For the second week in a row, the California Department of Transportation (Caltrans) at the last minute on March 10 postponed spraying herbicides along Highway 84 west of Skyline Boulevard, due to weather.

Caltrans spokeswoman Gidget Navarro said Caltrans will now spray on March 17 and 24, both Tuesdays, between Skyline Boulevard and Stage Road, about one mile east of Highway 1, on the westbound side of 84 (also known as La Honda Road), with no spraying between Hildebrand and Pescadero roads.

Ms. Navarro also issued a statement March 10, saying Caltrans will not give up herbicide spraying in San Mateo County, as residents had requested.

In 2012, the San Mateo County Board of Supervisors banned broadcast spraying of herbicides everywhere in the county except at its two airports. But Caltrans has continued to spray in the county, despite the fact that it has stopped spraying in other counties when asked to do so.

Local residents, who have formed a group called "Protect Our Watershed," say they fear the herbicide spraying will pollute local water sources and harm the health of local residents and others who use the roads, such as cyclists.

According to Ms. Navarro's statement, Caltrans has "evaluated mowing as an alternative to herbicide application and have determined that it is not feasible to implement this activity in a manner that is safe." The statement says the highway has limited sight distance around curves and mowing equipment cannot safely be dropped off and loaded.

"Herbicide application guarantees the eradication of the noxious weeds and promotes fire safety and can be done quickly," the statement says. "Mowing along Route 84 could take four to six weeks to complete, if it were safe to do so."

But those who study the toxicity of chemicals used as herbicides warn that those Caltrans plans to use have dangers.

Patty Clary, executive director of Californians for Alternatives to Toxics, said the herbicide Milestone has the active ingredient aminopyralid. "This chemical made headlines when — after compost caused stunting and maiming of crops — it was found to persist at toxic levels in compost containing grass or manure from areas where it is applied," Ms. Clary said.

The label "requires 'restricted entry' for 12 hours," she said. "The interested public should question why … (they) can potentially be exposed to a pesticide during the restricted re-entry period without warning," Ms. Clary said.

The herbicide EsplAnade 200 SC has the active ingredient of indaziflam, she said. The label says it is "classified as a high potential for reaching surface water via runoff for several months or more after application," Ms. Clary said. "It is toxic to fish and aquatic invertebrates. Is harmful if absorbed through skin or inhaled."

The third herbicide, Matrix SG, has the active ingredient rimsulfuron, Ms. Clary said. "It is a potential groundwater contaminant," she said.

For now, the only defense local residents have against the spraying, is to post "No Spray" signs. Ms. Navarro said properties with "No Spray" signs will not be sprayed, and no spraying will take place in the towns of La Honda or San Gregario.

Ms. Navarro said herbicide spraying takes place only once a year. Residents say it has occurred several times a year in the past.