Pesticides need to be used for the correct pest, in the correct dosage and at the correct time. Applying the wrong pesticide to the wrong pest at the wrong time is both ineffective and environmentally unsound. A recent national survey indicated that 98% of all applied by homeowners did not reach the intended pest.

To make the proper use of pesticides, the user must understand certain terminology. This is the first in a series of articles to address this issue.

The term "pesticide" covers all chemicals and other materials meant to kill a living organism. insecticides kill insects. Fungicides kill fungi. Herbicides kill plants. Although there are many other types of pesticides, these three are most commonly used in horticulture.

Herbicides are commonly referred to as "weed killers" since this is their intended use. However, in the strictest sense of the word, these are compounds intended to kill non-woody plants. The term "weed" is a human designation for a certain plant in a certain location. The chemical cannot tell the difference. It is up to the user to apply the proper herbicide for the job.

The following terms are often used when dealing with herbicides: 

- Pre-emergence. This type of herbicide acts only on plants as they come up from seed. Some herbicides soak into the ground and kill the seed while others provide a barrier that kills the tender seedling as it emerges. Pre-emergence herbicides have no effect on plants that are already up and growing. The annual weed, crabgrass, is often controlled with a pre-emergence herbicide.

- Post-emergence. These herbicides kill actively growing plants.

- Selective herbicide. Some herbicides kill only broadleaf plants while others kill only grasses. The common chemical 2,4-D will control broadleaf weeds but will not affect the grass in the lawn.

- Non-selective herbicide. This type of herbicide kills anything green that it contacts. Roundup is an example.

Perhaps the two most commonly used herbicides in the home landscape are 2,4-D and glyphosate.

2,4-D - For decades, this has been the herbicide used for lawn weeds across the world. It is a selective chemical in that it only kills broadleaf plants (dicotyledons) and does not kill grass (monocotyledons). Also, it is a post-emergent since it only kills plants that are up and growing and does not control seeds in the ground. It works as a growth stimulant and actually causes the plant to "grow to death" which is why plants treated with it show the symptom of epinasty that causes their stems to twist and turn.

Glyphosate - This is the chemical found in products such as RoundUp, KleenUp and many other herbicides used in the home landscape. If the label says glyphosate, it is a systemic herbicide. This means that the chemical enters the vascular system of the plant and moves throughout it. It is a non-selective herbicide in that it will kill any type of plant including both dicots and monocots. Finally, it too is a post-emergent herbicide which must be applied to green, actively growing plants to be effective. It works by shutting down the photosynthesis process.

Note: These two herbicides are shown as examples only in order to help explain terminology. As always, herbicides should be the final option in your integrated pest management strategy and should be used according to label instruction only.