If our goal is a pristine, weed free lawn, then it is obvious that weed control measures need to be put into place and become part of our lawn care program. Before trying to kill these unwanted plants, we should have a bit of an idea about how they grow. Weeds have many different life cycles, either annuals, biennials, or perennials. Winter annuals, and many other true annuals, germinate in the fall of the year and mature the following spring. We can predict when they are going to germinate based on soil temperature and apply pre-emergent herbicides that won’t allow them to germinate, meaning they won’t pester us in the spring. Pre-emergent herbicides are used to prevent the germination or emergence of many weeds. They do this by forming a barrier at the soil surface that affects susceptible species that can be found on the product labels. Because of this barrier and their ability to control weeds before we ever see them, pre-emergent herbicides can prove to be integral in your lawn maintenance program.
Apply fall pre-emergent with the active ingredient prodiamine or dithiopyr to your lawn when soil temperatures consistently drop below 70°F but before they are consistently below 55°F which is typically from late August to early October, depending on your region. Products containing prodiamine as the active ingredient can also be quite effective as Fall pre-emergent herbicides, however, many DIYers have used the maximum annual amount of these herbicides in the spring of the year when applying them for pre-emergent crabgrass control. Regardless of the product chosen, this timing based on soil temperature is crucial because it targets the germination window of winter weeds such annual bluegrass (Poa annua), chickweed (Stellaria media), downy bromegrass (Bromus tectorum) and henbit (Lamium amplexicaule), creating a barrier that prevents them from germinating and emerging.
Annual bluegrass (Poa annua)
Identification notes: (a) Generally a lighter, brighter green color; (b) often a ‘wrinkled’ appearance near the center of the leaf blade on mature leaves
Identification notes: Distinct seedheads that may appear in spring or fall.
Chickweed (Stellaria media)
Identification notes: (a) Opposite leaves that attach directly to the stem; (b) small white flowers; (c) mat like low growing appearance, especially when growing with limited competition.
Downy bromegrass (Bromus tectorum)
Identification notes: (a) Pubescent or hairy, especially obvious on the sheath; (b) sheath often has a purple tinge; (c) ligule (clear membranous projection) at the base of the leaf blade and junction with the sheath.
Henbit (Lamium amplexicaule)
Identification notes: (a) Square stem; (b) opposite leaves on the stem that attach directly; (c) distinct tubular pink to purple flowers.
Why This Timing Matters
Applying at the 70°F soil threshold ensures the herbicide is active before winter annual weeds germinate. If you wait too long, the product won’t be effective against weeds that have already sprouted. This is due to the fact that the herbicide breaks down (becomes ineffective) slowly over many weeks after it is applied. Full efficacy is typically in the range of 5-12 weeks depending on the product. After that time efficacy will slowly decrease as the herbicide degrades in the soil.
Properly timed pre-emergent applications form a chemical barrier just before or at the precise time weed seeds begin to emerge, providing season-long control. When the germinating seedling roots or shoots reach this barrier, it causes the young plant that has not yet emerged to die.
Stopping winter weeds in the fall makes for a healthier, weed-free lawn come spring, saving time and reducing the need for intensive weed control later. Pre-emergent herbicides, when timed correctly, also can significantly decrease the reliance of post-emergent herbicides that require weeds to be actively growing to be effective.
Check your MySoil Dashboard for your current soil temperature or use a soil thermometer at a 2-inch depth to track precise to lawn soil temperatures. Remember that air temperatures and calendars can be misleading as seasons and soil temperatures vary year over year. Regions or years with earlier cool-downs may require an earlier application.
How Much and What Spreader Setting?
Always read the label and follow the instructions and rates provided. Below are links to two pre-emergent herbicides, you can download the labels to better familiarize yourself with these products and their use.
Should I Use Liquid or Granular Pre-emergent Products?
Granular pre-emergents are straightforward to apply with a spreader and are a popular choice for homeowners, especially those with larger lawns. They generally offer a longer shelf life, reduced risk of drift into unwanted areas, and allow for easy storage between seasons.
One downside to granulars, however, is their reliance on thorough and timely watering-in to activate the barrier; without proper moisture after application, they may not provide effective weed control. In dry conditions or where irrigation and rainfall is inconsistent or unavailable, granular products can be slower to activate and form the protective layer needed in the soil.
Liquid pre-emergent formulations are praised for their quick soil activation and consistent, uniform coverage, especially valuable for small lawns and those that are irregularly shaped. They also work really well in targeted areas such as tree wells and property borders. Once sprayed, liquid products create an immediate chemical barrier, making them convenient in climates where rainfall is unpredictable.
They may offer better results when used with the right equipment, such as quality sprayers that allow for even application. However, liquid pre-emergents demand a higher level of skill to avoid missed spots, reduce risk of wind drift onto non-lawn areas, and require careful calibration to prevent uneven coverage.
Mixing and applying liquids may also pose more challenges for beginners, and any leftover mixed solution typically cannot be stored or reused for future applications.
Ultimately, granular and liquid pre-emergent herbicides both offer strong season-long weed control when used according to label directions. Homeowners often choose granular products for their simplicity and forgiving application, while liquid products appeal to those seeking fast results, precision, and flexibility, especially for targeting trouble spots or for integration with other products. The right choice often depends on lawn size, available equipment, comfort level with application techniques, and local weather or watering habits.
Key Tips
Do not apply pre-emergent if overseeding or if you’re establishing new grass. The herbicide will also hinder the germination and emergence of the desire grass seed species.
Water in the product thoroughly right after application for best results. One-half inch of irrigation water or rain is recommended to fully activate pre-emergent granular herbicides. Liquid foliar applied pre-emergents may not need to be watered in; see label instructions.
Be sure to mow prior to application. This is a scenario where bagging clippings makes sense as you want to be sure that all of the granules reach the soil surface. Some practitioners even mow the lawn much shorter than normal to increase the likelihood of the granules contacting the soil surface.
Avoid any activities that would disrupt the soil, therefore your pre-emergent barrier, after application. These activities would include dethatching, aerating, slicing, or aggressive raking.
Wait at least three days before mowing to ensure that you are not disrupting the granules as they begin to form the chemical barrier at the soil surface.
Summary:
Apply fall pre-emergent herbicides when soil temperatures fall below 70°F and before weeds germinate. If specified on the label, be certain to water in the product to create a barrier of weed prevention. Always follow label directions when applying herbicides. This proactive step prevents winter weed problems and helps ensure a stronger, healthier lawn next spring.
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