Tulips, daffodils, freesias, irises, pussywillows, mums, their heart-catching colors break through the grayness of a drizzly, excessively cold spring day and stop a passerby hurrying down S Street from Connecticut Avenue.

He pauses, looks, touches a petal, asks a price, sighs, turns away and turns back again, selects a bouquet and is on his way, another springtime example of a growing phenomenon. For the past several years Washingtonians, men as well as women, have been buying fresh-cut flowers for themselves and others more than ever before, according to merchants with rapidly expanding flower trades.

"People are indulging themselves more and more with flowers," said Joyce Chaconas recently at the 18th and I streets NW outlet of Peter's Flowerland, a chain of flower stands that her husband, Peter, started 12 years ago. "We have to do very little advertising; the flowers on the sidewalk are their own advertisement. People walk by, see them, and buy."

In three years, for example, the Chaconases have opened five Peter's Flowerland sidewalk stands in Washington's downtown business district. They are leaders of a new breed of sidewalk flower vendors who have tapped a growing, lucrative flower market: the impulse buyer.

No longer are funeral parlors and wedding caterers the main focus of interest for fast-tracked flower retailers. The spontaneous, self-indulgent buyer is to the flower industry what China was to Coca Cola: an uncharted land of opportunity.

The blooming of Washington is part of a national trend. The flower industry, described by Forbes magazine as "almost catatonic" just a few years ago, is increasing its sales by 10 percent a year by offering its products in supermarkets, elaborate sidewalk kiosks and just about anywhere that a funeral wreath is not to be found.

Banking on the impulse buyer, the Society of American Florists expects that 1981's total sales volume of $5 billion in flowers and potted plants nationally will double by 1985. Total sales figures for Washington are unavailable.

In Washington the boom is most visible on city streets, where vendors with buckets of flowers and more elaborate stands are breaking with the traditional ways of established flower retailers.

"It doesn't matter the price," says Moussa Chamma, known as "Mike" to the buyers at his stand on Connecticut Avenue and S Street NW. "I have good quality. People spend money to get what they want."

He keeps a diverse stock.

"Some flowers, mums, daisies, carnations, people aren't buying them like they used to," he said. "I have freesias, sweet william, eucalyptus...people don't know the name, but they ask, 'What do you do with these?' And they buy."

For Michael Caruso, whose great-grandfather started a flower business from a horse-and-buggy stand at Eighth and F streets NW, the expanse of extravagantly colored flowers in front of his M Street NW store is an advertisement, not a principal source of sales. Like most flower shop owners, Caruso depends upon telephone sales and on life's inescapable events, such as weddings, funerals, christenings, illnesses and holidays, for 70 percent of his trade. Yet he, too, is aware that his future is in the new customer.

"People are really getting into flowers," he said. "Lots of them are coming by once a week. They've put flowers on their shopping list."

"Several years ago there wasn't so much variety," Caruso added, standing in front of the filled, glass-fronted cooler in his store. Opening its door, he reverently removed a deep-blue iris. "Isn't it pretty?" he asked before replacing the flower among its jeweled brethren.

Caruso's store has a larger selection of flowers than his father ever sold. There are sky-blue delphiniums, shocking-red anthuriums and delicate windflowers among the array. Like every wholesaler and retailer interviewed, Caruso attributes the rich variety largely to Holland's pioneering work in selective breeding, centralized merchandising and overnight shipping of its year-round crop of hothouse flowers. Innovation has paid off for the Dutch, who last year sold $27 million worth of fresh flowers to the United States, or$13 million more than in 1981.

"There are a lot more flowers coming in," said a salesman at A. Gude Sons and Associates. "And Holland is sending more than they used to. They the Dutch have come up with weeds they make into flowers. Every year there are new ones, and customers are getting more sophisticated in their demands."

It is just before 10 a.m. on the 1300 block of I Street, and the salesman at A. Gude Sons is storing the fashionable alstroemeria, a lily developed by the Dutch from a wild Peruvian plant, in the store's room-sized cooler. The fragile blossoms look like baby orchids atop their tall stems, and Holland sells them in colors ranging from shell white to hot orange.

This is the hour when wholesalers scramble, setting flowers in buckets for vendors on their way to street corners, putting away the morning's fresh lot brought in from National and Dulles Airports and telephoning to California, New York and Amsterdam for the following day's supply.

Ted Johnson, the manager at A. Gude Sons, is speaking into a telephone and gesturing at his listener on the other end. Johnson orders most of his flowers from growers in California, and one of his salesmen explains that wholesalers run into problems when a flower lot is bumped from a plane in favor of a full load of passengers.

Air transport is crucial to the flower trade. Johnson can call a dealer in Amsterdam on a Tuesday and ask him to bid on a lot of freesias at the daily auction, then receive the fragrant shipment in his store Thursday morning, after a transfer through New York.

New York wholesalers shuttle gerbera daisies from Israel, chrysanthemums from Colombia, and orchids from Costa Rica to the District. The flowers, properly cut at the stem before each change of water, can be expected to last at least a week on a consumer's office desk or on the dining room mantel.

By noon, having placed the next day's orders and stored the unsold wares, A. Gude Sons is almost ready to call it a day. Other wholesalers rely on bucket vendors, equipped with $15 D.C. vendor's licenses, to buy flowers that have spent several days in the shop. Gude prefers to deal with decorators and florists.

"When the flowers start getting old, I get rid of them," Johnson said. "I'll call up a decorator that I know handles a lot of parties, who might need enough flowers for 50 arrangements, or a florist who does a lot of funeral work, and give them a big discount."

For the less specialized, the street market has a variety to offer: daffodils, roses, daisies. At a five-bucket stand at 15th and K streets NW, Lonnie Crawford, dressed in work coveralls, stops and selects a single rose for his love.

"Flowers are closer to her heart than what I can say," he explains.

At one of two competing stalls in front of the K Street NW exit of Farragut North Metro station, Nhung McLeland, a chemist who works nearby, studies and ponders before putting together with a painter's eye a perfect bouquet of one peach-colored lily, one cream-tinted fringed chrysanthemum, and one pink carnation, which she will place on her office desk. She is the flower merchandiser's dream: an upwardly mobile professional who buys flowers frequently and on impulse.

"I don't buy them for others that much," she says. "I buy them when I see them and when I have time. A good selection is so much more available on the street than it used to be."

Whatever the flower lovers' motivations, there is something seemingly extravagant about flowers, whether it is the single calla lily that Peter's Flowerland offers for $4, or the bunch of 10 daffodils for $1. There is something reckless and irresistible about the mad colors blooming against the concrete starkness of the urban sidewalk.

Dusty Morse, a flower salesman, offers perhaps the most telling motivation when he remembers how he used to buy flowers for his apartment as often as he could afford: "I decided I didn't feel like waiting for other people to buy them for me. So I started getting them for myself."