Pre emergent herbicide/preemergent is one of two broad categories of herbicide weed control. The other category of herbicides is Post emergent.
Preemergent herbicide kills a weed seed when it has germinated before the weed surfaces and 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 March, April and up to about May 20th. With exception, warmer climates are more likely to apply preemergent chemicals in the fall to control the weeds of winter.
Colder winter climates will use preemergents when spring is coming. There are two times that would be the best to get maximum benefits from applications. First apply in March when temperatures begin to rise. If there is still snow on the ground, that's OK. In April when you are cleaning your garden for the new growing season, apply herbicide again. After these two applications that's it for the year.
In most areas that would be late winter to early spring just before new growth appears. If winter lasts longer, simply shift the above by applying preemergent herbicide first in April, then after cleaning garden in May, apply a second time. The goal is to be past any more post-winter frosts.
Once applied, those areas should not be disturbed for a few weeks before planting or tilling. 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:
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.
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.
Outline of Herbicides - View the Big Picture of Herbicides
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