Preemergent herbicide/pre-emergent is one of two broad categories of herbicide weed control. The other category of herbicides is Post-emergent.
See related articles below for more information on this along with an outline to help view the big picture of understanding herbicides.
A) Pre-emergent Herbicide
1) Killing Weed Seeds at Germination
Pre-emergent herbicide kills a weed before any sprouting occurs or before the weed surfaces.
The chemical is applied when the weed seed has germinated. It's purpose is to prevent the newly germinated seed to be killed before photosynthesis occurs.
This method is rarely effective on dormant weed seeds, but a few products are marketed to do so. Overall these products are geared toward killing the newly germinated seed.
These herbicide products are best applied a week or two before growing season begins in spring. A good range to keep in mind for most areas with winter is April up to about May 20th.
In most areas that would be late winter to early spring just before new growth appears. Make sure you are past any more post-winter frosts.
Warmer climates are more likely to apply preemergent chemicals in the fall to control the weeds of winter.
Once applied, those areas should not be disturbed for a few weeks. The time it takes for the seeds to die depends on many different factors. These factors include soil, temperatures, time of year and the specific chemicals used.
This is a great video to help you understand:
Optimal timing in your seasonal area for a preemergent herbicide with the help of a Soil Thermometer
Understanding which products would be best for your needs
If you prefer an effective organic weed killer
Really helpful if you live in solid winter climates
Pre-emergent herbicides work well on annual weeds. They can work on some species of perennial weeds that grow solely from seeds. They are far less effective against weeds that spread through Rhizomes or Stolons...
Rhizomes are horizontal stalks and creeping root stalks (root-stocks) that separate into pieces where it can be easily reproduced for propagation. This is also known as Vegetative Reproduction
Graphic of Rhizome weed
Stolons, often termed runners, are a slender stem-branch that grows away from main plant running along or below the ground. They produce buds at the tips and can root.
Graphic of Stolon weed
I emphasize that a pre-emergent herbicide doesn't respond, or kill, as quickly as a post-emergent herbacide. They act slowly, even taking weeks to prohibit any growth. Avoid disturbing area of application for some time.
Ready to get started? Here is a good do it yourself preemergent video. I also like the idea of using mulch to help keep things undisturbed.
Related Articles
Post-emergent - The second category of herbicide types