Most people hold an ideal of a lawn as a smooth, unbroken expanse of green, much like a living rug. Others are willing to accept some of the more attractive competitors, tiny daisies and blue flowered Veronica, but few are pleased when dandelions appear. Weed killers are convenient, but remember that your best defense is a thick, vigorous mat of grass plants. A healthy lawn without bare spots rarely welcomes invaders.
Types of Herbicides
Herbicides can be divided into two types--pre-emergent and post-emergent--those that kill seedlings before they can establish themselves and those that are effective against young plants or adult weeds.
Pre-emergent herbicides are mainly used on newly seeded lawns with a lot of bare soil to protect. Siduron and Benefin are most effective against weedy grasses such as crab grass rather than broadleaf weeds. Isoxaben and Pendimethilin kill broadleaf weeds. All need to be used after grass seed is established.
The post-emergent chemicals can also be divided into systemics, those that are absorbed by leaves and transported to roots and other tissues, and nonsystemics, those that kill only the parts of a plant they contact. 2,4-D is the most popular systemic broadleaf weed killer; MCPP and dicamba are effective nonsystemic herbicides.
The first consideration in choosing an herbicide to use on your lawn is, of course, the potential damage to the lawn itself. Many herbicides kill everything they touch, though there are usually some plants that are more susceptible or resistant than others. Herbicides used in grass are usually chemicals that damage only broadleaf weeds, those only distantly related to grass.
Know the identity of the weeds in your lawn. Some herbicides are better at killing dandelions, for instance, than others, and some weeds require a combination of herbicides for elimination. You can take a piece to a local nursery or county extension clinic or use one of the weed identification sites online. Other considerations include toxicity to humans and the environment, ease of application and cost.
Acetic acid, or vinegar, is an effective weed killer at high concentrations, but kitchen vinegar contains only 5 percent acetic acid. It is best used on very young weed plants as a spray for small areas. At this concentration, damage to the lawn is minimal, but at higher concentrations it will kill anything it touches. I will not damage perennial roots.
Corn gluten meal, another organic product, is effective as a pre-emergent herbicide in new lawns but has no effect on already established plants. It could be used on bare patches to reduce invasion by weeds as you wait for the lawn to thicken.
It's important to remember that post-emergent herbicides are only effective when weeds are actively growing. Spraying in winter is just a waste of the product. Avoid weed and feed mixtures; the best time to fertilize is rarely the best time to apply an herbicide, and most lawns need only spot treatments, not complete coverage.
Any weed killer is a toxic material. Always use gloves and eye protection when applying an herbicide and avoid any kind of skin contact.