Many of the flowers and vegetables sold at the annual Washington Cathedral flower mart, to be held May 9, are grown in a battered high-school greenhouse by students in one of Washington's oldest vocational education programs.

Founded before World War II when Langston Golf Course on Benning Road NE was a large school farm complete with chickens, the horticulture program at Pehlps Career Development Center has gone indoors.

In a white plastic greenhouse at 24th and Benning Road NE, 55 to 65 students and two teachers grow thousands of flower and vegetable plants which they sell or give away throughout the city.

"We've got students who work half a day, some doing landscaping and maintenance at places like D.C. General Hospital, Washington Cathedral, the Naval Research Lab, the home for the incurables, a church and a commerical florist and sometimes the National Park Service and the Arboretum," says teacher Richard Taylor. Taylor managed the Howard University greenhouse before coming to Phelps.

"And the good thing is that many of these kids, who some people call deadend kids, are now out working in places like the park service, or Hechingers," said Taylor. "I've got two graduates now assistant managers at Hechingers, another getting a horticulture degree at the University of Maryland and one at Geroge Washington University Medical School."

Under the painted white plastic of the greenhouse, Taylor's students are growing hundreds of boxes of beefsteak and Better Boy tomaotes, as well as dozens of varieties of other vegetables. Long tables of flowering impatiens and geraniums make this hot-house classroom stunningly bright.

Until this winter when additional panels of heavy-duty plastic were installed to protect the greenhouse, there were break-ins almost every weekend. Vandals have thrown everything from pebbles to bricks from the hill above the greenhouse onto the plastic roof, said Don Juncal, a former golf course and Cincinnati parks department employe who has taught horticulture in D.C. schools now for 11 years. "But then a greenhouse, the nature of the building, is almost an invitation to throw rocks at," said Juncal.

Juncal and Taylor say they try to ignore these things and concentrate on the good that their program is doing, the students it has helped and the pleasure that plants bring to so many people.

Many of the school plants are sold, which Taylor says brings in about $2,000 a year for the Philps' landscaping program, which otherwise might founder on its meager $1,600 annual school budget. The landscraping program is part of the school's major construction and auto mechanics programs.

"What we teach in these tight-budget times is how to get by on little money," sid Taylor. "All these geraniums came from cuttings from that dozen plants over there. Seeds are too expensive. But aren't they beautiful? Look at this red variety . . . So many children grow today knowing nothing about plants, stepping on tulips and just not caring." Taylor said the program is trying to teach children to care.