A SURPRISING NUMBER of people ask about water lilies and pools, and many gardeners report nothing in the garden is so rewarding.

The fall is a fine time - the best time - to build a pool. It is not a project to be taken lightly, without thought. Mine holds 12,000 pounds of water, as I recall it, so something pretty substantial is called for.

Cast cement is best, as far as I am concerned, with walls 6 inches thick. Directions on just how to go about construction will be found in G. L. Thomas' book, "Goldfish Pools, Water Lilies and Tropical Fishes."

Needless to say, the more sun the better, but with three to four hours of sun on the water, you can get water lilies to bloom well - varieties doing well with this much sun include: 'James Brydon' (rose red), 'Chromatella' (primrose sulfur yellow) and 'Yellow Pigmy' (ivory yellow). All these are still blooming at the end of September as they have bloomed since the first of June. They ae perfectly hardy and just sit in their tubs in the water all winter, though ice freezes all around them.

This is a good time to make rose cuttings. Climbers especially root well from cuttings, but many bush varieties do too. For climbers, cuttings a foot long are sliced off with a knife or clippers, leaves are removed from the bottom eight inches and the cuttings are set right in the ground with four to six inches sticking up above ground.

Sometimes I put a jar on mine, sometimes not. The ones that root will leaf out nicely in the spring and can be moved to permanent quarters a year from now - late October or November is a good time.

You recall some months ago we dealt with peonies, and needless to say you followed directions. You prepared a place for them and ordered roots to be planned in October. Now that the time is here, you are rightly pleased with yourself. I think it wise to mulch the new planted peonies a couple of inches deep after Thanksgiving, removing the mulch in March. I do not much peonies except their first winter.

In the unlikely event you have done nothing at all that you were supposed to do, you can plant peonies all through October. Simply dig a nice hole the size of a bushed basket, making sure the earth is nice and friable, and plant the roots, taking great care that the growth buds (the reddish knobs on the roots) are not more than an inch below ground. It does not harm them to be at ground level, but certainly not deeper than an inch.

It is a mystery how house plants accumulate.

I do not care much for them, and an appalled at the amarylises, bromliads, gardenias, dracaenas, palms, zub, zub, zub all sitting hopefully out there in their pots waiting to come in, much like dogs.

Since the house is almost exactly the size of a farm's chicken coop, plants wind up all over the place except on top of the stove.

My wife grows African violets, which do their main flowering in August because they don't get the east light in winter. The gardenias were almost dispatched to a better world, and hearing of that, I suppose, bloomed their heads off during the summer.

In a former garden in Tennessee, where we had gardenias outdoors, I complained that bushes took a lot of space and never bloomed much. My wife counted the flowers the next year, and stopped from weariness after 572, but said it would be criminal to harm such hard-working bushes. So they stayed. Possibly gardenias know when they're about to be pitched out and start flying right for a change.

I know daffodils can tell. Repeatedly, in a crowded garden, decisions would have to be made as to which clumps of daffodils had to go to make room for new ones. Certainly nobody needs 'Mount Hood,' 'Rembrandt,' 'Unsurpassable,' 'Mrs. Ernst H. Krelage,' 'Lord Nelson,' and so on in comparison with the marvelous daffodils of today. Yet each of these varieties, in the spring, immediately following the decision to discard them, flowered so spectacularly, and so much finer than we had ever seen them, that doom was averted.

Once arrangements were made for the new daffodils, these old-timers settled back to their usual performance, not justifying the space they took. Still, I remember the year 'Lord Nelson' was far finer than 'Arctic Gold.' And the year 'Mrs. Krelage' was lovelier than 'Empress of Ireland.'

One year I took against 'Bahram,' and planned to remove it in May. That spring it outdid itself, blooming with an intense fiery cup I never saw before or since. So much for the perversity of Nature.

A suitable tree for replacing a dead weeping cherry is the crabapple 'Red Jade.' It is not exactly weeping, but pendulous enough to be considered melancholy. Gardeners always are planting trees too large for the site, and my own instinct is to suggest dogwoods, sourwoods and American persimmons. But I admit the crabapples have their moments. The average city place would be much better if there were fewer maples and more large shrubs - things like crape myrtles, fringe bushes and Venetian sumachs.

In a final word, let me suggest pokeweed be grubbed out in the fall, unless you are growing it as an ornamental (it is very ornamental). It comes up late in the spring, and you won't find it in late March when you try to dig up its vast root. So if you don't want it, dig it out in late October. Its splendid berries stain shirts in a royal and permanent way, but by late October the mockingbirds have usually finished them off.