Weed Control Explained

Mode of action refers to how exactly a product effects a plant. The categories here are broad. There is a lot more that goes into selecting herbicides in a resistance management program. At The Green Advantage, we take resistance management very seriously. Incorrectly using products can have devastating results in the long term on this industry, resulting in the production of new modes of action, often using products that have an unknown long term effect on the environment and health of people and animals. It is best that we all be responsible in the usage of herbicides, as the negative effects of being irresponsible are far reaching.  

Fertilization programs often contain weed control. For the most part, two types of weed control are utilized: Pre-emergent and Post-emergent. The names indicate when the application is applied, before the weeds emerge or after. The last type of weed control, Ground Sterilizers, are in a category of their own, and are used for long term vegetation control usually on industrial sites, parking lots, and other areas where no vegetation is desired.

Pre-emergent weed control is used to prevent weeds in lawns and landscaping. This is accomplished by preventing seeds from germinating. The main target for pre-emergent weed control are annual weeds. Annual weeds live for one year, so they depend on reproducing by seeds. The seeds from perennial and bi-annual weeds are also prevented from germinating, although pre-emergent applications will not prevent weeds that have already germinated.  

The application is made when ground temperature reaches about 50 degrees Fahrenheit. Timing is critical: If applications are made too early, much of the active ingredients will degrade or leach out of the treated area before the seeds are ready to germinate. If made too late, seeds will already have germinated, rendering the application useless. 

Another popular use of pre-emergent weed control is in landscape beds, as it does not affect the plants that are currently growing.    

This is not effective on weeds that have already emerged or have root systems that are living. This tends to be the biggest misunderstanding with this type of weed control. As the treated areas are observed, many weeds have not gotten out of their dormant state and are not visible. This application is usually made in the early spring, but as the temperatures rise certain weeds become visible; often dandelions. The weeds that seem to emerge (technically they are not emerging, instead they are leaving their dormant state and beginning their seasonal growth) after the application are often perennials that have over wintered, so their root systems are still intact.  

The second general type of weed control is post-emergent. It is applied after the weeds have emerged, and is most effective when the weeds are actively growing. There are two categories of post-emergent formulations: selective and non-selective. Selective herbicides target certain weeds or groups of weeds. Non-selective herbicides effect any vegetation. The most common in turf is a selective broadleaf weed control, as grass is nearly unaffected by the application (occasionally a slight browning of the lawn may result, but this is usually temporary if the application is made correctly).  

Selective and non-selective herbicides are available in systemic and contact formulations. Systemic weed control effects the internal workings of the plants, resulting in elimination. The results from systemic weed control are often seen over a short period of time, often 3 days to 2 weeks. Contact weed control externally effects the plants, the results are very rapid often within hours. The problem is the effective ingredients can be washed away fairly easily and the application made ineffective. Re-application of contact weed control is not uncommon when desired results are not obtained fairly quickly. It is also not uncommon for the weed root systems to survive the leaf kill and recover from the damage, requiring multiple applications.  

We often use a combination of Systemic and Contact weed control. This gives a fast but total kill; as the contact herbicide effects the top of the plant, the systemic herbicide goes to work on the root system.

This is a non-selective herbicide which renders the soil unable to support plant life for a period of time. Depending on the product used and the application made, this period can be up to two years. Often this type of herbicide is restricted to industrial sites, parking lots, railroad areas, and utility areas.  

As you can see there is a lot more to weed control than most would expect. Using the right equipment and knowing the products is critical to safe and effective weed control. Proper training, research, and experience are necessary to perform these services. Advice from professionals should be followed, as they work with these materials often enough to know the timing and methods to use in order to achieve the best results. 

 Feel free to contact me if you have any other questions.

Jon Ipema

The Green Advantage

219.779.9815