Sequential Preemerge Herbicide Applications Provide Long-Term Weed Control

If debris are left in the berms, preemerge herbicide cannot penetrate the soil effectively.

As problem weeds continue to develop resistance to glyphosate, weed management strategies must continue to evolve. Resistance management has focused primarily on annual weeds such as marestail and fleabane, but summer weeds, especially summer grasses such as junglerice, are becoming glyphosate resistant as well.

One way to manage these weeds without adding significant cost is with sequential applications of preemergent herbicides. While the use of a preemergence herbicide has always been an important part of a weed control program, they are becoming even more vital now as contact sprays become less effective on certain weeds.

“The goal is to extend weed control deeper into the late spring and summer months,” says Jeff Pacheco, technical sales agronomist for DuPont. Pacheco consults with PCAs and almond growers in the central and southern San Joaquin Valley of California. “Most preemerge herbicides provide four or five months of residual control. So if you apply in November, you’re only getting residual weed control until March or April.  Fleabane and marestail can emerge through late spring and early summer along with summer annual grasses and other broadleaf weeds when irrigations get going strong. What are you going to do from April through the rest of the year?”

This is where sequential applications come in. Pacheco recommends applying preemerge applications to the berms under the trees in November or December, when the rainy season begins. A second application in March or April, before the rainy season ends, allows the incorporation of the herbicide into soil to provide weed control during the warm spring and hot summer months. Northern California is a good fit for this strategy due to rainfall patterns that start earlier and end later than other parts of the state, but the Central and Southern parts of the Central Valley can also benefit.

DuPont’s preemerge herbicide, Matrix® SG, is ideal for this approach. It can be tank-mixed with glyphosate to improve early postemergence control of emerged weeds, and can be applied at a time when a grower would normally be spraying a contact herbicide anyway.

“Matrix® SG can be applied twice a year if you’re treating less than a 50-percent band,” Pacheco says. “You can use Matrix® SG in November or December and then just before the rainy season ends in March, make another application to those berms – while they are still weed and debris free. Preemerge herbicides don’t last as long when air temperatures get hot and soil temperatures warm. In the spring, as the rainy season ends, growers have to tighten-up the irrigation schedules. Irrigation water and higher soil temperatures break down herbicides through hydrolysis or microbial activity. We’re trying to get at least to harvest, which is typically in August. Our goal is to reduce dependence on contact sprays since they aren’t working as well as they used to.”

Pacheco says without this approach, many growers are applying a mixture of contact herbicides every four to six weeks, but with a sequential preemerge herbicide strategy only one to two cleanup sprays may be needed.

“When you add up the cost of repeated contact applications, plus the risk of drift onto the trees and the fact that contact sprays aren’t working as well on some key weeds, you can actually save money with a residual program,” he says. “Our recommendation is to always have a tank mix partner with Matrix® SG that meets the need of the situation, so we can broaden the weed spectrum, and and manage weed resistance. The choice of a tank mix partner will typically be different in the fall than the spring, and should be dependent on the species of weeds that may be emerged, and historical winter annual or summer annual weed pressure you are targeting.”

Matrix® SG provides effective control of important winter annual weeds such as fleabane, marestail, filaree, malva, chickweed, sowthistle, annual bluegrass and Italian ryegrass, with fall applications Pacheco says. With spring applications, Matrix® SG will provide extended residual control of late germinating fleabane and marestail, along with summer annual weeds such as puncturevine, purslane, spurge, barnyardgrass and foxtail.

“Matrix® SG also has some suppression of yellow nutsedge, a perennial weed, and that is a big advantage over other preemerge herbicides that are in the market,” Pacheco says. “A fall application followed by a good rainfall event will help get Matrix® SG into the soil and down by the roots and nutlets. Then when the nutsedge begins to emerge in March, you hit it with another shot.”


Safety and Flexibility Are Big Advantages

The biggest advantage of Matrix® SG is the flexibility and outstanding crop tolerance, Pacheco says. There are no restrictions in regards to soil texture, percent organic matter or GWPA (groundwater protection areas), and it can be applied in both dormant and non-dormant orchards.

“We had to do extensive testing to prove to our company that Matrix® SG was safe enough to use in high-value crops like trees and vines,” Pacheco says. “There are several products you can’t use after bud break, because they’re volatile. It’s also a great choice for weed control in young trees, which can be a terrible problem. Trees only have to be in the ground for one full growing season before Matrix® SG can be applied. It really helps growers to be proactive with weed control starting when the trees are young.”

About five years ago, the formulation was changed from a dry flowable (DF) to a water soluble granule (SG). This offers a number of advantages: once it’s dissolved in the tank it goes into full solution, may not need agitation, will not tie-up glyphosate, makes it much easier to rinse from the tank, and more efficient to use through micro sprinklers.

“After the rainy season, most of the weeds grow where the micro sprinklers are,” says Pacheco. “So we have some growers that will make a band application in the fall (less than 50% band) and then make a second application through the micro sprinklers.”

Asked what cultural practices help to optimize the efficacy of preemerge herbicides, Pacheco says to “start clean and end clean.”

“We can discover and develop fantastic products, but they are only as good as the preparation and accuracy of the application,” he says. “If there are large weeds or weed carcasses on the berm from the previous year, and/or lots of fallen leaves or debris on the berms, the preemerge herbicide will have difficulty reaching the soil uniformly. If you can’t get the spray on before the leaves drop, you need to blow them off and sweep away dead weeds. Starting clean is critical if growers want to maximize benefits”

For more information on how Matrix® SG can support your weed control plan, please visit