Kohlrabi is one of the least grown members of the cabbage family, but nonetheless one of my personal favorites. Just above the ground, the stem of kohlrabi swells into a round ball-shaped knob anywhere from an inch to a foot in diameter (depending on the variety and the growing conditions), and the leaves of the plant spring directly from the top and sides of this knob, giving it the appearance of some sort of satellite with leaves.
Kohlrabi is a quick-growing plant, and the spring varieties should be direct-sown in the garden a month before the last frost, 1/4 to 1/2 inch deep in rows twelve to eighteen inches apart. It should be thinned at the four-leaf stage so the plants stand four to six inches apart.
You can also sow it in beds, placing two or three seeds in spots six to eight inches apart, equidistant in all directions, and thin out the weakest seedlings. The larger, fall types can use a foot or more of space, and should be planted during a summer cool spell 60 days or so before the first frost, but no later than the end of August, except in the deep South.
Pests and diseases are the same as the rest of the cabbage family, but kohlrabi is rarely bothered in our garden by anything other than Harlequin bugs. The biggest problem we have is splitting, which is more or less prevalent in different varieties and controlled by consistent irrigation.
We let spring-sown knobs reach two or three inches in diameter before harvest, then cut the plants off just above the ground. Some fall varieties will get quite large and should be left until the danger of hard frost threatens (<25 F). If protected, and then stored in a high-humidity, low-temperature root cellar, they will keep for two to three months.