Herbicides and irrigation management keep infestations minimal

Photos Courtesy of Phil Busey, University of Florida.

Dollarweed, also called pennywort or manyflower, thrives in warm, humid climates, but occurs all the way north into Canada. Hydrocotyle umbellate is listed in the USDA Plants Database as native to the lower 48 states and Puerto Rico. It is a member of the parsley family and, because it is a rhizomous, water-loving species, can be resistant to permanent control treatments in well-irrigated turfgrass. It is recognizable by its round, fleshy, lobed leaves on upright stalks, and wherever the five-petaled, star-like flowers develop, there are many of them in white, umbrella-shaped clusters.

The weed can infest many species of turfgrass, but is particularly resilient in warm-season grasses. It is a raging pest in southern Florida, where it can take over St. Augustinegrass lawns. One method of infestation is from imported soil, particularly from nursery potted plants, and it can also move from wet areas and invade plots of turfgrass that are over-watered.

“I think it is the number one weed problem here in Florida,” says Phil Busey, University of Florida associate professor of environmental horticulture. He has run tests on dollarweed controls for nine years, and has found that neither herbicides by themselves nor cultural controls by themselves are completely effective. “The combination of irrigation management and herbicides is most effective,” he says.

Infestations of dollarweed can be preventedby treating it when it first appears. Dollarweed is a tough perennial broadleaf that can be controlled with a combination ofherbicides and irrigation management.

Busey has tried cultural practices, such as raised and lowered mowing heights and decreased fertilizer rates, to find a way to minimize dollarweed and encourage grass, but that has proved ineffective. The weed can be reduced somewhat by close mowing, but that promotes crabgrass growth.

The one cultural practice that shows promise is reducing irrigation rates. “If you adjust your sprinklers to once a week or every six days, you can lower your dollarweed population,” Busey says. In one study he showed that irrigating every day sustained a 30 percent dollarweed infestation. By simply changing irrigation to a seven-day schedule, the infestation could be reduced to a 6 percent level; that was without chemical treatment.

Eradication can occur when irrigation frequency is reduced and herbicides are applied. One chemical is metsulfuron (Blade and Manor), Busey says. He has tested the dry, flowable compound successfully in St. Augustinegrass, and it is labeled for a number of warm-season grasses. It is effective for a number of broadleaf weeds, and he notes that one application may be sufficient for dollarweed.

Metsulfuron is a slow-acting herbicide, and dollarweed will show a gradual yellowing over a couple of weeks. In three weeks, it should be dead, but if the turfgrass area is wet, this chemical is ineffective, Busey says.

Another herbicide is sulfosulfuron (Certainty), Busey says. Purchased as a wetable granule, this is also a slow-acting herbicide on dollarweed. One application should suffice, though a second application can be made if labeled rates for broadleaf weeds are not exceeded.

Cafentrazone (QuickSilver) is an herbicide that, when mixed with metsulfuron or sulfosulfuron, becomes an activator and speeds up the kill on dollarweed, Busey says. In his tests it enhanced the other two chemicals, though it gives no long-term benefits. It would be beneficial to applicators who want clients to see a more immediate result from treatment.

Busey says that 2,4-D in its many iterations is effective on dollarweed, but is not safe for St. Augustinegrass. It might be effective on the weed pest in other appropriate turfgrass species, especially in a manufacturer’s mix with other chemicals such as dicamba. He cautions turf managers to read the labels for active ingredients. He has found atrazine compounds, which once were the standard treatment for dollarweed, ineffective due to lowering of the active ingredient.

He emphasizes that the chemicals most effective on dollarweed are more efficient when combined with reduced irrigation. In the years after an initial treatment, spot applications may still be needed, and a drier lawn will be less likely to have new or continuing infestations.

“Irrigation management and appropriate herbicides properly timed can be effective over time,” Busey says. He notes that in the warm Florida climate he has had the most efficient control when chemicals are applied in the fall as the plant begins to grow. In northern regions, dollarweed may be more of a summer problem, but in the Southeast it dies back in the summer and reemerges from tillers in the fall. Dollarweed is best attacked when it first becomes vigorous and is in small patches.

Busey adds that fertilizer management can be important, and he has found the nitrogen/phosphorus balance in turfgrass to be a factor in dollarweed growth. His work on pastures has led him to conclude that less phosphorus is better in reducing dollarweed. Consider using a 4-0-2 ratio instead of the traditional 4-1-2 blend for lawns that have dollarweed.

Although dollarweed likes an aquatic situation, it may not require treatment as a pond infestation. That’s because among other major aquatic weed pests, such as hydrilla, it will usually be a minor problem. He hasn’t seen dollarweed creep up from an aquatic situation onto a dry lawn, but if it should invade, treat it quickly while it is still localized, and take care of the small spots in future years.

Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.