Now we come to the great season for the dahlia, which is a plant utterly easy to grow provided its few requirements are fully met, and which is not worth growing otherwise.

I've grown them both ways, superbly and slothfully, and I can say superbly is better.

With dahlias now at their peak, and the National Dahlia Show scheduled for next Saturday and Sunday in Falls Church, the gardener should take a sharp look at his space. He should reflect on his improvident ways and resolve to do better.

Specifically he should lay out a little space for next year's dahlias, remembering they require six hours of sun a day, and while small plants obviously require less space, still the big dahlias should be 30 inches apart in the row, and the rows three feet apart. In short, dahlias require the best spot in the garden, just as roses, irises and a good many other things do.

September and October are excellent times to dig a sunny patch for the dahlia tubers to be set in next spring.

The National Capital Dahlia Society Inc. recommends planting them five inches deep from mid-May through June. They may, of course, be planted sooner, but nothing much is gained and there is a substantial hazard that the roots will rot in cold damp ground.

Besides, most dahlia lovers do not want them to bloom before the end of June. A dahlia that is in full flower early in July is not going to keep producing good flowers in September, which is when most gardeners want dahlias most.

If feasible, the dahlia patch can be dug to a depth of two feet. Dahlias like a good friable loam, and plenty of leaf mould or other humus, as well as fully rotted horse manure, may well be dug in this fall.

In the spring (sometimes as early as February, and always by April) the dormant roots begin to send out tender sprouts, which are, by the way, brittle and easily broken off. This is true whether the roots have been stored (the society recommends vermiculite in a sealed container, if you are storing your own over winter in a cool, preferably 40-degree room) or whether new roots are bought from dealers.

Some gardeners like to pot up these roots and grow them in the house until mid-May, and if done properly the plants will be sturdy indeed and a foot high while still indoors. This system, which I have tried, has two drawbacks. First, it is a lot of trouble to keep the potted plants from becoming drawn and thin in response to the relatively dim light even of a sunny window. Second, the potted dahlias, once liberated in the bed outdoors, are in heavy bloom by early July and have pretty much run their course by September.

The society recommends planting the tubers directly outdoors, covered with five inches of soil, once the soil has warmed to 60 degrees. The stake is put in just before the dahlia is planted, not after the new shoots have emerged from the ground.

Some gardeners like to grow dahlias from cuttings an inch and a half or two inches long. These root easily and quickly in sand and are then planted in soil in small pots, and shifted to larger ones until ready for the outdoor garden.

But the society recommends planting the main tuber, just as received from a specialist or just as you get it in a plastic bag in a garden center or hardware store.

For the first six weeks after planting the dahlia bed should be lightly cultivated, just enough to keep weeds from gaining a stronghold. The first or second good dry day after a rain it is helpful to scratch the soil to keep a crust from forming. By August the roots will have begun to grow strongly upward, as well as downward, so no cultivation is done after early August or (some gardeners believe) mid-July.

Mulching is practiced by many, perhaps almost all, dahlia growers. It is applied once the young plants are 10 inches high.

Usually there is enough rain to keep the young plants growing along steadily. If there is a drought, the dahlias are soaked once a week with the equivalent of an inch of rain water. Often when the rows are made a slight trench results between them, and a slow soak with the hose takes care of the watering. But there may be restrictions on water, so a good three-inch mulch and a soil with plenty of humus in it will prove its worth in such summers.

Once flowering begins, the dahlias should be watered slowly and deeply once a week. The society recommends spraying once a week with a multipurpose insecticide.

It also recommends letting only one stem grow from the tuber. This results in the largest possible bloom. Gardeners not fully intent on the largest conceivable bloom may leave four stems, and of course they will have many more flowers but need not expect to win so many prizes.

Dis-budding makes perfect sense, unless the dahlia is being grown for garden color and is not intended for cut blooms. If the blooms are to be cut, for a show or for the house, it is pointless to leave the side flower buds, as these will never open once the terminal bloom opens and is cut. But for garden color, the side buds may as well remain, as in due time they will flower (the terminal bud is simply cut out once that flower has faded).

Even for garden decoration a reasonable amount of disbudding is good. As the young plant grows and has developed three good sets of leaves, the top growing point is pinched off. New shoots will appear. Keep the two new shoots nearest the main stalk and pinch out the others.

As the plant grows it will produce flower buds, usually a central one and two side ones. Pinch out the two side ones, as the central bud will make the best flower. The side flower buds should be pinched out as soon as they are large enough to be dealt with, smaller than peas.

The first week of July the society suggests giving each plant a handful of 5-10-10 chemical fertilizer, lightly scratched in and watered. The fertilizer should not come closer to the stem than six inches. As the flower buds appear the society suggests applying (three or four pounds per 100 square feet) a dressing of three parts bone meal, one part phosphate (0-40-0) and one part potash (0-0-60). No further fertilizer should be given, and make sure the plants have ample water, not light squirtings from the hose that merely dampen the surface.

Flowers are cut early or late in the day, not while the sun is on them and they are limp. They are put directly into cool water and kept cool. Keep the blooms separated, otherwise petals will snag on neighboring blooms and be torn. Really huge dahlias should have the stems supported after being cut so the weight of the bloom does not make the stem break.

Dahlias are allowed to keep blooming long after September, and until a real freeze blackens the leaves. Then they are cut to within two inches of the ground and dug up. Use a spade and cut a circle about 10 inches from the stem, and with some care lift the tubers. Keep cold or cool, but away from freezing, and let the dug-up clump dry naturally for a couple of days, then carefully cut individual tubers from the clump. Each tuber must get part of the old stem, as that is where new growth will begin in the spring. A tuber without part of the old stem is worthless and should be discarded as it will never produce a plant.

Dahlias like a soil less acid than most garden soils here. It is thought a pH reading of 6.8 suits them best. If lime is thought necessary, use ground stone (not finely ground powder) and apply it in the fall.

The society suggests, as a grand source of humus, that winter rye be sown in the dahlia bed once it has been dug in the fall, and this rye be dug under in the first half of April.

The national show will draw as entries blooms from as far away as Seattle and Holland. It will be held at the Fairview Park Marriott Hotel in Falls Church from 2 p.m. to 9 p.m. Saturday and 9 a.m. to 5 p.m. next Sunday, so local gardeners have a week to make plans to get there.

The classification of dahlias being shown may at first seem unduly complex, but it is simple enough. Dahlias are found in 15 color categories and in four floral shapes and in several sizes ranging from golf balls to dinner plates. As the flowers will be labeled, it will be easy to jot down the names of the varieties one especially likes, and the local society will provide names of leading nurseries and specialists. Susan M. Finch, editor of the Dahliagram newsletter, is good at answering knotty questions at (301) 933-7800.