents. The most popular non-selective herbicide is probably Round-Up (ai-glyphosate). Its new formulation Quick Pro adds the active ingredient diquat dibromide for fast-acting results within 24 hours. Non-selective herbicides tend to be used very sparingly in a lawn unless applied meticulously to individual weeds or wiping out an entire lawn due to be renovated.

How Are They Applied?

Herbicides are commonly applied in granular and liquid form. Granular herbicides are easily applied with a common push-type spreader and can come blended with a fertilizer. The downside of granular applications is that they must be broadcast over a large area, applying chemical in places that may not need it. Pre-emergent crabgrass control and "weed and feed" products are common granular forms of herbicide. Liquid herbicides are diluted with water and sprayed onto the plant with a hand-held or backpack sprayer. Spot spraying broadleaf weeds, like dandelions or clover with a selective post-emergent herbicide, is effective and saves the lawn from excessive chemicals