Culture
Celeriac or celery root (Apium graveolens var. rapaceum) is a cool-weather vegetable that dislikes the heat and humidity of a typical St. Louis summer. Best growing season temperatures fall within the range of 60-75°F. Celeriac tends to bolt if exposed to temperatures of 55°F. or less for continuous periods of 1-2 weeks (cover plants at night if this problem occurs). This plant is difficult to grow well. It requires a moist, rich, well-drained soil in full sun with regular fertilization and consistent watering. Mulch helps soils retain moisture while discouraging weed growth. Start seed indoors about 10 weeks prior to last spring frost date. Starter plants should be set out in the garden about 8-10" apart after last spring frost date. Celeriac generally requires a rich, well-drained soil in full sun. Roots are ready to harvest after 3-4 months. It may be stored 6-8 months in a root cellar. Celeriac is not commonly used in the U.S.
Noteworthy Characteristics
Apium graveolens is a biennial to perennial herb that is reportedly native to temperate Mediterranean climates in Europe, Asia and Africa. Wild plants typically grow to 24-36" tall and to 12-18" wide with aromatic pinnately divided leaves. In biennial form, the plant forms a basal rossette of leaves in the first year followed in the second year by a summer bloom of off-white flowers in umbels. Plants may naturalize in the landscape through self-seeding. This species is the progenitor of two different vegetable crops: (1) Celery (Apium graveolens var. dulce) which is grown for its edible crisp leaf stalks and leaves and (2) Celery root or Celeriac (Apium graveolens var. rapaceum) which is grown for its swollen, edible, enlarged, brown, turnip-like roots.
Var. rapaceum is know as celeriac or celery root and tastes like celery but with turnip-like additional flavoring. Roots are edible raw or cooked. Roots generally have their best flavor when harvested after several light fall frosts. Roots have a shelf life of 3-4 months if properly refrigerated (from 33 to 40°F.).
Genus name from Latin means bee (bees are reportedly attracted to the white flowers of the plant).
'Diamant' produces a clean, white, medium-sized, round but irregular root. It grows to 2' tall with glossy dark green leaves.
Problems
Aphids, earwigs, nematodes, celery worms and slugs. Root rot may develop in overly wet soils. Early blight and late blight can cause problems in some areas.