SOMEONE HAS given me two nice young seedlings of the Arabian thistle, Onopordon arabicum, and I gave one to my elegant friend who has great success with everything.

The other I planted, after some weeks of thought (or some weeks of avoiding thought), on the west wall of a shed. This thistle is precisely the sort of plant I admire - easy to grow, monumental and architectural in scale and stature, superb foliage like gray acanthus leaves with fierce spines.

And virtually impossible to site in a jam-packed garden. It is a biennial, having a two-year life cycle. Surely I do not need to warn gardeners aganist ordinary thistles of our pastures, which are immortal and cannot be got rid of in less than two centuries. Our wild thistles are relatively beautiful, but they spread underground and are a great terror.

The Arabian thistle is also a terror, but since its root is not perennial, and since it relies on seed to maintain itself, it is quite easily controlled.

It makes rosettes the size of a bushel basket the second year and sends up a flower stalk maybe 10 or 12 feet high. But the main thing is the silvery gray foliage.

If you happen to have a wall of clipped yew 14 feet high, as I do not, this thistle is very beautiful, planted about 4 feet in front of it.

The trouble is that its large basal rosettes, sitting on the ground, effectively smother any little plant in the way. Another trouble is that when the flowering stem reaches 10 feet, it is vulnerable to wind and can come crashing down like a small redwood in a summer storm, its spiny leaves doing moderate damage to any fleshy plant or animal in the way. Needless to say, a good stake, somewhat like a telephone pole, and very firm ties, like insulated wire, will keep it upright.

One small problem, needless to say, is where to find space for a such a plant.

Fortunately in during foundations for the shed several clumps of daylilies were smothered (my chart at the time showed I had moved tham, but some error transpired) so that is where the thistle went.

Not boldly in a fine well-prepared site. But niggardly, sort of under the eaves. Maybe it will not exceed 5 feet in height. I do not want it to flourish too much, or it will mean trouble for the Red Bengal on one side.

How much better a garden would be if it were planned in all details at the start, and the plan adhered to.

But when we start gardening we do not, as a rule, know any more about gardening than an architect.

We learn as we go and we accumulate as we learn. I have several times planted seed of this very thistle with no luck. So years ago I gave up the idea of growing it.

But when you get two nice young plants in pots, nobody can just say "Thanks, take them back, I have no space." At least I cannot, at least in the case of the Arabian thistle that I so admire.

Another case: When I moved to this new small garden I ordered the lily 'Black Beauty,' a rather foolproof dark red hybrid of Lilium speciosum. The lily specialist was unable to provide it, so I settled for something else.

No sooner did I have all the space filled up than someone presented me with several small bulbs of 'Black Beauty.' Naturally, in they went, somewhat jammed up against Clematis tangutica and a clump of the lily 'George C. Creelman,' or what is said to be Creelman, not that I believe it.

I had temporarily forgot, needless to say, that Astrantia major was on the other side. And I had not quite counted on the red valerian expanding northward quite as far as it did, and at that time I had no way of knowing I would add a lavender bee-balm.

It was in the way of some bulldozers. I only got a tiny root. Of course it grows superbly from a tiny root, and is heading north like Lee on a bender.

Another thing that seemed all right at the time was the propagation of a dozen bulbils of one of those yellow DeGraaff lilies (by now I have forgotten exactly which) and it took some thought to figure out what they were when they came up, somewhat indignantly no doubt, through the skirts of the yellow clematis that has of course grown a bit since the time the bulbils were put in.

The original plan, which I look at as one might look at the city plan for Ur, shows the cypress post with the clematis and the rose, 'Helen Traubel' beside it, and nothing else. The three clumps of lilies (the rose died, mercifully), the astrantia, kentrathus, alstroemeria, the daffodil 'Liberty Bells' and possibly a few other oddments that I see no point in mentioning have somehow all moved in.

I shall give another example:

Along one length of a lily pool I dug a shallow ditch to carry off any surface rainfall. Of course I did not want it to look like a trench. It is between some brick laving and the raised walls of the pool, faced with tiles from Spain. So I did not want a raw ditch.

It is only 8 inches deep, and the master plan calls for one clump of Iris tectorum at the pool corner, and a gentle rug-like cover of the Corsican mint (Mentha requienii) and yellow creeping Jenny on the surface of the shallow ditch. Very well.

The iris up and died (it flourishes in another spot not nearly so well suited to its needs) and so did the mint. The creeping Jenny wants more sun and has headed west, coveting a spot occupied by a blue geranium. Well away from the pool.

But in the meantime a friend wearied of planting primroses and gave me three white and yellow ones. These were stashed out.

These soon got in the way of the Corsican hellebore, and they increased mightily. So I dug them up, divided them and planted them at the base of the pool wall along the ditch.

In the ditch itself I planted the wonderful waxy skunk cabbage (Lysichitum americanum) that I had a chance to acquire, and two Italian arums somebody found in New York. Also, some seedling foxgloves were temporarily set in (two years ago) and it turns out that a lady's mantle (Alchemilla) managed to seed itself all over before departing this life.

Now I do not know any gardener who can pull up alchemillas when they are babies. Those tiny star-shaped leaves are as beautiful as anything I can think of.

In the space of two years, however, the original plan has been slightly altered to the extent that the wild iris, mint and so on have gone utterly, and a whole batch of unplanned things have arrived.

The trouble with master plans in gardens, then, is simply that they do not take into account masterful plants. Nor addled masters.