Preemergence weed control in warm season grasses is the key

Preemergence weed control in warm season grasses is as important now as it has ever been

Preemergence herbicides are the basis of all weed management programs in turfgrass. They should be used on a yearly basis, either applied in the late winter/early spring for summer annual weed control or late summer/early fall for winter annual weed control. Preemergence weed control is about prevention. By controlling weeds as they germinate, one can prevent weed problems from ever taking root.

Figure 1
Sureguard was applied to a surrounding bermudagrass putting green and moved laterally with surface water onto a creeping bentgrass putting green, causing injury.

If you do not use preemergence herbicides, you jeopardize the effectiveness of postemergence herbicides you use later. Without preemergence herbicides to at least decrease weed population densities, postemergence herbicides will have to control larger plants at higher population densities.

In my research program, we conduct weed control trials under the worst conditions — large plants, high population densities, with very little turfgrass. These sites do not receive any preemergence herbicide, thus the populations are excessive and not what one would normally observe in a managed turfgrass situation. In these high-density weed situations, virtually all postemergence herbicides underperform, necessitating the need to make multiple applications.

Five years ago when writing about preemergence herbicide use in warm season turfgrass, I would have written about four herbicides — prodiamine (Barricade), pendimethalin (Pendimethalin, Pendulum), dithiopyr (Dimension) and oxadiazon (Ronstar). These were the big four — the four primary preemergence herbicides used in turfgrass.

Prodiamine and pendimethalin are classic preemergence herbicides in the dinitroaniline family. They are mitotic inhibiting herbicides, with no postemergence activity, and are yellow to orange in color. Dithiopyr is also a mitotic inhibiting herbicide that is slightly different than dinitroanilines. Dithiopyr is known for its pre- and early postemergence activity on crabgrass species. Oxadiazon is completely different in its mode of action compared to the previous three. It is a protox inhibiting herbicide and is more similar chemically to sulfentrazone (Dismiss) than to the previously mentioned preemergence herbicides.

While known for its preemergence effect, its non-selective postemergence activity is something known to be avoided. Oxadiazon can injure green leaf tissue when applied as a liquid to green leaf tissue or granular to wet green leaf tissue.
 

In with the “new”

New is relative. Two “new” herbicides used in turfgrass may not be that new to you, as they have been on the market for two to three years now. But in the grand scheme of things they are still pretty new. The “new” herbicides are Sureguard (flumioxazin) and Specticle (indaziflam). I will refer to these by their product names. Older herbicides are referred to by their chemical names to give deference to generic products.

Like oxadiazon, Sureguard is a protox-inhibiting herbicide. And like oxadiazon, precautions must be taken to protect against potential injury to green, growing tissue. Sureguard is primarily sold for its preemergence effects in dormant warm-season turfgrass and it has the added benefit of having postemergence control on winter annual weeds, such as common chickweed, field pansy and henbit, which allows for flexibility in application timing.

Figure 2
Specticle treated on left compared to an overseeded golf course fairway containing Barricade and Revolver/Monument-resistant annual bluegrass.

As with all herbicides, one has to balance out the pros and cons with Sureguard. As for pros, it has excellent residual and early postemergence activity. It is excellent for landscape beds where no turfgrass can be contacted during application. As for cons, it can injure green turf and delay green-up if applied too close to green-up. It is not a stand-alone postemergence treatment on larger weeds. Sureguard can also move off-target through surface water run-off or foot traffic, on to sensitive cool-season grasses (Figure 1).

As stated previously, “new” is a relative term. As products enter the marketplace and become more widely adopted researchers and end-users begin to discover new uses or unique possibilities about products. With Sureguard it has been the possibility of extended residual activity throughout the summer from winter applications. In my research program we have observed summer-long smooth crabgrass control from applications made in December. This was an incredible observation to make. The problem is that it was not consistent in multiple years. So the possibility is there for extended control, but actually relying on it may be another thing.

The second “new” herbicide is Specticle. It contains the active ingredient indaziflam. It is a cellulose biosynthesis inhibiting herbicide, used at low rates (measured in oz. or fl. oz. per acre), and has excellent residual preemergence control. Specticle can only be applied to warm-season turfgrass and precaution must be taken when applying to high sand content soils, as these conditions have been correlated to potential turfgrass injury.

As with Sureguard, Specticle has its pros and cons. Its pros include long-residual preemergence activity, low use rate and great preemergence effectiveness against crabgrass, goosegrass and annual bluegrass (Figure 2). It has a few cons that can easily be avoided if understood. First, Specticle can cause root pruning in high sand soils. This is especially prevalent with late-winter to spring applications of Specticle when turfgrass is greening up. Second, Specticle can move or be tracked off-target onto sensitive species. Movement is similar to sulfonylureas such as Monument, Revolver and Sureguard, where the herbicide washes down a slope with surface runoff and is deposited on to a sensitive species where damage occurs. Tracking via shoes or equipment can also occur if Specticle is not watered-in properly.
 

Other preemergence herbicides

While these two herbicides are referred to as being “new” because the active ingredients are recent additions to the turfgrass industry, there are other herbicides that can be considered “new” due to their added benefits.

First, there is Echelon. Echelon is a prepackaged mixture of prodiamine and sulfentrazone (Dismiss). The addition of these two active ingredients has two potential benefits of increased goosegrass and sedge control. One potential strategy is to apply an initial preemergence application of prodiamine alone in late winter/early spring followed by Echelon, applied late spring/early summer as sedges and goosegrass are beginning to emerge. This application timing would have the added benefit of preemergence and early postemergence control of goosegrass and some sedges.

Table 1 (click to enlarge)
Preemergence herbicides commonly applied to warm season turfgrasses and their respective modes of action.
* In column three there is a list of mode of actions along with letter (Herbicide Resistance Action Committee) and number (Weed Science Society of America) codes associated with the select modes of action. These numbers or letters can be used to determine if one is rotating to a different mode of action. The use of the number classification system is becoming more commonplace on labels.

Tower (dimethenamid) can be used in a similar fashion to Echelon when combined with pendimethalin or other traditional preemergence herbicides. While not a stand-alone herbicide for season-long control, when used in combination with pendimethalin it can provide improved goosegrass and sedge control.

Other preemergence herbicides are available that are not widely used in turfgrass. Some examples are Gallery (isoxaben), Pennant Magnum (metolachlor) and oryzalin (Surflan and Oryzalin products). Gallery is another cellulose biosynthesis inhibitor, but it is unique in that it is known for preemergence control of broadleaf weeds. Where dinitroaniline herbicides primarily control small seeded broadleaf weeds and grasses, Gallery can control a plethora of broadleaf weeds of various seed sizes.

Pennant Magnum is a herbicide widely used in row-crop agriculture also known for its preemergence broadleaf weed control and also has the added benefit of preemergence sedge activity. Pennant Magnum has little to no preemergence grass control, thus it needs to be applied with a preemergence annual grass herbicide. Pennant Magnum can delay greenup when applied during warm-season turfgrass greenup.

Oryzalin is a dinitroaniline herbicide very similar to pendimethalin and prodiamine. Oryzalin can be as effective as standard preemergence herbicides, but issues with potential volatility and inconsistent control limit its use. I personally have seen it perform very well, but occasionally preemergence control can be underwhelming.

One should also not forget about Bensumec (bensulide). Bensumec is primarily used for preemergence control of annual bluegrass on putting greens. It is commonly thought that bensulide is identical to prodiamine or pendimethalin in its mode of action. While that is not true, the mode of action of Bensumec is not completely understood. While it is thought to be a mitotic inhibitor, it is also classified as a fatty acid or lipid biosynthesis inhibitor.
 

Resistance to Preemergence Herbicides

Annual bluegrass and goosegrass have developed resistance to numerous herbicides used in turfgrass. Annual bluegrass resistant to prodiamine is becoming more widespread, especially in the Southeast. Some populations resistant to prodiamine are also resistant to pendimethalin and dithiopyr. Goosegrass resistant to dinitroaniline herbicides, such as pendimethalin and prodiamine, was discovered over 30 years ago. Even more disturbing are reports that goosegrass has developed resistance to oxadiazon, leaving even fewer preemergence options for its control.

In my view, annual bluegrass and goosegrass resistance to preemergence herbicides has hastened the move by superintendents to herbicides such as Specticle. It is fortunate that there was an effective herbicide available to replace herbicides failing due to weed resistance.

There is no doubt however, that unless proper stewardship is undertaken with newer herbicides, the same resistance problems will develop over time. Proper stewardship means rotating modes of action or tank mixing different modes of action (Table 1). Rotating modes of action is more than just changing to a different herbicide, but rotating to a herbicide that is different in its herbicidal activity. Biologically, proper stewardship also means preventing or limiting seed production of uncontrolled weed escapes. There is likely not to be another active ingredient to replace herbicides failing due to resistance, so stewardship is our only course of action.

Photos: Scott McElroy