A meadow garden at 12th and O streets NW will be dedicated at 2 this afternoon, a year after the firs seeds were sown.

The site is a vacant lot adjoining a substation of the Potomac Electric Power Co., surrounded by a high chain link fence painted black -- a highly sensible step that makes the fence less obtrusive and easier to see through from the street.

Pepco paid for the planting, except for a row of purple-leaf plums along the street, paid for by the Logan Circle Community Association. The land was originally flat, and I would have left it that way if it were mine, but everyone has his own notions, and a good many tons of earth excavated from a Smithsonian Institution project were dumped and shaped into hillocks.

There is nothing wrong with the idea, and the dirt was free, I was told, but I am a great believer in leaving a grade alone. The bumps are thought, however, to add interest to the site, and the result is not unpleasant.

The designer is Larry Frank, who worked up a plan for the garden as a civic contribution, and Marvin Moss, representing the local citizens, has headed a group that has spent many hours turning the place into a meadow of flowers.

This past week the prettiest thing in bloom has been a flax, of lovely sky blue, collected from the Black Hills of South Dakota. It is called 'Appar Lewis' flax and was selected for its exceptional vigor and ability to compete with weed grasses, as well as for its beauty. It is rare for a plant so graceful and delicate to be as obliging as a dandelion, and I would happily give it a spot in a garden where only plants of stunning beauty are included.

Another showy, if weedier, flower of the season is the dame's violet, or sweet rocket, with deep mauve flowers. Sometimes the flowers are sweetly scented toward dusk, and in gardens it seeds itself about and is an agreeable weed. If it did not bloom in May, when the whole world is in bloom, it would be more esteemed.

There are a couple of kinds of wild coreopsis, some blue bachelor's buttons, and a pretty catchfly (Silene armeria), a handsome medium magenta now making a pretty display visible for some distance. Also conspicuous is the annual Siberian wallflower (Cheiranthus allionii) in brassy yellow-orange, and later in the season will be some evening primroses (Oenothera lamarckiana) with a long succession of light yellow blooms, and still later black-eyed susans.

The corn poppies have not succeeded, but a toadflax (Linaria maroccana) is coming along. The kind they have is said to be pink or violet. I once grew an assortment from exotic places like Bulgaria, but the one that seems to me prettiest is the yellow and white kind I used to see on steep banks in waste places, but do not see nowadays.

All these things come up in the grass and sow themselves about. Within a few years, as they find the places they like best, the effect will be utterly natural, I think, and far more pleasant than an open vacant lot colonized by beer cans.

This spring, thanks to unseasonably warm days in late winter, has been quite early for some things. My hardy waterlilies are a full month ahead of themselves, and some garden day lilies are already in flower, though I never expect them before June 16. On the other hand, the clematis and pink locust and pansies are right on schedule. Irises are two weeks ahead of schedule and so are the early rugosa roses. 'Agnes,' my earliest one, is already out of bloom.

People downtown get much comfort from the National Geographic Society's splendid new garden fronting on M Street west of 16th. There should be a special commemorative medal given by the city for this effort, the work of Urban Associates. I have regretted, in a mild way, the choice of rhododendron and a certain tendency to plant things in lines rather than clumps, but these are matters of individual preference, merely. I also loathe the huge rocks sprinkled about and regarded as sculpture, but feel that future generations will become sufficiently annoyed by them they will be removed in time. In any case, they are hardly the garden designer's fault. The absurdly narrow little canal also seems to me a mistake, since water is abundant here and it might have been a pleasant pool with waterlilies.

All the same, the huge 'Bradford' pears are exquisite at all seasons, however ubiquitous they are becoming, and great thought has been given to contrasts of texture. My spirits rise every time I pass this garden and general public thanks are due to James Urban and the authorities of the Geographic.

Their pansies failed this spring, because of an outrageous winter of below-zero temperatures. I thought they had used rather too-large plants in the fall. These always suffer more than smaller plants in cold weather, but they were a joy when they flowered around Christmas, and another year they should be successful and I hope will be planted again.

In Europe they do not seem to be familiar with the Washington thorn, though anyone can see it is much lovelier and a far choicer garden plant than any of the European thorns. The new Geographic garden inclues a fine clump of them, which partly atones for having cut down a superb row of them on M Street (except for one, which remains) and this thorn is also newly planted in the Pepco meadow garden. It is always reassuring to see a first-rate shrub or small tree planted, like this thorn, instead of something nondescript.

It is my fate to live on a street planted with perhaps the ugliest maples visible in the temperate world. They are short-lived, thank God, and don't even color in the fall or bloom handsomely in the spring. A certain flair is required, probably, to choose a tree so wretched. Fortunately most of them are dead or dying, but I often visualize (in moments of evil fantasy late at night) the imbecile who chose them, though he probably is, or was, not as revolting as I picture him when I think of his maples.