(For details on growing many other vegetables and fruits, visit our Crop at a Glancecollection page.)
One of the newest members to join the cabbage family crops, kohlrabi was bred in Germany in the 1500s. Instead of growing a head like cabbage, broccoli and cauliflower, closely related kohlrabi develops a rounded “bulb” in its stem, just above the soil line.
Growing kohlrabi is easy as long as you use the best planting dates for your climate. In most areas, you can grow spring and fall crops of fast-maturing kohlrabi varieties, and end the season with a few storage-type kohlrabi, which grow unusually large.
Types of Kohlrabi
Kohlrabi varieties vary in growth rate and color. The earliest-maturing varieties are mostly classically bred hybrids (noted by “F1” below), while the best storage varieties are open-pollinated.
Green kohlrabi varieties such as ‘Korridor’ (F1; 50 days) and ‘Winner’ (F1; 45 days) are fast growers that thrive in a wide range of climates. You can plant green and purple kohlrabi together to create a beautiful bed.
Purple kohlrabi varieties such as ‘Azur Star’ (58 days) and’ Kolibri’ (45 days) are even easier to grow than green kohlrabi, because cabbage worms and other insects avoid the purple leaves.
Storage kohlrabi varieties include’ Kossak’ (F1; 80 days),’Gigante’ (up to 130 days), and ‘Superschmelz’ (60 to 80 days). When given wide spacing and regular water, these storage varieties produce very large bulbs that are up to 10 inches across. The bulbs will store for weeks in a refrigerator or cold root cellar.
How to Plant Kohlrabi
For your first spring sowing of kohlrabi, start seeds indoors about six weeks before your last spring frost date. Kohlrabi is not as cold-tolerant as other cabbage family crops, so wait until two weeks before your last frost date to set out seedlings. This is also the best time to direct-sow seeds in a well-prepared bed.
Kohlrabi plants are heavy feeders that demand moist, fertile soil with a pH between 6.0 and 7.5. Micronutrients are important, so take the time to amend the soil with compost and a standard application of a balanced organic fertilizer before planting. Thin or transplant seedlings to 8 inches apart, but allow more space when growing large, vigorous storage varieties. When growing kohlrabi in spring, use a biodegradable mulch of grass clippings or coarse compost to insulate the roots from summer’s heat.
Start seeds of storage varieties in midsummer, at about the same time you would sow broccoli or cabbage for fall harvest. This is usually about 90 days before your average first fall frost date. Harden off the seedlings before setting them out in well-prepared soil, and plan to cover them with lightweight row cover or tulle to exclude insect pests.
For recommended planting dates for your local climate — and to design your garden beds — try our Vegetable Garden Planner.
In spring, you may need to protect kohlrabi from cold winds with cloches or a tunnel covered with row cover or perforated plastic. Featherweight row cover held aloft with hoops or stakes is the easiest way to protect actively growing kohlrabi from insects. Should the little plants refuse to grow even after the soil warms up in spring, drench them with a high-nitrogen fertilizer, such as fish emulsion, every two weeks.
Harvesting and Storage
Pull up entire plants when kohlrabi bulbs reach 3 inches in diameter. Cut off the roots and leaves and store the bulbs in your refrigerator. Kohlrabi will keep in the refrigerator for several weeks, or you can blanch and freeze uniform chunks of peeled kohlrabi.
Some people enjoy eating the young leaves taken from the tops of kohlrabi as cooked greens, especially in fall when the leaves tend to be sweet and tender. This is seldom true of leaves taken from kohlrabi that has grown in warm weather.
As biennials, kohlrabi plants produce yellow flowers followed by elongated seedpods in their second year. Seedlings that are exposed to more than two weeks of cold spring weather may bloom in their first year. After the flowers fade and the seedpods dry to tan, gather them in a paper bag, and allow them to dry indoors for a week. Shatter the dry pods and collect the largest seeds for replanting. Under good conditions, kohlrabi seeds will store for up to 4 years.
For growing advice for many more garden crops, check out our complete Crops at a Glance Guide.
Contributing editor Barbara Pleasant gardens in southwest Virginia, where she grows vegetables, herbs, fruits, flowers and a few lucky chickens. Contact Barbara by visiting her website or finding her on Google+.