What Are Pre-Emergent Herbicides?

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  • Written By: Mary McMahon
  • Edited By: Nancy Fann-Im
  • Last Modified Date: 20 September 2019
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Pre-emergent herbicides are gardening chemicals that act by preventing germination. They are most effective against annual weeds, such as crabgrass, and come in a number of formulations. Gardening and home supply stores typically carry pre-emergent herbicides along with supplies for applying them, and can order them by request from customers if they are not commonly held in stock. As with any chemicals used in gardening for weed and pest control, it is important to follow directions carefully.

These herbicides contain chemicals that block enzymes that seeds count on for germination. It is necessary to use them before seeds start sprouting. Once unwanted weeds have begun to appear, it is too late for pre-emergent herbicides, and gardeners will need to explore other options for weed control. The chemicals also need to be applied each year to prevent emergence, as seeds can drift from other locations even if no crop of weeds had an opportunity to set seeds in the garden the year before.

Timing the application of pre-emergent herbicides can be tricky. Gardeners can take note of when weeds tend to appear, and count back approximately three weeks to determine when they should treat their lawns and gardens. It is also important to monitor weather conditions, as wet, warm weather can facilitate early germination, and may allow the weeds to get a head start before the gardener has an opportunity to add herbicides.


The packaging should contain information about the types of seeds the herbicides work against, and how they should be used. Some may need to be diluted before application, while others should be applied straight to the garden. A sprayer is usually the tool of choice, and it is advisable to wear protective garments and a face mask to limit contact with pre-emergent herbicides. If the chemicals get on the skin or eyes, they should be flushed with warm water.

These chemicals can work against wanted seeds, making it important to apply them carefully. They are most classically used on lawns, where established turf can keep growing after the application of pre-emergent herbicides. The chemicals can prevent the unsightly appearance of intrusive weeds that can ruin the look and feel of the lawn, as well as choke out the turf. As turf matures and develops a thick, strong network of roots, it can be more difficult for weeds to get a foothold, and the application of lawn treatments may become less necessary.



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