Fertilization Weed Control: How do Pre-emergents work?
Introduction
Perennials and annuals germinate under different environmental conditions and at different times of the year. Crabgrass and foxtail are some of the common annual grassy weeds common in turfgrass in the Spring. Poa annua is an example of grassy weeds that germinate in the Fall. Crabgrass germinates first followed by foxtail because of its temperature requirements and the time it requires before germinating. There has to be adequate soil moisture and sunlight for the annuals and perennials to germinate.
How pre-emergents work
Once a seed has germinated, its primary roots emerge. After the application of a pre-emergent, the young root absorbs moisture from the soil. Based on the specific active contained in the pre-emergent, the root or first leaf stops developing, and the plant dies. The process takes place under the canopy of the turf and is not visible to the human eye. Dithiopyr 2L is an example of a pre-emergent that has the capability of providing limited control until after the plant develops 2-3 leaves.
Pre-emergent creates a barrier on the top of the soil, and this covers the seeds preventing it from germinating and growing. In as much as they work on different types of weeds, they are mostly used to reduce crabgrass.
When is it most appropriate to apply pre-emergents?
Pre-emergent is an insoluble chemical and this is important because you want the active to be near the surface of the soil since this is where most weeds germinate. A proper application is made 7-14 days before the weed seed germinates. The period allows the pre-emergent to form a barrier on the surface of the soil. It is good to go earlier and not later because pre-emergents are broken down by microbes. There is no microbe activity when the soil temperatures are low.
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