Pre-Emergence Herbicide Basics

A pre-emergence herbicide is designed to control weeds by interfering with seedling germination and emergence. These products form a (mostly) impenetrable barrier in your soil during critical seasons when weeds are most actively germinating and growing. These critical seasons occur when your turf is transitioning into dormancy or vice versa.  

All pre-emergence herbicides (often also referred to as pre-emergents) are classified by their mode of action (MOA), which refers to the location within the plant where the active ingredient (AI) interferes with its development. Pre-emergence herbicide active ingredients may have different modes of action or manners in which they work. Most of the active ingredients have a mode of action known as a mitosis inhibitor. These AIs inhibit cell division, resulting in seedlings that are stunted, deformed, and unable to emerge as healthy plants.

What Are the Benefits of Using Pre-Emergence Herbicides?

• These herbicides help protect your turf stand going into and out of dormancy when the turf is less able to compete with the weeds (spring and fall).
• These herbicides are typically the most effective chemical options for controlling historically difficult-to-control weeds like crabgrass, sedges, annual bluegrass, Virginia buttonweed, and annual blue-eyed grass to name a few.
• Typically, with an effective pre-emergence herbicide program, the number of post-emergence herbicide product applications are reduced to maintain a healthy turf stand.
• When used as directed by the label, many pre-emergence products are safer to use in turf and landscape areas than most post-emergence products would be. 

What Are Factors To Consider Before Choosing and Applying a Pre-Emergence Product?

• Pre-emergence herbicides can injure new sod or sprigs if the wrong product is chosen and applied too close to planting or too soon after planting. Always follow the label recommendations and rates. 
• The performance of pre-emergence herbicides can vary greatly based on application timing and environmental conditions (precipitation, soil temperature, soil structure, irrigation, water quality, etc.).
• Properly timed and applied spray applications of pre-emergence herbicides tend to perform better than herbicide-impregnated granular products (with and without fertility added). If cost or lack of a sprayer is an issue, a granular pre-emergent application is completely satisfactory and better than not doing anything at all.
• Applying granular pre-emergence herbicides/fertility should be avoided if the turf isn’t actively growing, as it wastes plant nutrients.

When Do I Apply Pre-Emergence Herbicides?

For the most success, it is best to make two main applications of products using different AIs and MOAs during the season. These applications can be done during the spring and late summer/fall seasons. The spring application targets summer annual weeds, while the late summer/fall application targets the winter annual weeds. Pre-emergence herbicides are most effective when applied based on soil temperature. Soil temperature plays a critical role in weed seed germination. Spring applications should take place when soil temperatures have reached 55°F for several days consecutively. Fall applications should be made when soil temperatures have decreased to around 70°F for several days consecutively. Applications can vary by several weeks year-to-year due to overall climactic factors based on your region, especially in the South.

Soil Temperature Monitoring

Soil temperature monitoring can be accomplished yourself with a soil thermometer or even a digital meat thermometer. Measure the temperature in the top inch of soil where most weed seeds will be concentrated. It should be noted that some seeds will be close to the surface or lying on the surface; These seeds will germinate earlier than those deeper down. Your local agricultural extension service can help you, as well. They tend to monitor soil temperature at a variety of depths in many areas in your state. Your local extension agent can give recommendations for the best sources of soil temperature data for your area.

Common Pre-Emergence Herbicides

Product Recommendations

For fall pre-emergence herbicide applications, I tend to favor Specticle here in Texas due to our varying climate, soils, and the lack of yearly climate consistency. The split applications (two or three applications) in the label tend to be more successful in controlling our most troublesome weed-annual bluegrass. I do like to rotate yearly with Barricade, Pendulum, and Surflan to prevent resistance. This may or may not work depending on your seed bank and resistance to the older chemistries. If you experience breakthrough or would like to follow up, you may do applications of Princep or Kerb after the others stated previously. Irrigation is very important immediately following Kerb applications. For spring pre-emergence herbicide applications, I tend to favor Barricade, Pendulum, Surflan, and Ronstar. I like to rotate them every season to prevent resistance issues. In the early summer, I like to get an application of Dimension out there, too, especially if crabgrass, virginia buttonweed, and nutsedge are issues.  

The most common weeds targeted with pre-emergence herbicides in the south are annual bluegrass, crabgrass, and goosegrass. There are a lot of broadleaf weeds also covered by the same products, but these vary much more by region and by state. If you can name the broadleaf weeds you have issues with, it is good to make sure that the products you choose cover them in addition to your grassy weeds.

Best Management Practices

• Check out your local (or preferred) university turf program for assistance with weed identification, recommendations, and possible field research trials.
• Always follow the label. The label is the law. Read and consider all parts of the label, including the recommended PPE, as well as the rates, timing, and irrigation requirements. If you have questions, ask the manufacturer’s territory rep or your sales representative.
• Combine your pre-emergent application with a soil surfactant (penetrant), preferably. This allows you to efficiently irrigate the application to the desired location (depth) in the soil profile. It is also beneficial to plan your application around your irrigation needs/plans and the weather so you attain the maximum efficacy of your application.
• If you have slopes that can drain back (sheet flow water from heavy rain) to your greens, consider using a granular product like Freehand or Crew in those areas (from the collar out) until you reach the crest of the slope. Use marking paint or irrigation flags to mark your tie-ins for spray applications. From personal experience, I recommend this if you are using Specticle.
• If you do your pre-emergence applications in-house with your crew and sprayer(s), consider using letter-sized copies of course maps (or hole maps) and markers to color in areas as they are completed—especially if there are multiple sprayers going at one time. Have the team split up. For example, have one team do the odd-numbered holes and the other team do the even-numbered holes. Communicate areas that have been completed or need completing, follow up, or personally observe the process. I’ve seen some instances of sickly, severely injured, or dead turf because of lack of a communication or not using a simple aid (like a colored-in map), which led to an area being sprayed twice at the high label rate.

When in doubt, consult with your local agriculture extension agent, manufacturer territory representative (for the product), or ATS sales representative for questions on pre-emergence products.