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Weed Science

Broadleaf Weed Control Reseach Update

Since the following research summary was written in 2002, the sulfonylurea herbicide Certainty® has shown very good results in controlling dollarweed, and there have been changes in the labeling of several products. Lontrel is no longer available for landscape use, and the SpeedZone products have had major changes in their labels so they are no longer available for use in Floratam St. Augustinegrass in Florida.

Since the 1950s, atrazine has been the standard postemergence herbicide for broadleaf weed control in St. Augustinegrass turf.  More recently, four new broadleaf herbicides have been labeled for the same purpose:  Lontrel® (1999, Dow AgroSciences); Manor™ (2000, Riverdale Chemical Company); SpeedZone™ St. Augustine Formula (2001, PBI Gordon Corporation); and Blade™ (2002, PBI Gordon Corporation).  Unlike atrazine which is generally harmful to other grasses, Blade™, Lontrel®, Manor™, and SpeedZone™ St. Augustine Formula are also labeled for certain other warm-season turfgrasses.  The purpose of this web page is to provide a brief update on the relative weed control usefulness of all four products, and pertinent history.

Damage to cultivars of St. Augustinegrass from summer application of atrazine on a sod farm.  Bitterblue and other cultivars are severely injured at rates that do not visibly affect Floratam.

The observations on which weed control usefulness is based are replicated experimental plots that I established and evaluated from 1999-2002 in Broward County and Palm Beach County, Florida.  The data demonstrate relative efficacy of control of selected subtropical weeds, and phytotoxicity to St. Augustinegrass turf, both in lawn areas and sod.  This is not meant to be comprehensive of all the weeds controlled by these products, nor is it meant to represent weed control usefulness outside the experimental area of subtropical South Florida.  The herbicides are also labeled for use on other subtropical turfgrasses, but this report focuses on St. Augustinegrass, the main turf of Florida.  First, some history.

Atrazine

Atrazine was adopted in the 1950s as the main postemergence herbicide for broadleaf weed control in St. Augustinegrass, after sod producers observed damage from phenoxy herbicides containing 2,4-D.  After consulting with Dr. Evert O. Burt, weed scientist at the University of Florida, Ralph W. White (Agricultural Extension Turf Specialist at the University of Florida) tested low rates of triazine herbicides for the control of broadleaf and grassy weeds.  It was discovered from research that White did in Davie, Florida, that simazine and atrazine were effective for selective weed control in St. Augustinegrass.  This was a major breakthrough for growers to produce clean sod for the consumers.  Atrazine can also be used on centipedegrass.  Unlike the other herbicides discussed here, atrazine has considerable preemergence weed control activity.  The Floratam cultivar of St. Augustinegrass is so resistant to atrazine that, after its release in 1973, Floratam became the main turfgrass grown for sod in Florida.  Weeds are always a major problem in turfgrass production, but this was most clear when growers expanded their acreage in the rich humus soils of the Everglades Agricultural Areas.  Something unusual about atrazine and St. Augustinegrass is that most turfgrasses are injured by atrazine, and cool-season turfgrasses are severely damaged at even low rates of atrazine, so St. Augustinegrass is fairly unique.

Unfortunately, there are drawbacks to atrazine.  It is relatively highly water soluble, therefore in some parts of the United States, such as the Midwest, atrazine has been discovered in groundwater.  Secondly, not all St. Augustinegrasses are equally tolerant of atrazine, and summer applications can be destructive.  Bitterblue and FX-10 St. Augustinegrasses have less tolerance for atrazine than does the Floratam cultivar of St. Augustinegrass.  Manufacturers have elected to label professional liquid formulations of atrazine as a Restricted Use Pesticide, so that only licensed users may apply those formulations, with the stipulation that they may not be applied after tax day, April 15th, until October 1st, although different labels describe this differently.  (Surprisingly, homeowners can purchase both granular weed-and-feed mixtures as well as certain liquid atrazine formulations, and do not have strict prohibition from April 16 through September 30th).  The third problem with atrazine is that pest control operators have reported inconsistent results from using it to control certain important broadleaf weeds, such as dollarweed.  Dollarweed is the most common broadleaf weed in St. Augustinegrass in Florida, in part because of the excessive irrigation used by homeowners, and the fact that dollarweed is a water-loving plant.

	Time-course of weed control by several broadleaf postemergence herbicides applied to control dollarweed, Hydrocotyle umbellata (pennywort). Manor™ and Blade™ are identical in active ingredient and concentration, but they differ in packaging. The container of Blade™ contains a measuring cylinder with attached funnel that it appears would be effective in measuring the small quantities used. Trimec Southern is not labeled for use on St. Augustinegrass.  Not shown are additional mixtures such as Manor+Lontrel, which was slightly more effective than Manor or Lontrel, and a three-way mixture of 2,4-D, dicamba, and mecoprop, which was less effective than the same mixture with carfentrazone-ethyl.  Plots were treated on 25 January 2002, wind speed was less than 5 miles per hour.  The day was sunny and the high temperature was 85.5 F (29.7 C)

Lontrel®

Each of the "new" broadleaf herbicides has some history.  Lontrel® contains the active ingredient clopyralid, which appeared previously in the turf product Confront, a right-of-way material which also contains triclopyr.  Lontrel® has excellent turf safety, and is particularly effective against several families of weeds, especially the Asteraceae or sunflower family including groundsels; but also the Fabaceae, or pea family including the clovers; the Polygonaceae, or knotweed family; and the Solanaceae, or nightshade family.  In Florida, California, and New York, the maximum use rate is 2/3 pint per acre per growing season, which is only one-half of the suggested application rate for some difficult weeds.  Some applicators have gotten improved results by mixing Lontrel® with other herbicides, but one must be cautious to carefully test new mixtures in small areas.

Manor™ and Blade™

Manor™ and Blade™ have the active ingredient metsulfuron, which was used previously in the herbicide DMC marketed by O. M. Scotts and Sons briefly in the 1990s.  Metsulfuron has been the active ingredient of the agricultural herbicide Escort, also labeled for unimproved turf, and which in July 1997 received a 24C Special Local Needs label for St. Augustinegrass sod in Florida.  Although I wrote a letter in support of the 24C label, most of the credit goes to Dr. Bert McCarty of Clemson University, based on research that he had done while with the University of Florida.  Manor™ and Blade™ are very slow acting, but are effective against some particularly difficult weeds such as alligatorweed, a problem in some South Florida sod farms.  Manor™ and Blade™ are effective at 0.25 to 1.0 ounces per acre, although 0.50 ounces per acre is commonly used.  Manor™ and Blade™ may not be used on bahiagrass, except to control it as a weed.  Because such small rates of application of Manor™ and Blade™ are effective, extreme precautions must be taken to accurately weigh the small amounts needed.  This may require an analytical balance.  The packaging of Blade™ contains a built-in measuring cylinder that appears would be effective in measuring the small quantities used.  (The other broadleaf herbicides discussed here are liquids.)

SpeedZone St. Augustine™ Formula

SpeedZone™ St. Augustine Formula contains the new turf herbicide carfentrazone-ethyl, which the EPA considers a Reduced Risk Pesticide, effective in herbicide mixtures at rates as low as 0.02 pounds per acre.  Because of this, the EPA fast-tracked the approval process.  PBI Gordon obtained the exclusive U.S. non-agricultural marketing rights to carfentrazone-ethyl in combinations from FMC Corporation.  In mixtures such as SpeedZone™ St. Augustine Formula, carfentrazone-ethyl quickly kills difficult broadleaf weeds such as clover, spurge, Old World diamondflower, and alligatorweed.  Weed foliar injury is often noticed within hours after application, and most of the weed control is accomplished within 4 days, based on my experience.

Carfentrazone is in the triazolinone family which is believed to disrupt cell membranes similar to the diphenyl ethers by inhibiting the enzyme protoporphyrinogen oxidase (the PROTOX enzyme), thus they are called PPO-inhibiting.  This activity leads to peroxidation due to accumulation of a singlet oxygen in the presence of light, and death of contacted leaf areas upon
exposure to light.  Other PPO-inhibitors include oxyfluorfen and oxadiazon.

SpeedZone™ St. Augustine Formula is also unusual in containing the herbicide 2,4-D which was generally thought to be damaging to St. Augustinegrass.  In the mid 1980s Dr. Wayne Currey, then turfgrass weed scientist for the University of Florida, observed considerable difference in the St. Augustinegrass injury among different phenoxy herbicide products.  The differences were due to the concentration of herbicides other than 2,4-D, particularly mecoprop (once called MCPP), in the mixtures.  From many experiments that I have performed in South Florida, mecoprop and not 2,4-D appears to be the killer of St. Augustinegrass.  SpeedZone™ St. Augustine Formula contains a reduced concentration of mecoprop, and I have not seen injury to St. Augustinegrass from labeled rates applied at any season of the year.

The fourth component of SpeedZone™ St. Augustine Formula is dicamba, another common component of phenoxy herbicides for turf, and it helps boost the control of perennial broadleaf weeds.  The 2,4-D is in an ester, and because of this there is less odor, but more potential from volatile movement into the air, though as the isooctyl ester it has less volatility than other esters.  Use rate is 2-4 pints per acre in St. Augustinegrass.

Methods

Dollarweed areas were chosen at the Fort Lauderdale Research and Education Center, based on nearly solid coverage.  There was a thin understory of bermudagrass.  Plots 4 ft X 8 ft (122 cm X 244) cm were assigned to six replicates based on condition.  Herbicide treatments were applied with a CO2-pressurized backpack sprayer calibrated to deliver 22.6 gallons per acre (678 L/ha) at 30 psi (207 kPa) with 11002 flat-fan nozzles.  The herbicide treatments were Atrazine 4L (2 pt/A), a low rate; Lontrel® (3/4 pt/A); Manor™ (0.5 oz./1000 ft2); SpeedZone™ St. Augustine Formula (3 pt/A); Trimec Southern (1.5 pt/A); water control, and two mixtures (not shown in the graph above) Lontrel®+Manor™; and 2,4-D+mecoprop+dicamba.  Plots were evaluated visually for dollarweed injury each day for 10 days, and twice per week thereafter.

Results

Within 3 weeks, atrazine, SpeedZone™ St. Augustine Formula, and Trimec Southern (not labeled for St. Augustinegrass) were equally effective in controlling dollarweed, averaging about 90% control.  SpeedZone™ St. Augustine Formula was faster than atrazine and Trimec Southern, reaching a plateau of weed control by 5 days after treatment.  Manor™ caused slow kill of dollarweed, but at 21 days had achieved only about 60% control.  Lontrel® was not effective, and caused only 25% injury to dollarweed.  These results were similar to others obtained in other experiments, including more difficult weeds such as Old World diamondflower and alligatorweed.