Operas by Jules Massenet don't grow on trees. If the truth be known, operas by other composers have seldom been observed to vegetate in this manner either, but that's beside the point. What's not beside the point is Massenet's thoroughly beguiling "Cendrillon" ("Cinderella"). The reasons for its prior neglect in this country can have had little to do with musical quality, accessibility or stageworthiness, because this charming comic romance is notably strong in each of these respects.

It would be hard to imagine a more auspicious vehicle for the launching of the Washington Opera's 1979-80 season, especially in the entrancing, handsomely mounted production introduced at the Kennedy Center Opera House Saturday night. All the more so because uncovering unfamiliar gems should be and is a prime order of business for the troupe.

The opening night performance was also a worthy tribute to George London, to whom the current Washington Opera seas has been dedictated, upon the announcement of his formal retirement as general director of the organization due to the state of his health. This "Cendrillon" forcefully illustrates the very conjunction of values-- musical, intellectual, dramatic and poetic-- which London so movingly embodied in his own renowned career as a singer and an impresario.

Such a concurrence of values calls for superior artistry in a multiplicity of media, and this was conspicuously in evidence Saturday night. The pit conductor was Marli Bernardi, who is musical director of Canada's National Arts Centre in Ottawa, where the present production originated. It would be hard to overpraise the authority, refinement and vibrancy of his conduting, which not only held the singing to an ideal balance and flow, but also elicited uncommon sonority and solidarity from the Opera House orchestra.

Equal praise must go to the stage direction and choreography by another Canadian, Brian MacDonald, who has devised movement and gesture that is lucid, tasteful and full of witty invention, yet never unmindful of the romantic underpinnings of the opera.

No operatic performance, of course, can be more effective than its vocal cast, and here again, the Washington Opera production could scarcely have been more fortunately endowed. The singing of Frederica von Stade in the title role was itself the stuff of fairy tales, the kind of svelte tone-spinning and dynamics that send telltale chills up the back of one's neck. A promising neophyte only a few seasons back, she's plainly emerged as an operatic artist of the first rank, and it's not just her voice which so testifies, but her entire portrayal, in its warmth and confidence and tact.

Massenet anticipated Richard Strauss by making Prince Charming, like Octavian in "Der Rosenkavalier," a "trouser role" for a mezzo-soprano, and much of "Cendrillon's" vocal enchantment comes from the ecstatic intertwining of the two female voices-- the Price and Cinderalla-- in lithe, lustrous consonances. Von Stade is ideally "partnered" in this instance by Britisher Delia Wallis, whose elegantly phrased and effulgent singing blends perfectly with that of the heroine.

As the Fairy Godmother, coloratura Ruth Welting presides over her retinue and the lovers with a voice of silvery accuracy and fleetness. Maureen Forrester, Elizabeth Pruett and Judith Christin latch firmly onto the comic opportunities of the stepmother and her unseemly offspring without squeezing the life out of them, and John Reardon is his usual dramatically persuasive self as the father, Pandolphe.

Cinderella has been a popular subject for opera and ballet since at least the early 18th century. Massenet's version, a considerable success in its Paris premiere in 1899 during the composer's 57th year, is no earthshaker, but its melodic and harmonic felicities, the impeccable instrumentation and overall craftsmanship, insure an enduring appeal. Moreover, the peaks of the production coincide with those of the score-- the magical appearance of the coach in the ballroom scene, for instance, and the ensuing lovers' duet; Cinderella's monologue and the interpolated dream scene of the third act. In all of this, Henry Bardon's storybook sets, Suzanne Mess' costumes, Neil Peter Jampolis' lighting and Charles Elsen's makeup are potent assets, as we are the dancing of the corps de ballet and the singing of the Washington Opera chorus throughout.