THIS TIME of year nurserymen sell perennials in cans and pots or flats, and it is perfectly sensible to buy them that way rather than not get them at all.

Field-grown clumps, planted in the fall, are best, since most gardeners refuse to get around to tending to such matters then, it can all be tended to now. All perennials that I can think of require a while to settle down and show what they can do.

Phlox, for example, or baby's breath or baptisia or hostas or daylibes or peonies - you name it - need to be in the garden two or three years before anything approaching their full beauty is to be seen.

Virtually all perennials require sun. Some, like soldiers and sailors (pulmonarias) or the big forget-me-nots (cynoglossums) plus ferns, of course, and hostas and bugle weeds (ajugas) accept woodland shade, since woodlands are quite sunny in the early spring.

For those dark places where the sun never shines, the best that can be hoped for is good-looking paving with ivy, pachysandra ferns and hostas, and even these will benefit from coddling in the way of careful watering and good soil.

Foxgloves (digitals) are too often overlooked for positions in dappled light.

Most foxgloves are short-lived perennials, dying out after two or three years, but incongenial places they seed themselves. So do many poppies, thogh they require the nearest thing to full sun the gardener can offer. The soundly perennial oriental poppies (in white, orange, crimson, scarlet, rose) like the same sort of position that corn does - plenty of sun, and good, deep rich soil. There is no point expecting them to look like much for a couple of years.

I have enjoyed my white and yellow primroses this spring. They are very easily grown, but are not plants that can be stuck in and then forgotten.Every year, or every other year at maximum, they must be dug up, separated and replanted in good rich soil. This is done in May. Mine grew at the east end of a raised pool, and are shaded most of the day. If left alone, they get thick and then start to die out. I replanted every year (it is good to have them out of the ground no longer than it takes to pull them apart and reset them) they go on forever, increasing at a great rate. They will not take being dry in the summer, by the way.

For some reason columbines never seem to seed themselves with me, probably because the earth is disturbed by scratching around before they have time to sprout. Columbines sometimes settle down for five years or so, but often they die after two or three years. Like everything else, they take a good bit of space (like a bushel basket) but their foliage is handsome even when the plants are not in bloom, and they flower from late April right through May.

The coral bells (heucheras) are among the most rewarding perennials for gardeners with small places, since they bloom for two months or more, knee-high slender stalks hung with delicate bells of red, rose, white or green. They have handsome leaves the size of a small hand, usually with a reddish tint, and these are much appreciated in November, say, when most perennials are sad. They flower from late April into July.

They are admirable in clumps along a walk, forming neat hummocks. The only problem with them is that they easily heave in the winter, and need to be pushed down when it thaws. They make little offsets, and should be kept coming along from these, since old plants get increasingly woody and do not do so well. But I would not want to say anything that might discourage the planting of these splendid flowers which contrast so well with almost everything else in the garden.

Everybody, I suppose, likes hollyhocks. I often come to grief with them because I jam other things up against them, and hollyhocks like plenty of space and air and full sun. The alleys of Washington are sometimes showy with them, for they are not fussy as long as they have room and sun.

Shasta daises are far more agreeable and lovely than one thinks they are going to be.

The average collection of daylillies, incidentally, would be much brightened by a few clumps of these white daisies to bloom with them in late June. These daisies are among those plants that should be dug up, separated and replanted every year or so, in April or else in September. It does not do at all to separate them in November, because they die over winter. I do not see why anything so hardy should not settle in promptly, even in November, but experience convinces me to separate these daisies either in April or around Labor Day.

Peonies do quite well planted from cans in the spring. I myself would always buy the bare roots in October and plant them then, because they are cheaper and usually better. But I have seen the fine peonies from spring plants, which can look splendid after three years.

This is a great time, by the way, to prepare daffodil beds for planting next fall. In the garden where space is short, daffodils can be planted eight inches apart and left for two or three years before being dug up, separated and dried (in June or early July) and replanted in September. So a good many will fit in a small space.

If the earth is deeply dug, with peat mixed in and a bit of rotted manure (an inch or two, dug in and mixed with the soil) you can grow vegetables there this summer, clearing them away and digging again lightly in late September and planting daffodil bulbs in October. As any gardener knows, who has tried various methods, daffodils do far better in a bed prepared the spring before, instead of being stuck in any which way in the fall.

There are certain terrible grasses that form massive clumps, so that when they are dug up with a spade a whole bushel of dirt comes up also. These grasses dearly love to sprout among daylilies, shrub roses, or anywhere else they can get a foothold. They must be got up now, or they will form truly enormous clumps this summer, and nothing - literally nothing - can grow through them. Again in early fall they should be dug up, if any have managed to get started during the summer.

I have been getting some out of old clumps of daylilies, and it meant digging the entire daylily clump up, working all the grass out, and replanting. There is no point thinking the daylilies will shade these clump-forming grasses out. They will not. Fortunately the grass comes up clean (along with half the dirt of the garden) so once the clump is up, you don't have to worry about oddments sprouting all over the place.