DAHLIAS ARE A considerable comfort in September but, like most comforts, need some forethought.

Now is the time to stir. So many gardeners get dahlia roots at garden centers or grocery stores that I will suggest two ways of handling them.

The commonest way, which I do not like much, though I have tried it and it works, is simply to dig a hole, set the roots in more or less flat, at five or six inches - that is, the top surface of the root is five or six inches beneath the ordinary ground level.

These roots should be covered with two inches of soil. They are planted now. Once the shoots are growing vigorously, fill in the hole with another two inches of soil, and a few weeks later, add the rest of the soil. A stake rising three or four feet above ground will have been set when the roots were planted - 20 to 30 inches apart for the ches-high sorts, or 15 inches for the for the lower kinds that only grow a foot or two in height. These very low kinds do not need stakes.

Depending on weather, growth will be rapid or slow, and gardeners worry in cold spells in May.

If dahlias are planted outdoors, dormant, as the lilacs fade, however, the cold rainy spells that come along will not be disastrous though the gardener (especially the first time he tries dahlias) is sure to complain that everything must be rotting.

Really heavy growth does not occur until late May and June at the earliest. It is only in late July that you see the stakes are none too tall, and only in late August (when incredible winds are likely) that you are finally convinced the stakes were a great idea.

Possibly it should be said here that the dahlia stalk is tied with strips of old shirts or sheets to the stake. Once I heard of a gardener whose dahlias were laid low by September storms, though staked. Nobody had told him the dahlias had to be tied - he assumed they sort of leaned, like a drunk with a lamp post.

The great foot-wide or 10-inch wide dahlias that are not so popular nowadays, grow to shoulder height if they are carefully pinched and groomed, or six or more feet high if left alone and merely disbudded. They will need stakes a minimum of five feet high, and one-by-two lumber will be found a lot safer than light stuff.

Until you have seen it (and seen a vast dahlia plant snap off) you may have trouble imagining the stress a stake must bear when a 50-mile wind tangles with a fleshy luxuriant mass of leaves six feet high and three feet wide.

So resign yourself, if you grow the tall sorts, to providing adequate stakes at planting time. Do not imagine you can safely drive stakes in on July 19.

Now the other way of starting dahlias now is to set the roots flat in a three-inch cardboard box - the kind garden centers usually set small plants in when you buy them will hold about six dahlia roots - and pack damp peat moss around them.

Peat moss should not be dry or wet either, but about like a wash cloth wrung out and set on the side of a bathtub.

Shoots, will pop up in a day or a week. At this point, you can do one of two things - you may keep the roots damp in the moss (once the fine hair roots start growing from the big tubers, more water is necessary) and set the whole root mass out in May, say May 10, just as you set out the dormant roots in April.The only difference is that the plant has started growing, and you avoid the slight risk of a prolonged spell of damp cold weather which greatly delays growth of the dormant roots. (Thus by early June, the dahlias started in peat moss and planted out in May are likely to be more vigorous than dormant roots planted outdoors in April).

But now back to the box of peat with dahlia shoots three or four inches high. If you like, you can also make cuttings of these shoots. Plant them firmly in a sand-peat mix or potting soil mix and keep slightly damp.

They root in a couple of weeks, as I remember, and when nicely rooted they are grown in pots - first three-inch, then five-inch - until they look sturdy enough to plant in the open garden.

Dahlias rooted this way in April will make full-sized plants with full-sized blooms by Labor Day.

But if you do not want to fool with cuttings, just set the whole root mass in the six-inch hole.

Three or four shoots only should be allowd to grow - the dahlia root may send up more, but these should either be used as cuttings or else pinched off. Nothing is gained and a lot is lost by letting an old dahlia clump send up a forest of stems like a peony.

In buying dahlia roots, it is best if the eyes or growth buds are just barely visible, but often they will be already aprouted. If sprouted, remember they are brittle and snap off easily. Do not be too upset if they are pale greenish white - with care and a bit of sun they will soon straighten up and look wholesome. But do not cover a root with long pale shoots with five inches of dirt - the shoot may very well rot.

If you have a root with six or eight tubers dangling from a central point (the stub of last year's stalk) like sweet potatoes, you should let it sprout in the peat moss, then divide it into sections.

Each section must contain at least one sprout.

Often some of these sweet potato-type tubers will break off. If they contain part of the old stalk, they will send up a shoot, but if they do not have part of that old crown, they will not do anything. Throw them out.

Do not divide the root system until you see where the new shoots are. If you just pull the tubers off, none will produce plants, but if part of the old crown is attached to each tuber or small cluster of tubers, you may get three or four plants from one cluster.

It is not always easy, by the way, to "divide" the tubers, even when you can see the new shoots, since the old crown is both woody and spongy, and the dickens to cut through, sometimes.

Until I tried a dahlia cutting (three inches) I did not believe so tiny a creature in late April could make a plant as good as one supported by six fat tubers. But it did.

The best kind of dahlias for most gardeners are the ones about four feet high eventually - often the catalogs or labels on the tubers will say ("32 inches"). The semi-cactus types (curved quill-like petals) or the informal decoratives (small disordered mops, not too dense in petalage) are usual. There are also singles, anemone-types, collarettes, pompons - all suitable for the border or for the small garden where a few cut flowers are wanted.

Do not assume that the pompons, singles or collarette types are low growing. Some are and some not.

I notice at several garden centers the label on each root gives the name of the variety, the color, and the height, which is extremely sensible.

You may wonder if the pictures look much like the actual flowers. Yes. Exactly.

Dahlias come along as the daylilies and phlox wane. Many dahlias of the chest-high border type (and the smaller ones) bloom from July on, but it is from late August - you know how suddenly you feel a hint in the air that summer is passing - until frost that they are in their glory, when few other things are.

The commonest error with dahlias, if I may speak from my own experiene, is failure to give them the water they require in summer. A profound soaking every week or so is useful, from July onward. And they need sun, as much as possible. In woodsy places, forget dahlias.