For the second week in a row, the California Department of Transportation (Caltrans) has at the last minute 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 on 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 the county has no authority over Caltrans. Although Caltrans has stopped spraying herbicides in other counties when asked to do so, the agency continues to spray herbicides alongside its highways in San Mateo County.

Local residents, who have formed a group called "Protect Our Watershed," have protested Caltrans' spraying, saying they fear it 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.

"In general, the use of herbicides is more effective and efficient," the statement says. "Herbicide application guarantees the eradication of the noxious weeds and promotes fire safety and can be done quickly. 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, says the herbicide Milestone has the active ingredient of 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 herbicide's label "requires 'restricted entry' for 12 hours" she said. "If a worker is to re-enter earlier, they are to wear coveralls and waterproof chemical-resistant gloves." "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.

"Milestone is very persistent with a half life of 50 to more than 550 days depending on environmental conditions," she said.