Celeriac (Celery Root)

Sow 1/4 inch deep or press seed onto the surface with only a light sprinkling of soil over it, and keep it constantly moist. Germinate at 70-75F, then hold seedlings at 65-70F once they are up. Expect two to three weeks for sprouting, and do not expect every seed to come up. Celeriac needs roughly 100-150 days to size a root, so it has to be well along before it ever reaches the garden. The seed is tiny and light-dependent for germination, which is why burying it deeply or letting the surface dry out is the usual cause of a failed flat. Seed can also go dormant above 80F, so a hot windowsill works against you.

Move seedlings out once frost risk has passed and nights have settled, spacing them 6-12 inches apart in rows 18-36 inches apart. Do not chill plants to harden them off. If you use mulch film, choose bare ground or white plastic rather than black. Celeriac bolts instead of forming a root if young plants sit below 55F for more than about seven days, and a plant that has begun to bolt splits its petioles and never sizes a knob. Celeriac is also more sensitive than celery to the heat trapped under black plastic, which pushes top growth at the root's expense.

Keep the top few inches of soil moist but not wet at all times, ideally with drip irrigation. Water early in the day so the foliage dries, and avoid the pattern of letting the bed go dry and then flooding it. Celeriac is shallow rooted, and any interruption of growth permanently shrinks the root. Wild swings in moisture also drive blackheart, in which the innermost growing points die because the plant cannot assimilate calcium during the growth surge that follows a drought broken by heavy water. Wet foliage overnight is separately what lets the leaf blights infect.

Work compost or rotted manure in before planting, then side-dress once at 3-4 weeks after transplanting and again at 7-8 weeks after setting out. Base the total rate on a soil test rather than guessing. This is a long-season heavy feeder that has to hold steady growth for three to five months, and the root only bulks while the tops are actively feeding it. Splitting the nitrogen into two later doses matches the crop's demand instead of dumping it all at planting, when the seedling cannot use it.

As the crown begins to swell, snip away the side roots that form around it. You can hill soil over the knob to blanch the outside white, or leave it unhilled and standing proud of the soil. Side roots split the plant's effort and leave a knob that is harder to peel and trim at harvest. Hilling is a genuine tradeoff rather than a rule: it blanches the skin white, but roots left to push above soil level with no hilling may finish larger.

Look over the foliage twice weekly. Pick off and destroy yellow leaves and any showing the small holes and tunnels of celery leafminer. Remove and destroy plants with curled, fan-like leaves and twisted petioles. Do not follow celeriac with its own family — carrots, parsley, parsnip, and celery share its pests and residue-borne diseases; rotate the bed to an unrelated crop such as onion or lettuce for at least 2 years. Remove or till under crop debris after harvest. Small yellow spots on the leaves are early or late blight, and late blight lesions carry dark pycnidia in their centers. Both survive on crop residue and on the seed, and both run fast once foliage stays wet above 70F. Leafminers themselves do minimal damage as long as their numbers stay in check, so handwork is usually enough in a garden.

Lift the roots with a garden fork once they reach 3-4 inches across or slightly larger, cutting the stems close to the knob and trimming the side roots. Leave the crop out through at least the first frost of fall before digging in earnest. Celeriac does not reach its full flavor until after the first frost, which is why digging on size alone in early fall gives you a blander root than waiting. It holds fairly well in the ground, but leaving it too long in wet conditions invites internal hollowing and rot, and a plant carried into a second year as the biennial it is just throws a tall seed stalk and a pithy root.

Cover the plants with straw or leaves in fall and pull roots as you want them. Where you would rather store them, trim the roots and stalks and hold them at 32-34F at 95 percent relative humidity, where they will keep for three to six months. The roots take frost, so the practical limit on a fall garden is frozen ground rather than the plant itself. A straw blanket keeps the soil diggable and lets the bed act as the root cellar. In cold storage, humidity is what keeps the knob from shriveling, and how well it holds depends on variety and on how much disease it met before harvest.