THE QUITE LARGE garden of the Averell Harrimans was much improved, they think, by chopping down a large tree on the lower level. Mrs. Harriman, who is English, turned that part of the garden into a place for homey flowers -- tulips are having parades on that ground at the moment -- which would never have survived under the tree.

At the same time they imported a quite large evergreen magnolia and some nice young (but possibly 20 feet tall) cryptomerias. (The Harriman garden is one of eight on the Georgetown Garden Tour Saturday, a benefit for the Georgetown Children's House. Another group of nine gardens will be open Friday.)

From the house -- they still take pains to polish the brass and keep things in a great state of good order -- doors open to a terrace carpeted with what I think is called Houston grass, that manufactured turf on which professional athletes play in Astrodomes, and the effect is reasonable and handsome. Mrs. Harriman found some old provincial garden chairs (the backs hinge down to cover the seats when not in use) and had them refinished gray.

Further steps lead down into the garden past an extremely large old box bush that must have seen 150 winters at least. You take a turn to the right to go down the stairs and are delighted to see the falling terraces connected with masonry steps.

Mrs. Harriman thought the upper part was a bit heavy looking with so many evergreen plants -- rhododendrons, magnolias, camellias, and introduced a large willow tree. It is the devil's own task to keep snipping it back so it doesn't overwhelm the garden, but it serves a lightening purpose and is very beautiful.

Along the way to the bottom of the garden, you notice little square beds set in the brick pavement and edged with box which appears to be 'Kingsville,' a very dwarf sort. Roses used to grow here, and Mrs. Harriman said she never quite had the nerve to get rid of them until they died naturally. Which, of course, the pampered hybrid teas of today all do, soon enough.

These empty box-edged square beds will be crisscrossed with regular dwarf box, and the triangles filled in with begonias. It will be neat and colorful, and when the begonias come up next fall it would be surprising if she stuck in thick clots of early spring bulbs and polyanthus primroses, which would provide eight weeks of color in late winter and early spring, followed again in late April with tender bedding plants for the summer.

Such small beds might also have agapanthus and tuberoses for summer or, if the begonias began to seem tiresome after a few summers, Japanese memones and small chrysanthemums, with sternbergias and Aster frikarti for late summer and early fall. the possibilities are endless -- there could be lilies --box edging and brick pavement would always look good, no matter how the flowers came and went.

But I have dawdled too long on the square beds, and now race further down the steps holding to iron rails.

Visitors should be careful with these railings, lest they upset the pink Clematis montana growing on the spindles. this is a robust but manageable vine with small flowers in early spring, before most other clematises are about. My own view is that people tend to bump into virtually anything not surrounded with sabers, and I wish the clematis well. I, at least, was very tender with it.

We pass a little bastion of camellias, a number of plants of the waxy semi-double starry 'Magnoliaeflora,' a great favorite of everybody's, and which has been full of bloom despite the winter which entitled it to sulk.

The Harrimans did not have the usual Georgetown agonies over gardening space, since they bought an additional house and land to the east. Mrs. Harriman surreptitiously sneaks off from time to time to putter in her underground greenhouse. This is in a basement, fixed with what I suppose we may as well get used to calling Gro-Lights.

They are superbly efficient for replacing sunlight, and the Harriman streptocarpuses (which they bring in to decorate the rooms of the house during winter, along with the orchids) are most successful. Space is found to winter a cherished plant of lemon verbena, which is not quite hardy outdoors in Washington. There are some coleus plants, minus their usual mealy bugs, because Mrs. Harriman is not one to tolerate mealy bugs, and a lot of seedlings in flats. So many summer flowers are best started indoors up here.

There is a spectacular double flowering cherry, with clusters of flowers the size of canteloupes. No matter what one thinks of double flowering cherries, I defy anybody to deny the spectacular beauty of a very well-grown tree at the peak of its bloom. A more delicate shadblow is nearby, for those who require purity at all times.

The swimming pool would make a beautiful lily pool for tropical water lilies. It is just a thought.

No doubt it does very well as a swimming basin, too, and I doubt the local dolphins have any intention of giving it up. Less athletic guests can doze, I would think, in the green shade of a pretty paved garden on a terrace even lower than the pool. It is overhung with trees, paved with soft gray stone, and bright with tufts of green and white variegated hostas.

Here you can look up the handsome flights of easy steps all the way back to the house, admiring from a different angle the trees and other beauties one has met on the way down.

The little gray gazebo is a bar, if one veers a few feet to the west, and behind it dressing rooms for the pool. The whole garden has a good feeling to it, of having been there always, and not trying very hard for effects, though full of pretty things and pleasant little surprises like the clematis on the stair railings.

One of the marvelous gardens of the Western world is that of Lester Collins, the noted landscape architect and scholar, but it's not at all what you probably expect.

It is only 19 feet square, and Collins expects visitors on the Georgetown Garden Tour Friday will keep moving, through the house, through the Back 40 and out the side gate, otherwise he is in trouble. The house is 14 feet wide, so there's not much backup space there for dawdling.

The first startling thing you see is a grape vine at the Collins front door. Do you expect some great rarity from the Chinese-Tibetan borderland? Not at all, it is the wild fox grape of New England, rising the full height of the house, with a stem 10 inches thick.

It would be rude to ask the Collinses why this grape does not pull the little house down.

Off the living room you see the world's greatest greenhouse, 39 inches wide, nine feet high and 10 feet long. It has a Plexiglas roof that hinges, so in the summer god's rain falls right in, and in the winter (for the plastic has gone a bit swayback in the middle) it collects a bit of water for cardinals to bathe in. In only a few places in this capital can you see cardinals bathing above your head. Collins admires them because they are perfectly red underneath -- "so many birds run to gray," and of course it makes a difference if you're seeing them from beneath.

This greenhouse (heated by a fan blowing across the copper coils of the regular hot-water tank for the house) boasts a creeping fig about 15 years old. It festoons walls and some of the roof. In the summer, with the roof off, it shoots up to the top of the dwelling.

It gets whacked back in winter, and has to make do with growing inside the greenhouse until the next summer when it zooms forth again.

In the back garden you will notice a gnarled trunk of the native aralia, much encumbered with a vine or two but no branches to speak of. That is because the Collinses let them sprout up to the top of the house in the summer --wonderful -- but saw them off in the fall, to catch the winter sun. Just now the aralia is sprouting for its dizzy ascent this summer.

There is an iron park bench, the supports of which end in either a mythical beast or the head of a cock, in cast iron. You can sit in a lot of parks, even in France, without seeing its like.

There is also a white marble chair, perhaps of the Italian Renaissance. It is just what Savonarola might have had for his enthronement if he had not run into trouble.

The floor of this garden is strewn with pine needles, gathered from such a romantic spot as a local garden center where they sell them in bales.

Several kinds of bamboo are somehow squeezed into the garden, along with a Korean fan palm (which laughed at the winter, though it had no protection) and a curious little Holland oak, which is evergreen but does not look like the usual evergreen oak at all.

Note the little standard of English holly, small but lovingly tended over the years, and the weeping cedar grafted on a standard. There is another weeping cedar, too, except it is not in such grief and does not weep so much.

Collins thinks that in congested places like Georgetown it is all the more important to have a garden as another room to sit in and look at outdoors. He has got over the gardener's usual anguish of not having enough space for his favorite plants. In a garden smaller than a livingroom restraint and discipline is imposed.

Nevertheless, it is good of Collins not to carry on about not having space -- as most gardeners do.

There are a few thick sky-blue tiles, just 21 of them, for a touch of color. The weeping beeches (there are two of them, believe it or not) will be in young leaf -- nothing in the whole vegetable kingdom is more likely to knock the gardener cold than these trees in their beauty. The bit of pavement is of granite blocks rescued from the old M Street streetcar tracks. The striking sculpture is simply a New England granite fence post, and those Henry Moore-type holes are merely where the fence rails went through.

This garden shows, for the millionth time, that a garden need not be large to be lovable (god wot) and the greenhouse shows you that life in 39 inches may perfect be.

All the gardens on tour have some particular charm or merit, and it seemed a good idea to dwell a bit on one so tiny since it shows as well as any what can be done.

The vast garden of the Huntington blocks, by contrast, boasts a cross-axis of 30 feet, perhaps 60 feet in depth.That may be stretching, but anyhow it is enormous compared to the Collinses' forest. The Blocks are on the Saturday Georgetown Garden Tour.

Here you will see the "historic" rock on which dolley Madison is said to have stood to gain a better view of the city of Washington. some say she sat on the rock as well. In any case, there it is, and formerly there was a Victorian garden here with a big holly and a somewhat daft tangle of herbiage. This was cleared away -- the Blocks like the sunlight -- and behold, you can now see the sky. You can also see the rock ("My, think what it cost to put that rock in," mercenary people often say to the Blocks) which of course has been there since the world first cooled and which is entirely too large to blast out.

Not that the Blocks wish to be rid of it. On the contrary, it is the garden's great ornament:

Stone stairs of sensitive design curve past it, with simple stone-paved sitting places at either end. The rock's mass is allowed to flaunt it, discreetly draped here and there by lemon thyme, the one that is gold-variegated and smells like lemon, with a few little heathers, quite dead at the moment but expected (by Gustin's Nursery, at least) to revive, and sure enough one or two of them have a bit of green, while others seem not to have been hurt at all.

There are some young (as yews go) weeping yews, a screen of hemlock, an edging of sarcococca, a young mahonia and some modest non-gaudy rhododendrons that will not offend even the most pure and prim eye.

The house, having been cleared of much of its Victorian junk, especially its entrance hallway which was the sort that makes you think The Chair is at the end. this hall was thrown into the living room, new windows were cut in the side wall, and it all looks civilized now.

Fortunately these great liberating glass windows are on the south wall, so in winter the sun streams in, gilding the off-white elegance of the place and affording the most stimulating views of the garden with its beautiful stonework. The Blocks say they know somebody who thinks the house and its garden have been bastardized -- that friend worships at a brownstone shrine -- and it is true the place conveys none of that dark tubercular stateliness that used to impress ribbon clerks. The garden is a beautiful example of letting the sun and the air in, instead of huddling (like so many urban gardens) beneath dank sordid maples.

Of course, some like the Victorian style better than others, and as for dense shade in gardens, slugs and cockroaches have always thought well of it. But most humanoid visitors are likely to admire this sunlight, this freshness, this careful construction and this simplicity.

The Georgetown Garden tour of 17 families benefits the Georgetown Children's House, a day care center, and advance ticket sales are accepted by the house at 3224 N St. NW 20007, phone 333-6252. Tickets are $6, or $5 in blocks of 10, each ticket good for either Friday or Saturday -- you get two tickets to go both days.

Gardens open Friday are those of the following families: John Warner, Abe Fortas, Mansfield D. Sprague, Mrs. Poe Burling, Mrs. E. Hood Phillips, Lester Collins, Dr. Frank L. Stroud and John R. Wallach.

On Saturday the gardens of the following will be open: Capt. Peter Belin, Huntington T. Block, Mrs. Charles E. Bohlen, Carl Weiss, Tom Scanlon, Mrs. Virginia W. Harrison, Mrs. Christian A. Herter, W. Averell Harriman and John U. Nef.