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.
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 of crabgrass prior to tillering.
>>>> Oxadiazon is another important preemergent option for grassy weeds, including crabgrass and goosegrass. It inhibits the activity of PPO, an enzyme needed for chlorophyll production and chloroplast membrane integrity, both of which are found in shoots. In other words, oxadiazon stands apart in that it does not directly target roots, which can be an important distinction in younger turf stands or situations where turf may need to recover from damage. Oxadiazon also has a good broadleaf weed control spectrum including doveweed.
>>>> 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.
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:
These dates are based on historical soil temperature averages. 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.
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.
Ian Rodriguez Technical Service Manager Quali-Pro