What Are Pre-Emergent Herbicides: Tips On Using Pre-Emergents

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By Bonnie L. Grant

Even the most vigilant gardener will have a weed or two in the lawn. Herbicides are useful in the battle against annual, perennial and biennial weeds, but you have to know when to use them and which ones are most effective against a particular weed problem.

Pre-emergence weed killers are used on established lawns as part of an annual effort to combat plant pests. What are pre-emergent herbicides? These chemical compositions are used before weeds take hold to kill off infant root systems and keep them from growing. Learn how pre-emergent herbicides work so you can decide if they are the right method for you.

What are Pre-Emergent Herbicides?

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Pre-emergence weed killers are used before you see the weeds to prevent them from showing up in the garden or lawn. This doesn’t mean the chemicals interfere with germination but rather they stop the formation of new root cells in baby weed plants.

Without weeds, the seedlings cannot continue to feed and grow and they just die back. This whole process happens at the soil level under the blades and thatch of the grass so you don’t ever have to see the sprouted weeds. Timing, weather and the type of weeds that are problematic in the garden will dictate the exact formula and application for using pre-emergents.

How Pre-Emergents Work

The chemicals in pre-emergent weed killers are not effective on vegetative buds that sprout from existing roots or rhizomes. They also cannot be used on a prepared grass seed bed because their root stunting action in young plants will also affect sprouting grass.

Established plants have nothing to fear, as their root system is already developed and the plant is hearty and healthy. Pre-emergent info indicates that it is the sensitive root tissue of newly germinated seedlings which is killed off, resulting in complete plant death.

Perennial weeds develop thick persistent adult roots that re-sprout in spring, which makes them difficult to control with a pre-emergent formula. Annual weeds are in two classes: winter and summer annuals. The timing of a pre-emergence weed killer for each must match the germination period for the variety of weed. Biennial weeds, like dandelions, are not controlled by a pre-emergent because they produce seed that germinates nearly year around.

Pre-Emergent Info for Applications

As with most plant chemicals, the weather and type of weeds will affect the application method. When using pre-emergents for winter annuals, apply in fall because that is when the seeds germinate. Summer annuals germinate in spring and that is the correct time to apply a pre-emergent. If you are unsure what type of weed is the most troublesome, it is a safe bet that a springtime application will control the majority of the pests.

Pre-emergent weed killers require water to activate them and carry the chemical down to the root systems of newly sprouted weeds. Never apply an herbicide spray when there is a wind to prevent injury to other plants. The ambient temperature must be above freezing and the soil should be workable. Consult the manufacturers label for the varieties of weeds the product is effective against and the method and timing of application.

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