Kohlrabi (Brassica caulopara pasq., the Cruciferae family) is a biennial plant. During the first year, it develops a short stalk, which expands to form a round or oval stem-crop 6 to 15 cm in diameter. During the second year, it develops floriferous shoots, blooms, and produces seeds.
The edible part of the plant is the stem-crop, which is light green, light purple, crimson or dark purple in color and has white juicy flesh. As it nears overmaturity, the stem-crop becomes tough and uneatable.
At industrial ripeness, the stem-crop contains 3.5% protein, up to 8% sugars, 40 to 100 mg% ascorbic acid - as much as the lemon and orange fruit do - as well as carotene and vitamins B1, B2 and PP.
Kohlrabi is less demanding than other cabbages in terms of growing conditions. This crop grows well on all types of soil. It is a cool-season, moisture-loving crop. Under low soil moisture and air humidity, the stem-crops elongate and toughen, and with excess watering they burst. Due to its short growing season, kohlrabi fails to respond to fresh organic fertilizers. Therefore, only mineral fertilizers are applied to kohlrabi, at the same rate of application as for cabbage.
Being a very early-maturing crop, kohlrabi is successfully grown in spring, spring-summer and autumn culture.
Growing tips
The best preceding crops are annual legumes, cucurbits, winter cereals, early tomato and potato, and onion. Healthy, uniform transplants are a key to early harvest. These should be 35-50 days old, depending on the growing technique, and have 4-6 well-developed leaves. The planting pattern is 40 x 40 cm or 40 x 50 cm with a seeding rate of 70-90 seeds per 10 m2. Cassette or potted seedlings become more readily established because no root system injuries occur during transplanting and, hence, there are no diseased plants. To obtain high yields of kohlrabi (3,5-4 kg/m2 or 35-40 ton/ha), fertilizers are applied during the growing season at a rate of 400-500 g ammonium nitrate, 250 g superphosphate and 350-400 g potassium salt per 10 m2 (N 170, P2O5 50 and K2O 160 kg of active substance per hectare). The phosphate-potassium fertilizers are applied during autumn soil cultivation and the nitrogenous ones as supplementary fertilizing.
Systematic irrigation is a crucial factor in the cultivation cycle. Kohlrabi plants are given 5 to 8 irrigations with the water application rate of 350-400 l per 10 m2 (350-400 m3/ha) per irrigation.
To avoid unmarketable stem-crops, kohlrabi plants, unlike other cabbages, should not be hilled throughout the growing season.
During the growing season, the crop may be attacked by a number of pests such as blue fleas, cabbage aphid, cabbage butterfly and cutworm, cabbage moth, and thrips. Therefore, chemical control is an essential component of the kohlrabi production technology.
Control measures: on discovering the first signs of infestation, sprays with insecticides such as Sumi-alpha, Zolone, Sherpa, Karate, Aktellic, Volaton, etc. should be made systematically in cycles of 7-10 days. Considering that the cabbage leaf has a thick wax coating, it is advisable to admix adhesives (OP-7, skimmed milk or household soap in amounts of 30-50 g per 10 l of water) into the spray solution.
The cabbage root fly, which is the most harmful insect pest attacking cabbage plants, deserves special mention. It starts to fly in mid- or late April when the soil has warmed up to 12-13°C. The fly lays eggs on the root neck or in the soil around it. The hatching larvae penetrate the roots and gnaw passages in them. As a result, the plants begin to wilt (especially in dry, hot weather), the leaves turn leaden-blue, the growth is retarded or completely terminated and, not infrequently, the plants die.
Control measures: good cultural practices, preventive sprays (at the beginning of flight of butterflies) with the above-mentioned insecticides. Where the plants are attacked by larvae of the cabbage root fly, soil application of 0,15-0,2% Bi-58 or Basudin at a rate of 0,25-0,3 l per plant is recommended. A particularly effective control measure is preplant furrow application of pelleted phosphamide or pelleted superphosphate treated with phosphamide (Bi-58).