ONCE AT Longwood, the lavish garden in Delaware, I noticed a festoon of pink flowers falling from the roof of a small low building and wondered what sort of showy vine it was.

It turned out they were ivy-leaved geraniums growing in boxes on the parapet.

Now as a rule it does not work well for gardeners to treat a tree as a bush or a vine as a bedding plant. But the annual geranium (it is not hardy over winter, but of course can be wintered over indoors) naturally sprawls about, and shows itself off best when the plant is above eye level. So the Longwood boxes on the roof were in keeping with the natural growth habit of the geranium. But the effect was of a constant-blooming rose carrying on after the fashion of a bougainvillea.

Some gardeners do not like geraniums of any kind and I can see why. For too many years the geranium has been used as the easiest way out, for providing all-season color in sunny or half-sunny spots.

I myself had a belly full of them when I once spent a summer in California, a dry, dusty place with no trees to speak of and a lot of cars and glass and hamburger joints and geraniums.

Still, there is no point cutting off your nose to spite your face, and there are times that a deep-seated prejudice against the geranium should be overcome. They overcame at Longwood, and the effect was full of charm; and I mention all this to justify my recent crime of acquiring three plants of the ivy geranium 'Sybil Holmes.'

A crime because they cost $2,50 each, an insane price, that only a fool would go along with. Let me say that ususally I am not so gullible.

Anyway, there they are. They will live in a copper tub.

Assuming they do well-and under my care it is not a foregone assumption-nobody is likely to be reminded, seeing them, of the garlands of Longwood that inspired them.

This may be the place to say that great gardens (any by great I mean merely large, expensive, with ample labor, and proper maintenance and good design and some attention to richness of variety) are always useful to small gardeners.

The roof parapet full of procumbent geraniums may become a mere tub of them, in the small garden, but often the general effect is first seen and admired in a lavish garden.

So that it is not true at all to say of great gardens that they have no useful lessons for the gardener on a small city lot. My own cat-run owes something to the greatest gardens and to monuments of architecture as well.

My system of four-by-four posts through which you see the pool and then the horse trough and then the garage owes a good bit to the great church at Canterbury.

This also may be the place to say it is well not to echo or "quote" the original source (Canterbury, for example) too insistently.

Posts and horse troughs (though no horse, thank God, has yet wandered in to drink) give me the elements I admire at Canterbury, the light and perspective and illusion of lenght and so on. But I would be wary of using Caen stone and 12th-century. For fitness and rightness is the thing, and my modest back yard cat-run should not look not much like the cathedral. I think I may safely say that nobody, peering at my back yard, would think of the aisles and ambulatries of Canterbury (Canterbury does not have many weeds in the transepts or the choir, as I recall) and yet I would have done the thing differently if it had not been for my admiration of the 12-th-century churches and Edfu and Karnak and so on.

This also may be the place to disclose my notion that a garden is rather a monumental thing. I do not understand anything of gardens made only for sitting down in and wandering about in.

Once somebody said my garden reminded him of a parking lot in Burma (not detecting the Canterbury-Karnak source, apparently) and his criticism was fair.

But it is, of course, for me, not him, that it was built, and it is to my vision, not his, that it must increasingly conform.

If there is one merit I have, it is that I know better than most people the gardener's tensions and impatience and occasional despair at how slowly the great design takes shape. This is true whether the garden is Bodnant or Villa d'Este (to name two big ones) or a trashcan-studded 20-by-30 garden in the middle of town.

I think of some gardener of the 17th century who fidgested four years after he planted the acorns at how long it was taking his 2,000-foot avenue of oaks to get goint. And I think of the gardener of Capitol Hill who bravely sawed down the Norway maple and is now waiting, in some anguish, for the camellias to achieve some size.

It's not the finished glory that counts. The readiness is all.