Theoretically we now have the most glorious weather of the year, but that doesn't mean it will be warm.

In Washington there is more than a month's difference between the frost-free season at National Airport, say, and the northern suburbs of town. One year all by basil froze in September, utterly black, though in general I can wait till the end of October to rescue it.

Indoor plants should come indoors, if they have not already done so, and there should be great activity outdoors planting bulbs, getting out the Bermuda grass and other weeds that have sprouted since the end-of-August cleanup. Nowhere does it say the gardener may sit out on a sunny Saturday and doze.

On the other hand, nowhere does it say this is against the law. This year I have sat a good bit on an old iron chair on the brick walk near a clump of the sedum called "Autumn Joy," which is a Great Plant though people who don't garden may wonder why all gardeners think so.

It has thick succulent leaves, grayish like a cabbage but rather oval and about an inch and a half long, marching right up the steem in neat order. In August you can see this sedum intends to bloom, and its greenish flower buds display themselves in flat or rounded clusters at about knee height. They only open very gradually. It may be the middle of September before they are fully colored.

The dense clusters of bloom are soft rose-bronze, not at all bright or brilliant, but very soft in effect, making a handsome contrast with the blue-green leaves.

This sedum is not too different from spectabile , which comes from Japan and which gardemers jave grpwm fpr ages. But S. spectabile is much brighter, and does not last in flower so long as "Autumn Joy."

Like sedums in general, it is easily increased by cuttings a couple of inches long, just stuck in the dirt in April. It is a very good plant for growing in those small shallow wooden windows boxes that gardeners keep acquiring and in which very few things flourish. Let me give an example:

Sometimes there is a walk from the kitchen, edged for a few feet by an iron railing to keep you from tumbling down a stairwell that leads to the basement. Since there is no earth there, the effect of iron and concrete or brick may be a little cold. And yet there is not room enough on the walk to set large pots or tubs.

Often there is room to set a couple of shallow narrow wooden window boxes. There may be only 5 inches of soil in them. Still, you can grow parsley in them, convenient to the kitchen, and you can grow sedums in them. Even this small touch of greenery can make things look better.

"Autumn Joy" grows straight up, it doesn't flop. At the edge of the box it is easy to root a few other sedums such as "Dragon's Blood," a deep bronzy red or almost crimson-flowered from of S. spurium . It leans out and weeps down, and looks fine with the taller "Autumn Joy," and sometimes blooms a few weeks earlier, so that for six or eight weeks you have a touch of color. And even when not in bloom the sedums look near and finished.

They die down in the winter and produce fat little rosettes of leaves again in the spring, like gray Brussels sprouts sitting on the ground. At this time it is possible for a few spring-blooming plants to flower and die back before the sedums spread out.

Everybody admires polyanthus or bunch-flowered primroses that in sunny spots in a town garden may send forth a few flowers in January and which hit a good stride of bloom in March and April, dying back in the hot summer.

When they die back, the earth looks awfully bare. Hence the sedum. Such an arrangement -- which, by the way, I do not consider especially remarkable or anything to get fiercely excited about -- has at least the merit of providing flowers when they are most welcome, early in the spring and in late September or October. Furthermore, the combination looks rather neat through the year, giving the impression that the gardener knows what he is doing.

Often in small town gardens I think wee mourn too quickly the effects we cannot have, and too often overlook simple touches (like the shallow boxes of primroses and sedums) that we could squeeze in here or there without cluttering open space too much.

Last spring I thought my Japanese irises were getting too thick in the oak tub I grow them in, so in a fine spell I transplanted them to roomier quarters in additional whisky barrels. To my sorrow they all died but two. I believe this was simply because they were moved before settled weather arrived. I am always alarmed I shall wait too late in the spring to tend to things, and here was a case in which I acted only a few days too eariy, but the results were fatal to the irises. A real loss, since one of them was most beautiful, a large white with sky-blue style arms in the center.

This may be the place to say that seeds of Higo Strain Japanese irises can be planted outdoors now. They will sprout like radishes in the spring, and they should be spaced a foot or so apart, where they will bloom in startling glory in June 1983. I got my seed from Park's (Greenwood, S.C. 29647).

A thing I have done in the past with some pleasure involves growing Japanese irises in pots or tubs which in April are set in the lily pool so that the water covers the soil in the pot by about an inch. When the flowers come in June, the effect is pretty, and you don't have to feel guilty that you are not watering them as much as they like.

In plain fact, there is enough water most of the time in Washington to float an ark, and these irises flourish in an ordinary garden border without any supplemental watering at all. But I think they are taller and finer with virtual flooding from April to July. They do not like to be in pools, or kept unusually wet, during the winter. But I have grown them in tubs without drainage holes and found they did not drown during the winter.

A thing I have never done, but which the Japanese sometimes do, is plant one seed to a 3-inch pot, and set these pots in shallow pools or basins before they come into bloom. That seems to me to be tempting fate, growing them in containers so small, and I expect you must be very careful in their culture then, not letting them ever dry out. But it does mean you could get a great many plants in a small pool. I may try it, only using 6-inch pots.

These particular irises are said to abhor lime, which is said to be fatal to them. I have never put it to the test. I always grew mine in the normal soil of the garden (which is acid and fine for azaleas), giving a 2-inch surface mulch or rotted stable manure in January.