Pre-Emergent Herbicides, How Will You Lose Yours?

By: Jacob Hueste

When imagining a perfect lawn, landscape or garden, weeds are not usually in the picture. To accomplish this feat we need help from post and pre-emergent herbicides. Post-emergent herbicides are used to control the weeds that are already present in the landscape whereas pre-emergent herbicides are used to control weed seeds that are in the soil yet to germinate. However, after applying pre-emergent herbicides (and some post-emergent), it is impossible to reseed and sometimes replant until the herbicide has left the soil. There are many ways that a pre-emergent herbicide can be broken down in the environment. Learning more about your herbicides and which modes of degradation are common will help you better gauge the longevity of your pre-emergent herbicide in the landscape.

 There are several factors in the environment that degrade herbicides. They are as follows:
  • Plant Uptake- Occurs when the plant’s roots imbibe the chemical from the surrounding soil particles and store them in the plant tissue.
  • Runoff and/or erosion- Occurs when there is an excess of water collected at the soil surface which runs off taking the herbicide or the soil particles containing the herbicide away from the application site.
  • Microbial activity- Occurs when certain microbes use the chemical as a food source. Microbial activity is optimal at soil temperatures between 50 and 104ºF, and more so in a moist environment. Therefore during hot, dry summers and cold winters, you can expect little to no microbial degradation.
  • Volatization- The definition of volatization is the physical change from a liquid or solid state to a gaseous state. Vapor pressure of the herbicides determines the volatility. This occurs more readily at the soil surface at high temperatures. Some herbicides need to be incorporated into the soil to prevent this. Read your label to see best application practices.
  • Leaching- Occurs when water moves through the soil dissolving the herbicide and moving it down the soil profile away from the root and germination zone. This occurs more easily with herbicides that are highly water soluble.
  • Photo degradation- Occurs when sunlight comes into contact with the herbicide. The energy adsorbed by the herbicide changes the chemical makeup thus rendering the herbicide inert or ineffective.
  • Soil adsorption- Occurs when the herbicide adheres to the soil particles where it is unavailable to the surrounding area. This happens more readily in soils high in organic material and/ or clay particles. Therefore an herbicide applied to a sandy soil would be less likely to be adsorbed.
 Herbicides consist of active ingredients and inactive ingredients. To know how persistent the herbicide is in the soil it is best to know the active ingredient in the herbicide you are about to apply. Knowing the active ingredient will show you which group, or family, the herbicide can be classified under. This information will show you what factors of herbicide degradation will affect the chemical you are using. The table below is a snapshot of such information.
* Be sure to check the product label to see if the chemical is labeled for use in the area you are to apply.
To find the active ingredient of your herbicide you will need to refer to the chemical label that is attached to the container. You will also find application rates as well as the lifespan of the chemical after it is applied to the soil. The lifespan is a guideline, and can differ depending on how the chemical is degraded and the climate and soil conditions of your location. For example a person applying Marengo in a dry growing season could have an extended length of pre-emergent control as opposed to the same person applying Marengo in a wet growing season due to the fact that the main degradation factors are leaching and runoff.
Knowing your herbicide as well as the history of the weather and soil conditions in your area will help you judge how long the herbicide will stay active in the soil. This helps determine when to reseed or plant and/or when to re-apply herbicides without damaging your lawn, landscape, or garden. Awareness of this will aid in creating a healthy looking yard and becoming the envy of your neighbors.