JULY IS full of daylilies, which have progressed from their former state of being merely useful to their present state of substantial beauty.

I am much taken with the soft yellows and muted mulberry purple-rose sorts together. Years ago we had 'Potentate,' a smallish purple flower on tall stems, and I thought it actively ugly and dull, but many grow it to provide contrast to other colors.

Many dark daylilies misbehave if the weather is cold, but in good steady hot seasons they are sumptuous. 'Wild Wine' is a darkish purple, and 'Valley Forge' is nearer black-purple then red, and they look fine with sharp lemons like 'All Is Calm.'

In daylilies, as in the case of other flowers that inspire gardeners to collect many varieties, vast numbers of new sorts come along every year, and breeders wind up with many pretty sorts they cannot bear to toss out. The result is that many cannot be identified by ordinary gardeners without a label.

A while back, 'Winning Ways' won the highest award given daylilies, the Stout Medal. It is a wide-open flower, about seven inches in diameter, of lemon-primrose. The flowers are borne at the top of the stalks; it is not very well branched. Never mind, it struck judges as very beautiful. And promptly there appeared many seedlings of it.

'American Craftsman' and 'All Is Calm' are both similar, though varying a bit in greeness of throat and poise of petals. They are much of a muchness.

Results are best if daylilies are given a good deeply dug bed in full sun, with plenty of leaf-mould dug in, or composted cow manure or peat moss.

They need water from late May onwards, if the season is dry. With plenty of water they usually exceed the size and height that catalogues say they will reach.

They do not like a lot of fertilizers, in my experience, but they do like good soil. Any gardner may try to and see for himself the differnce in the flowers between two clumps of the same variety, one of which is very well grown in full sun and rich earth, and the other struggling along without attention for a few years.

The amazing thing is that even neglected old clumps will bloom. But, especially in small gardens, they may as well be given the best of everything.

The collection of daylilies at the National Arboretum is well worth visiting.

They have a fine assortment there, labeled, and you get a fine notion of what daylilies are like today.

Too often, probably, it is said daylilies are fragrant and have a long season of bloom. Some -- not many -- are quite fragrant; and while daylilies in general may be said to bloom from April till frost, the average well-established clump blooms three weeks then is done for the year. There are early sorts, midseason sorts, and late sorts. If a garden boasts many varieties, chosen for their season of flower, then the garden is bright with these flowers for about six weeks.

But it is unreasonable to expect three or four clumps of daylilies to provide masses of color throughout the growing season. Count on three weeks, and choose early and late kinds. It is true that some varieties bloom in July then send up additional flower stems in September.

They are not very reliable about it. The ones called "reblooming" may produce a second crop of flowers but usually they do not.

If well watered they are more likely to rebloom. Another endearing quality of some varieties is the sending up of additional bloom stalks just about the time the first set of stalks is in flower. Thus the season for that clump may be a week or two longer than usual, even if it does not bloom again in the fall.

In choosing daylilies you should, of course, select the ones you think are most beautiful. That would seem obvious, but often gardners choose varieties that are highly regarded and are then annoyed. I myself, years ago, acquired 'Rare China' because it grows well, blooms madly, makes brave show and so forth. It is medium yellow flushed at the edges with tawny rose. I do not like that combination and was quite put out when it bloomed with, of course, its typical blooms.

Increasingly I have come to distrust flowers that merely complete the spectrum or add variety. It is pointless, really, to acquire the best day-glow oranges if you don't like brilliant orange, or the black-reds, if you don't like dark reds.

'Master Touch' is typical of "pinks" that are to my mind wonderful but not what many people call pink. They have a salmon cast, and they hint a bit at canteloupes too, thanks to their fiery throats. But if you plant these sorts supposing you will get flowers like baby ribbons, you will be disappointed.

Flora colors are far richer and subtler than words seem adequate to pinpoint. A visit to a good collection, as at the arboretum, is valuable not merely to avoid colors you may not like at all, but to startle you with the discovery there are many colors you were not aware of that you like enormously. CAPTION: Picture 1, no caption; Picture 2, no caption; by Ken Fell -- The Washington Post