To grow celeriac, start the plant indoors from seed, water and fertilize frequently, and add mulch material as needed to keep the root ball covered. Your efforts to grow celeriac will be more successful when you provide plenty of moisture and rich soil for this plant, which is a heavy feeder. Celeriac is typically started from seed, as seedlings are not typically available for purchase at nurseries or garden centers.
Seeds should be started indoors in containers or trays, approximately eight weeks before your area's last predicted frost. Start by soaking the seeds in water overnight to encourage germination. Use loose potting mix to plant the seeds with only a light amount of mix covering each seed. A seed cloth or plastic wrap with ventilation holes can be used to maintain a moist environment for initial sprouting. The ideal temperature for germinating celeriac seeds is 70 degrees F (21.1 degrees C).
Wait for outdoor temperatures to reach 50 degrees F (10 degrees C) before transplanting celeriac seedlings to your garden. Once the seedlings have grown to about 3 inches (7.6 cm) in height, you can plant them outdoors with at least 24 inches (61 cm) of space between seedlings. Moist, rich soil that is slightly acidic is the best environment to grow celeriac. The plants will most likely need to be watered by every third day to maintain adequate soil moisture. Celeriac also appreciates a dose of fertilizer about every three weeks.
A full-sun location is necessary to grow celeriac. If necessary, this root vegetable can tolerate a small amount of shade during the day. You can place it with companion plants such as English peas, lettuce, and spinach, but do not attempt to grow celeriac adjacent to squash, pumpkins, or cucumbers.
With the arrival of cooler fall weather, it will be time to cover the bulb tops with mulch or soil. This process will help ensure a sweet flavor in the root. If you wait to dig up celeriac until after a light frost, its taste will be sweetest due to the conversion of some of the root's starches to sugars. It is best, however, to harvest celeriac before a hard freeze occurs.
Celeriac, or Apium graveolens rapaceum, is alternately known as celery root, turnip-rooted celery, or knob celery. Although the leaves can be used for flavoring in savory dishes, this root vegetable is primarily known for the potato-like texture and light celery flavor of the root portion of the plant. Freshly harvested celeriac will keep for several weeks in a cool storage area or when refrigerated.