Pre-emergent Herbicide Application Guidelines

From BASF: The Chemical Company Website

Weed management involves numerous strategies (such as exclusion, hand weeding and using herbicides) and should continue throughout the year and not be considered a once a year event. Controlling weeds within and around the production facility will greatly reduce the introduction of weeds seeds, insects, diseases, and viruses into the crops and the costs associated with controlling these problems.

Pre-emergent herbicides are effective tools at preventing weeds from becoming established when they are applied properly. These products form a chemical barrier (up to 1-inch thick) over the surface of the growing medium after they are applied (when applied properly). As weed seeds germinate and grow within the chemical barrier, their growth is either inhibited or the seedlings are killed. With the chemical barrier containing the herbicides being bound to the top inch of the growing medium, the roots of established plants (such as perennials, shrubs and trees) are largely unaffected.

Herbicides are either applied with either granular or spray formulations. Both application methods can be highly effective at controlling weeds provided the applicator achieves good coverage; application uniformity is paramount to obtaining good weed control. Granular formulations are generally safer to apply to containerized plants than spray formulations.  The degree of safety is achieved because the granules tend to fall to the media surface without excessive exposure to the plants leaves. An exception to this is when granular herbicides are applied to crops with wet foliage; in these situations, the granules stick to the leaf surfaces, often resulting in significant injury to the tender leaves from direct contact of the foliage with the concentrated active ingredient. Granules should only be applied when the foliage is dry, and then irrigated after application to remove the granules from the foliage.

As eluded to above, spray formulations are more prone to cause injury to existing crops than are granular applications. In most instances, growers should avoid applying herbicide spray formulations directly over actively growing crops, unless the herbicide’s label specifies otherwise. If pre-emergent herbicides are applied to the foliage of actively growing plants, it is recommended to wet the leaves prior to spraying the herbicide and to rinse off the foliage immediately following the application to reduce the risk of causing injury to the crop. 

Pre-emergent Use Guidelines 

  1. There are currently NO pre-emergent herbicides that are labeled for application inside enclosed structures. Pre-emergent herbicides tend to volatilize; if they are applied in an enclosed structure, the risk of injuring plants is greater through the direct exposure to the herbicide vapors or from herbicide condensation on the foliage. Pre-emergent herbicides can safely be applied to crops several weeks prior to moving them into an enclosed environment.
  2. In most cases, pre-emergent herbicides do NOT control established weeds. Therefore, it is necessary to remove any existing weeds before applying the herbicide. If weeds are pulled after a pre-emergent application has been made, the chemical barrier will be disrupted leading to reduced effectiveness or longevity of the herbicide application.
  3. Uniform coverage with the appropriate application rate is critical for both controlling weeds and for crop safety. Providing poor coverage and/or using lower than recommended rates will greatly reduce the efficacy (weed control) of the herbicide and decrease the length of time weeds are being controlled. Conversely, over-applying herbicides could potentially injure crops, adversely affect crop appearance and development, and may lead to the development of herbicide tolerance of various weed species.
  4. After herbicides are applied, it is important to incorporate or move them into the top layer of soil with irrigation. The normal recommendation is to apply approximately ½ inch of water shortly after the herbicide application. Watering in pre-emergent herbicides is important to reduce degradation and volatilization and to increase the overall efficacy of the application. Omitting this step will decrease the effectiveness and longevity of the application and may lead to injury to the crops being produced.
  5. Pre-emergent herbicides, although very effective at controlling germinating weed seeds, may cause severe injury to many plant species. It is important to match the herbicides up with the plants that can tolerate them. Herbicides that are perfectly safe on some crops are extremely toxic to others. Each herbicide label will provide a listing of the plants to which the herbicide has been applied to safely in experimental tests. These listings are not complete listings of all the plant species the herbicide might be applied to. When considering the application of herbicides to plants that are not on the label, test plant tolerances to herbicides on a small scale before applying them to an entire block of plants. Growers should follow each herbicide’s labeled recommendations, conduct in-house plant safety screens, and keep good records to determine which crops will be tolerant of herbicides applications. 

Most pre-emergent herbicides will provide several months (2-3 months generally) of control before the level of control subsides. The primary factors determining the length a pre-emergent herbicide will be effective are: the chemical used, the rate applied, the uniformity of application, temperature, rainfall/irrigation practices, and herbicide degradation. The guidelines specified above are very important and necessary to improve the performance of pre-emergent herbicides. Herbicides are an effective tool that can greatly reduce weed pressure, decrease the cost of your current weed control strategies, and will ultimately lead to less shrink and more salable product.