Both these plants are biennials, and related in the same way as beets and chard; that is, they are two varieties of the same species: Apium graveolens. Stalk celery is a relatively difficult plant to grow well, and might be best avoided by beginners; root celery, or celeriac, though, presents no problems to the average gardener except the amount of time it requires to mature, which is only a matter of patience.
Because they are grown for different parts of the plant, we put celery and celeriac into different parts of our rotation: the former is treated as a leaf crop since it is a notoriously greedy feeder; the latter is treated as a long-season, early-feeding root crop like leeks (even though it is no more a root crop than leeks or kohlrabi). Both celery and celeriac, however, are started for transplant at the same time, about 8 to 10 weeks before the frost-free date.
The seed is very small, so it should just be pressed into the surface of the potting soil, watered, and the flat covered to preserve moisture until the seed sprouts, which is likely to take a week or two. If you don’t need a full flat, put your parsley and other slow-growing herbs in the same tray. To sow thinly enough, you can use a pen or pencil to pick up the seed: just touch the tip of the pencil to your tongue, then in the seed; the moisture will cause a few seeds to stick to the pencil tip; then you can push them off into the waiting flat with your finger. During their time in the tray, keep the plants above 50º F to avoid triggering their biennial bolting reflex. As soon as the plants have their first true leaves. Thing to the strongest plant per cell of the tray, or to an inch apart if you are using an open tray.
Set the plants out 12 inches apart once the weather has become reliably mild. An occasional light frost won’t hurt them, but persistent daytime temperatures below 50º F will cause them to bolt, or at the minimum slow down their growth enough to hurt the quality of the final harvest. If planting in rows, allow 2 feet for a path. We give both root and stalk celery a shot of fish fertilizer at transplant time to get them off to a good start. The stalk celery will continue to receive periodic side dressings of this type, while the root celery, after its initial feeding, will be left to fend for itself. I don’t blanch celery; the tight planting will take care of that to the extent necessary.
Stalk celery can be harvested once the stems reach usable size, either by cutting off the entire plant at ground level, or by pulling individual stalks from the outside of the plant and letting the younger, inside stalks continue to grow. There is usually little time to do this in our garden, as the crop isn’t ready until fall, and it is not hardy enough to withstand frequent frosts.
Root celery, on the other hand can be hilled with soil and will survive even a hard frost. We mulch the beds to help keep down weeds and to help regulate moisture. To get the smoothest celery roots, pull away the mulch and soil once the knobs have reached an inch or so in diameter, rub off the small feeder roots above ground level with your finger, and then hill them to blanch that part of the root. Celeriac roots are suitable for harvest from the first fall frosts until temperatures get below 20F, and will keep up to a couple of months in a root cellar, or under moist refrigeration.
The most common cultivars of stem celery are variants of Utah 52-70, but there are also some Pascal types, which are more bolt-resistant. So-called “self-blanching” celery is really just a yellow petiole type, which some gardeners prefer, though it is not a vigorous as the green petiole types. Some catalogs list red cultivars, mostly from England, that are quite hardy, but only slightly red. Southern gardeners can grow celery in the fall and winter, and should choose the “Florida” cultivars that have been developed for their winter conditions.
The old standard cultivars for celery root are Alabaster and Giant Prague, but they are not nearly as vigorous or reliable as the French cultivar called Diamant. There are also some cultivars of celery that are grown for the leaves, which are dried and used for seasoning. They are listed in most catalogs as “cutting celery” and are grown just like parsley.