Celeriac, often called celery root, is a unique vegetable that offers a nutty, celery-like flavor distinct from its stalk-growing relative. Unlike traditional celery, this plant is cultivated specifically for its knobby, underground storage structure, or hypocotyl, which develops into a large, edible bulb. Growing this cool-weather, long-season crop successfully requires careful timing and specific maintenance techniques. This guide provides a step-by-step approach to cultivating celeriac from seed to harvest in the home garden.
Starting Celeriac Seeds Indoors
Celeriac is a slow-growing crop that requires a substantial head start indoors to ensure a harvestable root before the end of the season. Seeds should be sown approximately ten to twelve weeks prior to the average date of the last expected spring frost in your region.
The seeds are best started in shallow trays or containers filled with a sterile seed-starting mix. Because celeriac seeds require light for optimal germination, they should be pressed gently onto the surface of the soil rather than buried deeply. A thin layer of vermiculite can be dusted over the top to help maintain surface moisture without blocking light.
Maintaining consistent warmth and moisture is necessary for successful germination, ideally keeping the soil temperature around 70°F (21°C). Once the delicate seedlings emerge, they must be immediately placed under bright artificial grow lights or in a south-facing window. Insufficient light during this early stage will result in leggy, weak plants that struggle to establish themselves outdoors.
Seedlings benefit from being thinned to stand about one inch apart once they develop their first true leaves. The indoor environment must remain consistently moist, as drying out can severely stunt the young plants.
Preparing the Growing Site and Transplanting
Before moving the starts outdoors, the young plants must undergo hardening off, gradually exposing them to outdoor conditions over a period of seven to ten days. This acclimation to wind, direct sunlight, and cooler temperatures prevents shock when they are transplanted into the garden bed. Transplanting should occur after all danger of heavy frost has passed, usually when the seedlings are about four inches tall.
The outdoor growing site should receive full sun exposure throughout the day, as this promotes the vigorous foliage growth necessary to fuel the developing root. Celeriac is a heavy feeder and performs best in rich, deep, and well-draining soil that has been amended with plenty of organic matter, such as compost or aged manure.
Seedlings should be spaced between 12 and 18 inches apart in the row, with rows themselves spaced 24 inches apart. Planting too closely will restrict root development and result in smaller, underdeveloped bulbs. Digging a hole only as deep as the current root ball ensures the crown remains level with the surrounding soil surface.
Essential Care During the Growing Season
Celeriac root structure is highly susceptible to environmental stress; even brief periods of drought can cause the developing bulb to become tough, woody, and intensely flavored.
Maintaining soil moisture is best achieved through regular, deep watering, supplemented by a layer of organic mulch around the plants. Mulch conserves water by reducing evaporation and helps to maintain a consistent, cool soil temperature. Fluctuations in water availability can also lead to cracking or splitting of the hypocotyl as it rapidly expands following a dry spell.
Celeriac is a demanding plant, particularly in the early stages when it is establishing its large canopy of leaves. An initial application of a nitrogen-heavy fertilizer can promote vigorous vegetative growth immediately after transplanting. Once the root begins to swell noticeably, usually mid-season, transitioning to a balanced fertilizer with lower nitrogen content is appropriate.
A specific maintenance practice involves removing the tough, lower lateral roots that sometimes emerge from the developing bulb, a process often referred to as “shaving” the root. Gently mounding soil or mulch around the base of the plant can also help to blanch the developing root, keeping it tender and lighter in color.
Removing a few of the older, outer leaves can also be beneficial, especially those that begin to yellow or lie flat on the ground. This selective pruning improves air circulation, which helps deter fungal diseases, and directs more energy toward the expanding root. Vigilance against common pests, such as celery leaf miners or aphids, is also necessary, as damage to the foliage can severely limit the plant’s ability to produce a large, healthy root.
Harvesting and Post-Harvest Handling
The optimal time to harvest celeriac is in the late fall, after the foliage has experienced a few light frosts but before the ground freezes solid. Exposure to a light frost can improve the flavor of the root, concentrating its natural sugars and enhancing the nutty sweetness. The bulb is ready for harvest once it reaches a diameter of approximately three to five inches.
Harvesting requires a gentle approach to avoid damaging the root. Using a garden fork to carefully loosen the soil around the perimeter of the plant allows the bulb to be lifted without snapping the remaining roots.
After harvesting, the foliage and any small, fibrous rootlets should be trimmed away, leaving only the clean, firm bulb. The trimmed root should be allowed to air dry slightly before being prepared for storage. Celeriac stores exceptionally well when kept in a cool, dark environment with high humidity.
Cleaned roots can be layered in a container filled with slightly damp materials like sand, sawdust, or peat moss. Stored in a root cellar or cold basement between 32°F and 40°F (0°C and 4°C), the bulbs can last for several months throughout the winter.