The head of a national lawn care company, under pressure for two years from citizen groups and county officials, unveiled yesterday a plan to warn Montgomery County residents after pesticides have been sprayed in their neighborhood.

The voluntary plan announced by ChemLawn to post signs after chemically spraying lawns comes in the wake of legislation proposed by County Executive Charles W. Gilchrist to force chemical companies to post such signs while spraying and penalize them if they don't.

Gilchrist said he thinks ChemLawn's proposal is a step in the right direction and that his bill may have made the firm offer voluntary measures that some residents were demanding two years ago.

Council President Michael L. Gudis said that if ChemLawn's plan works and other companies in the area follow suit, the council might vote to make such signs mandatory after spraying.

"Now all we need is the rest of the lawn care industry and private individuals to put up signs," said council member Esther P. Gelman.

At a news conference yesterday, ChemLawn President Jack Van Fossen, flanked by Gudis and Gelman, said Montgomery County will be the first area in the country to have the 4-by-5-inch yellow flags available for private customers.

Van Fossen also said his company will provide government officials with information about the chemicals used and will maintain a list of names of people who request notification in advance of spraying. He said ChemLawn will use the list to notify customers and their neighbors, who may be hypersensitive to certain pesticides, the day before lawns are sprayed.

Von Fossen said he expects his workers to start placing the signs on 20,000 customers' lawns in two weeks.

The yellow sign reads: "Lawn Care Application. Keep Off Until Dry. Customer: Please remove when application is dry."

The signs are similar to ones now placed in parks and other chemically treated property in the county. The council decided six weeks ago to place orange signs on public land to notify the public that the lawns had recently been sprayed.

ChemLawn sprays customers' lawns five times a year.

Marjorie Smigel, ecology chairwoman for the Springfield Garden Club, praised the action by ChemLawn which she said her group had sought for two years.