Prevention is the Key to Annual Weed Control
At the start of a new year, most of us know that a little thinking ahead can prevent a lot of headaches down the road. While this sentiment holds true for many things in life, most turf managers would agree that it applies to a good weed management strategy. A solid preemergent herbicide approach can stop many of the most problematic weeds before they establish and become more difficult to control, while maintaining the quality of the desired turf.
Apply in the Winter to Prepare for the Summer
How they work:
Preemergent herbicides should be selected based on the chemical class and mode of action (MOA) of the product, the types of weeds being targeted, and more site-specific factors like turf type/use and seasonal climate. Understanding the options available will help ensure success.
Prodiamine is in the dinitroaniline (DNA) herbicide family, with a MOA resulting in the inhibition of root growth. Weed shoots and roots can absorb prodiamine, but the main effect is inhibition of normal root development, often resulting in “clubbed” tips. Without a functional root system, overall weed growth is halted, and the seedling dies. Prodiamine is a very affordable option with good prevention of grassy weeds such as crabgrass, Poa annua, and tropical signalgrass.
Dithiopyr is pyridine herbicide with a MOA similar to prodiamine, interfering with root development. It differs in that it controls a wider range of broadleaf species and can provide good preemergent crabgrass control along with post-emergent control prior to tillering.
Isoxaben is benzamide herbicide with a MOA that inhibits cell wall production and typically stunts root growth. This herbicide does not control grassy weeds but has a very good broadleaf weed control spectrum including chickweed, oxalis, and spurge. It is typically combined with another preemergent to cover annual grass weeds.
All three choices are also labeled for use in landscape beds and can be excellent tools for minimizing the need for labor-intensive hand weeding and spot spraying during the growing season.
Timing is Everything
Of course, the main principle of preemergents is that application must precede the germination window of the targeted weeds. This can vary broadly with weed species and region. Soil conditions favoring germination and other indicators have been established for many of the major weed species. For example, the following graphic can be found on the Quali-Pro label for Prodiamine 65 WDG:
Crabgrass is public enemy number 1 in most regions when it comes to lawns due to limited selective postemergence options. The germination dates above are based on historical soil temperature averages and the fact that crabgrass is known to begin germinating when soils warm to 55°F for several days. Additional information regarding germination windows specific to your region such as growing degree day (GDD) calculators and other prediction tools may be available from your state’s extension service.
Duration and Reapplication:
Preemergents begin breaking down soon after application, so the length of control is longer when higher rates are applied. Even at higher rates, season-long control will require multiple applications to cover spring-, summer-, and fall-germinating weed species, so planning ahead is key to maintain coverage that coincides with your biggest issues. Of course, repeated use of the same product is setting yourself up for resistant weeds, so rotation between MOAs should be maintained. Once again, knowing your weeds and when they germinate is key.