An herbicide is a product used to kill unwanted plants, much the way a pesticide is used to kill pests. Most often, herbicides are used to kill weeds or to clear an area of brush. There are synthetic and organic herbicides. Whatever type you choose, they should all be used sparingly and with great care. Just because a product is organic does not mean it can't be toxic to people, pets, livestock, and "good" plants.
There are several types of herbicides available to homeowners and it pays to know both what type of plants you want to get rid of and what type of herbicide you are using. Some herbicides are very targeted and others will kill any plant with which they come into contact. Since sprays can often drift while being applied, you can do a lot of unintended damage to your yard and garden with an herbicide.
Nonselective or broad-spectrum herbicides are used to kill all vegetation in an area. Broad-spectrum herbicides do not discriminate between garden plants, lawn, and weeds. Some of the more popular broad-spectrum herbicides contain a substance called glyphosate, which has been banned from use in some areas. You shouldn't need a nonselective herbicide very often and if you do, use it with extreme caution.
There are also acid-based broad-spectrum herbicides. These burn the leafy portion of the plant but do not always kill the roots and the plants eventually regrow. The best use for these organic products is on annual weeds and when treating weeds growing near cement, on a hot, sunny day. The heat from the cement intensifies the burning caused by the acid.
Even these acetic products should be used only as the label specifies. Runoff can leach into the ground and water supply. As for using regular household vinegar as an herbicide, the percentage of acid is too low to be effective as a weed killer.
Herbicides can also be selective and used to kill only targeted plants, for example, crabgrass killer. These generally cause some form of hormone disruption in the target plant. Unfortunately, we are seeing plants develop resistance to selective herbicides.
A pre-emergent herbicide works by preventing seed from germinating, and so is more effective in controlling annual weeds. Preen is a popular pre-emergent herbicide for homeowner use. Corn gluten meal is gaining followers as an organic pre-emergent.
Pre-emergent herbicides need to be applied at just the right time. They only work on ungerminated seeds, not existing plants, so they are most effective on annual weeds in the lawn, in garden paths, or in existing beds. Do not apply them where you will be direct sowing because they will also inhibit the germination of the seeds of whatever you are planting. The best time to apply these products for the weeds you are targeting should be explained on the product label.
Safe Use of Herbicides
Synthetic or organic, herbicides are all strong substances that should be used sparingly, with care and caution. If the problem is small enough to be eradicated with a little hand weeding, that should be your first course of action. If you absolutely must use an herbicide, for example, to get rid of an overgrowth of poison ivy, start with the least toxic option first. Remember, more is not better. Always follow the label instructions.
Keep yourself protected. Use rubber gloves when pouring. Goggles are a good idea to protect your eyes when spraying. And if you get any herbicide on your skin or clothes, clean immediately. Take care when storing and disposing of herbicides, just as you would with any garden chemical. Make certain the bottles are clearly labeled and out of reach of children and pets. It is smart to have a designated sprayer for herbicides so that you don't reuse the sprayer with something else, like a fungicide, and spray residual herbicide on your plants by accident.
Understanding Herbicides. University of Illinois Extension