If you fancy soup, try planting celeriac

If you're into soups and stews - and what food gardener isn't - you might consider growing celeriac this year. It's a first cousin, or more appropriately, a half brother to stalk celery.

Where conventional celery is grown for its edible stalks, celeriac packs all its edibility into a less-than-handsome root. But ugliness, in this case, is only skin deep. Once peeled, all that delicious celery flavor is yours for the eating, either raw or cooked.

Celeriac imparts a particularly pleasing flavor to soups.

The English, as well as other Europeans who have known and loved celery in this form for decades, often refer to it as celery root or celery knob. Botanically, it is known as Apium graveolens rapaceum compared to Apium graveolens dulce for conventional celery.

On the other side of the Atlantic, celeriac is widely available in stores, and is only moderately expensive; here it is a rare commodity. When you can find it at all, the price is usually $2 a pound or more.

The seed packet that arrived the other day from the Geo. W. Park Seed Company , Greenwood, S.C., (celeriac is available from most mail-order seed companies and garden centers) cost 85 cents for one-sixteenth of an ounce. If that sounds expensive, consider that there are some 71,000 celeriac seeds to the ounce. Thus , this one packet will supply me with all the celeriac I will need for several seasons to come.

Celeriac is considered the easier form of celery to grow in the home garden, because it requires no trenching and no blanching. Its other great advantage is that it stores readily over the winter, as do so many other root vegetables.

In Europe, celeriac seed is frequently sown outdoors early in the spring, and the same practice can be adopted here in the United States. On the other hand, it can also be started indoors. Many growers consider this the best approach in the US, where the cooler days of spring (celeriac's preferred growing weather) don't last that long.

The seed packet suggests sowing seed indoors 10 to 12 weeks before setting out. It warns, too, that germination can take between 3 and 4 weeks when soil temperatures range between 60 and 65 degrees F. Higher soil temperatures speed up germination quite considerably.

After hardening off, set out the plants 12 inches apart in every direction. Plant in well-worked soil enriched with manure and-or compost. A side dressing of fertilizer will prove beneficial once the plants are well established.

A mulch will keep soil temperatures from fluctuating and keep down weeds until the plants have become established. The bulk of the celery root, however, grows above ground somewhat like the onion, and most garden books suggest that the bulbs not be covered with either soil or mulch.

I have seen it suggested that celeriac plants need little watering during the growing season, but an English gardening book of mine says to ''water copiously.''

Start pulling the roots when they are 2 to 21/2 inches across. Store in a refrigerator in the short term, or in moist sand in a root cellar at around 40 degrees F. for the long term.