THE COMMONEST cause of despair among gardeners who pay attention to garden effects (that is, the effect of light on various masses, textures, contrasts) is that everything changes.

Everybody knows, of course, that pansies and tulips that are glorious in April are wretched looking by June, and nobody takes it hard.

It is something else when a major tree or shrub starts going back, or failing, or (equally awful) starts bounding forth in all directions, outgrowing its allotted space.

The only good thing I can think of to say for Norway maples and elms is that one can chop them down without the least pang. But I know no words to comfort the gardener whose tremendous old white oak finally dies. The garden will never be the same without it.

The new gardener should be instructed, very gently, that all plants not only die eventually (a fact that, however well-known, causes more gardening astonishment than any other) but also change and often change out of all recognition while still living.

Sometimes you see a very beautiful garden, and this has the very bad effect on innocent gardeners of leading them to think of their own gardens in terms of static pictures.

Ha, the youngster commonly says, all I need is a big Atlas cedar here a tangle of yellow azaleas over yonder, so he plants them and prepares to wait a few years for the great effect he imagines in his head. It may take 80 years for the cedar to do what he dreams of, of course, but he is brave about it.

What floors him is that the cedar does not grow as he thought it would -- it is softer, sappier, than he supposed and it grows up instead of reaching out in noble horizontal layers. When it is 20 years old a storm takes the top out of it.

Another cedar (if we may proceed even farther into horror) that was already a mature tree when he got the place, was handsome indeed in 1960.

All it had to do, as far as the gardener was concerned, was to stay just as it was, at a height of beauty. Instead, it has grown unbelievably in the past 20 years and now over-balances all the rest of the garden. Furthermore, a storm of 1970 that destroyed a major branch has interfered with its symmetry.

These are trifling crises, common to all gardeners who live long enough.

In great gardens -- not the little city lots we squeeze joy from, but great gardens -- it is a terrible thing when 120-year-old camellias have be sawed down, and when treasures of great interest or rarity have be grubbed out while still living. There is no great garden anywhere in the world, however, where this sort of agony is not routine.

Most gardeners start out innocent as little children, wrongly thinking in the back of their heads that their favorite plants are like favorite furniture. t

They have an easy life, the curators of museums, since the Persian glazed pots and their other stuff does not change.

But gardeners are on a far more complicated track, where nothing stays the same even for five years, and where the relationship between the old willow and the young yews changes every year. The patch of dusky green-black that was fine in 1970 has become oppressive and gloomy by 1980. The accent of the golden cypress that enlivened the scene a few years ago, has suddenly turned garnish and gaudy as neon, putting everything else out of countenance.

The azaleas that flamed in the woods have suddenly come to dominate it and no longer shine like welcome beacons in the soft light, but writhe everywhere in clots of sinister color.

The holly that dignified the entrance stairs has gradually (gardeners are quite tardy noticing such things) blocked the path and obscured the very picture it was meant to enchance.

It's absurd to say, as some of our more annoying teachers do, that we can avoid these pains by taking thought.

"Allow plenty of space," they are forever saying. But there is no such thing as space that will accommodate a cedar properly over a century.

Gardening is not in the least like painting a picture or writing book. It is very like running a government or coddling along a civilization.

Nobody, least of all the gardener (thank God) knows in the beginning what wars will come, what hostages will be taken, what amendments will be rammed through, what failures of the judiciary will at last be seen.

One of the most obvious (and least suspected) changes of a garden is a change of light. As a tree grows, as shrubs grow, as vines grow, the shade increases in the garden and the light alters past recognition.

Sometimes it is better to saw off limbs, chop out old mock-oranges, with every leisure breath. Other times it is better to accept the new light, or the new dimness, and forget the bright popies that used to be so wonderful along the path. Either way, changes will not cease and adjustments are going to be made, by Nature herself if not by us.

There is, however, one extremely important thing to add:

Just as growth and change alter gardens for worse, making it necessary again and again to stand helpless as the blue grass by the pool fails, in the increasing shade, and as the great locust splinters at last in a storm, so do time and growth improve a garden in ways so happy it would never have occurred to the gardener to plan them, or even to think them possible.

In how many gardens has an Alexandrian laurel, tucked in years ago for the hell of it among oddments, developed into a specimen worth traveling a long way to see? Or a holly, once meant to thicken a screen of shrubs, that has taken on a life of its own and now stands majestic as a centerpiece, long after the shrubs it was once meant to blend with, have died?

Again and again, if you examine the world's greatest gardens, especially in the British Isles where most of them are, you will be struck by the marvel that plants (perhaps set out a century ago) have composed themselves into so magnificent an array. You think what skill, what brilliance, sited them so perfectly, with such foresight.

But of course it is far otherwise:

The effects the orignal planter intended have been so long forgotten, so long eclipsed by time and accident, that now we give him credit for planning he never planned.

All the great treasures (it may easily happen) have long since vanished. Quite a new and quite unintended groupings have taken their place. The planter never knew the crytomerias would do so well, and never knew his collection of witch-hazels (that were intended to be the glory of this dell) would sulk and dwindle away.

It's lucky for gardeners that Nature is more subtle (as well as cruel, more gross, more clumsy, etc.) than even the most sophisticated gardener. The common ivy fails, and the gorgeous mutisias grow like weeds. The common grape dies, but the pink clematis drapes a 40-foot dogwood -- though neither plant was ever intended to be "important" in the garden.

You plan it and plant it with love and in ignorance. What indignation over the years. What surprising flurries of pleasure. Some day it will be very beautiful, only not as you planned it. Almost as if you had nothing whatever to do with it and had never been there.