How to Plant, Grow, and Save Seeds from Kohlrabi (Brassica oleracea)
Kohlrabi, sometimes called German turnip or turnip cabbage, was developed by selection for an enlarged edible stem. The edible part of the plant, which can be eaten raw or cooked, is either green or purple.
Sow kohlrabi seeds ¼” deep. Seeds will germinate in 3-10 days. Seeds can be sown outside just after the last frost, or inside several weeks before the last frost and then transplanted outside. If planting kohlrabi as a fall crop, direct sow seeds around 90 days before the first frost date. In the spring, transplant kohlrabi seedlings outdoors around one to two weeks before your last frost date. Space plants 9-12” apart. Harvest kohlrabi plants when the bulbs reach 3” in diameter.
Kohlrabi is susceptible to pests such as flea beetles. It is easy to deter these beetles by covering kohlrabi plants with a thin row cover.
Eating and Storing
Kohlrabi bulbs can be used in a variety of ways. Raw kohlrabi can be grated and used in salads, the bulb can be chopped finely and incorporated into soups, shredded kohlrabi can be made into fritters or vegetable pancakes, roasted kohlrabi is a great winter treat, and steamed kohlrabi can be added to soups. Kohlrabi greens can also be eaten when young. Bulbs will store in the refrigerator for several weeks.
Kohlrabi is a biennial crop. Biennials typically do not flower in their first growing season. They must first experience cold weather before they flower, set seed, and die in their second growing season.
When saving seeds from kohlrabi, separate varieties by at least 800 feet-½ mile in their second year. To ensure viable seeds, save seeds from at least 5 plants. To maintain a variety over time, save seeds from between 20-50 plants.
To save seeds from kohlrabi, learn how to vernalize your plants. Vernalization can happen in the field or in storage. Overwinter cabbage in the garden if you will have 10-12 weeks of cool weather (around 50 °F) without regular temperatures below 35 °F.
When plants cannot be successfully overwintered in the garden, they can be vernalized in storage. Before the first frost, dig up the entire plant, roots and all. Trim off outer leaves but keep the growing point intact. Replant these trimmed plants into containers filled with slightly moist potting mix or sand. Then, find a place to store your plants. The optimum storage conditions for kohlrabi vernalization ranges from 34-39 F and 80-95% relative humidity. A traditional root cellar is ideal but garages, sheds, and other unheated structures work well in some climates.
In the spring, when the soil can be worked, replant kohlrabi in your garden. Space plants at least 36” apart. After flowering in their second year of growth, mature seed pods become dry and turn brown as the seeds inside also mature and brown. As with many of the Brassica crops, the window of time for an optimal harvest may be short as mature pods will begin to shatter and bird predation can become a problem.
Seeds can be gathered by cutting entire branches or by harvesting whole plants. Because of this species’ tendency to shatter, the harvested material should be placed on drop cloths or in containers to prevent seed loss. Branches of mature fruits can be threshed by rubbing the pods between one’s hands or by hitting the brittle pods against any surface that will cause fruits to break open. If the pods are dry, they will release their seeds easily when threshed.
Store kohlrabi seeds in a cool, dark, and dry place in an airtight container to keep out moisture and humidity. Properly stored kohlrabi seeds will remain viable for several years.