Celeriac (Apium graveolens var. rapaceum), also called celery root, is a close relative of stalk celery in the carrot family, grown not for its stringy stems but for the swollen, knobby root that forms at the base of the plant. It's a cool-season crop that traces back to the same wild celery as its stalk-growing cousin, selected over generations for a bulb rather than a bundle of petioles. Celeriac is a patient crop rather than a difficult one: the seed is slow to germinate and the seedlings are slow to size up, so success comes down to starting early and giving the plant a long, steady season rather than any special skill. Gardeners who've grown stalk celery will find celeriac's needs familiar, since the two share nearly identical culture.
Sowing and planting
Celeriac seed germinates slowly and unevenly, so it has to be started indoors well ahead of the garden season. In zone 6b, sow seed indoors 10 to 12 weeks before your last expected frost, which usually means starting in late January or early February for a late-May transplant date. Sow seed just barely covered, no more than 1/8 inch (3 mm) deep, since it needs light to germinate well. Keep the starting mix consistently moist and warm, around 70°F (21°C); even under good conditions, germination can take two to three weeks, and patience here pays off later.
Once seedlings have a few true leaves, prick them out into individual cells or small pots so they can develop a sturdy root system before transplanting. Harden the young plants off over a week or so before setting them in the garden, once nighttime temperatures are reliably above 45°F (7°C) and the worst frost risk has passed. Space plants 8 inches (20 cm) apart in rows about 18 inches (45 cm) apart, setting them at the same depth they grew in their pots. Water in well after transplanting, since celeriac seedlings are easily set back by a dry spell right after the move.
Growing on
Celeriac needs steady, even moisture for its entire season to form a smooth, well-developed root; letting the soil dry out and then soaking it again produces cracked or hollow bulbs, so aim for consistent watering rather than a feast-or-famine schedule. A layer of mulch around the plants helps hold soil moisture and keeps down the weeds that would otherwise compete with celeriac's shallow root system. Feed with a balanced fertilizer at planting, then again once or twice during the season, since celeriac is a long-season crop and draws steadily on soil nutrients the whole time it's growing.
As the bulbs begin to swell, usually by midsummer, pull back the soil and strip off the lower leaves and any small side roots growing from the bulb's surface. This step, sometimes called "shoulder clearing," exposes the top of the root to light and air and keeps it from turning shaggy with fibrous roots, giving you a cleaner bulb at harvest. Repeat every few weeks as the root continues to size up. Weed carefully by hand close to the plants, since celeriac's shallow roots don't compete well with a hoe working nearby.
Harvesting
Celeriac is ready around 110 days after transplanting, when the bulb has reached 3 to 5 inches (7 to 13 cm) across. Unlike many root crops, celeriac tolerates light frost without harm and actually improves in flavor after a touch of cold, so there's no rush to dig the whole bed at the first chilly night. Lift roots as needed through autumn by loosening the soil with a garden fork and easing the bulb free, then trim off the leaves and fine roots, leaving an inch or so of stem attached.
For longer storage, dig the remaining roots before a hard freeze sets in, since a deep freeze in the ground will damage the bulbs. Brush off loose soil but don't wash the roots before storing them. Kept in damp sand or a perforated bag in a cold, humid spot near 32°F (0°C), such as a refrigerator crisper drawer or root cellar, celeriac keeps for several months, usually getting sweeter and more mellow in flavor over time.
Problems and pests
Seedlings fail to come up or take forever to appear
This is usually celeriac being celeriac rather than a sign of trouble: the seed germinates slowly even in ideal conditions. Keep the starting mix consistently moist and warm and resist the urge to resow too early. If nothing has emerged after three weeks in warm, damp conditions, check the seed's age, since older celeriac seed loses viability faster than most garden seeds.
Rust-colored, cracked patches on the stems and leaves
Celery leaf blight and similar fungal diseases show up as brownish spots that spread quickly in humid weather, especially when foliage stays wet overnight. Water at the base of plants rather than overhead, space plants for good airflow, and remove badly affected leaves promptly. Rotating celeriac away from the same bed each year reduces the disease's carryover in the soil.
Tunnels or blotches inside the leaves
Celery leafminers burrow between the leaf surfaces, leaving pale, winding trails. The damage looks worse than it usually is and rarely affects the root itself. Pick off and destroy badly mined leaves, and cover young plants with a floating row cover (a lightweight fabric laid over the bed) to keep the adult flies from laying eggs in the first place.
Root cracks open or turns hollow in the center
Irregular watering is almost always the cause: a dry spell followed by heavy watering or rain makes the bulb grow too fast and split, while hollow centers often point to a boron deficiency in the soil. Keep moisture even all season, and if hollow roots are a recurring problem, have the soil tested and add a boron supplement if it's lacking.
Small, ragged holes in the leaves
Flea beetles chew shot-holes in young celeriac foliage much as they do on other vegetables. Established plants generally shrug off the damage without any real effect on the root. A floating row cover over young transplants keeps the beetles off during the vulnerable early weeks if an infestation is heavy.
Varieties worth growing
- Brilliant – A reliable, widely available variety producing smooth, round roots with good flavor and reasonably clean skin.
- Giant Prague – An old standard variety known for large, dependable roots and a long storage life.
- Mars – A smaller-rooted variety that matures a bit faster, useful where the season is on the shorter side.
- Monarch – Produces large, round roots with fewer side roots than many varieties, making it easier to clean at harvest.
- Diamant – A dependable modern variety with smooth, well-shaped roots and good resistance to bolting.